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Subject: Re: [virtio] Groups - Action Item "Create text version of virtio 0.9.5 document" added

Rusty Russell <rusty@au1.ibm.com> writes:
> -----------------
> Action Item Subject: Create text version of virtio 0.9.5 document

OK, I've attached this below.  This is exactly the spec as 0.9.5
reformatted into text so we can work it.

I'm still waiting for virtio to be added to the OASIS issue tracking
system, where I will be prompting you all to open an issue for every
change we want to consider.

The obvious issues I want to open are:

o Major rework to make PCI an appendix, and core bus-independent.
o Update spec with changes/fixes since 0.9.5
        - This is easy where contributors are already members,
          trickier for others.

In addition here's a brain dump:

        o Endian for config space
          - LE everywhere?
        o Endian for ring
          - LE as well?
        o Allow arbitrary descriptor layouts / message framing.
        o Method to stop activity on a queue?
        o Size descriptor table independent of ringsize?
        o Remove VIRTIO_F_NOTIFY_ON_EMPTY?
        o Remove limit on # indirect descriptors
                - Some other limit?
        o Simplify indirect desc
                - No return to top level on end of desc array
        o Allow chained indirect desc
                - indirect bit use to chain?

        o Remove VIRTIO_NET_F_GSO?

        o Remove VIRTIO_BLK_F_SCSI?
        o Revisit flush/barrier semantics?

        o New capability layout
        o Allowing non-zero revision numbers?
        o Remove 'align' and use explicit addresses for used/avail.

        o Fix endianness
        o Remove outgoing page queue?


This document describes the specifications of the “virtio” family 
of PCI devices. These are devices 
are found in virtual environments, 
yet by design they are not all that different from physical PCI 
devices, and this document treats them as such. This allows the 
guest to use standard PCI drivers and discovery mechanisms.

The purpose of virtio and this specification is that virtual 
environments and guests should have a straightforward, efficient, 
standard and extensible mechanism for virtual devices, rather 
than boutique per-environment or per-OS mechanisms.

  Straightforward: Virtio PCI devices use normal PCI mechanisms 
  of interrupts and DMA which should be familiar to any device 
  driver author. There is no exotic page-flipping or COW 
  mechanism: it's just a PCI device.[1]

  Efficient: Virtio PCI devices consist of rings of descriptors 
  for input and output, which are neatly separated to avoid cache 
  effects from both guest and device writing to the same cache 

  Standard: Virtio PCI makes no assumptions about the environment 
  in which it operates, beyond supporting PCI. In fact the virtio 
  devices specified in the appendices do not require PCI at all: 
  they have been implemented on non-PCI buses.[2]

  Extensible: Virtio PCI devices contain feature bits which are 
  acknowledged by the guest operating system during device setup. 
  This allows forwards and backwards compatibility: the device 
  offers all the features it knows about, and the driver 
  acknowledges those it understands and wishes to use.

1.1 Virtqueues

The mechanism for bulk data transport on virtio PCI devices is 
pretentiously called a virtqueue. Each device can have zero or 
more virtqueues: for example, the network device has one for 
transmit and one for receive.

Each virtqueue occupies two or more physically-contiguous pages 
(defined, for the purposes of this specification, as 4096 bytes), 
and consists of three parts:

| Descriptor Table  |   Available Ring     (padding)    | Used Ring |

When the driver wants to send a buffer to the device, it fills in 
a slot in the descriptor table (or chains several together), and 
writes the descriptor index into the available ring. It then 
notifies the device. When the device has finished a buffer, it 
writes the descriptor into the used ring, and sends an interrupt.


2.1 PCI Discovery

Any PCI device with Vendor ID 0x1AF4, and Device ID 0x1000 through
0x103F inclusive is a virtio device[3]. The device must also have a
Revision ID of 0 to match this specification.

The Subsystem Device ID indicates which virtio device is 
supported by the device. The Subsystem Vendor ID should reflect 
the PCI Vendor ID of the environment (it's currently only used 
for informational purposes by the guest).

| Subsystem Device ID  |   Virtio Device    | Specification |
|          1           |   network card     |  Appendix C   |
|          2           |   block device     |  Appendix D   |
|          3           |      console       |  Appendix E   |
|          4           |  entropy source    |  Appendix F   |
|          5           | memory ballooning  |  Appendix G   |
|          6           |     ioMemory       |       -       |
|          7           |       rpmsg        |       -       |
|          8           |     SCSI host      |  Appendix I   |
|          9           |   9P transport     |       -       |
|         10           |   mac80211 wlan    |       -       |

2.2 Device Configuration

To configure the device, we use the first I/O region of the PCI 
device. This contains a virtio header followed by a 
device-specific region.

There may be different widths of accesses to the I/O region; the
“natural” access method for each field in the virtio header must be
used (i.e. 32-bit accesses for 32-bit fields, etc), but the
device-specific region can be accessed using any width accesses, and
should obtain the same results.

Note that this is possible because while the virtio header is PCI 
(i.e. little) endian, the device-specific region is encoded in 
the native endian of the guest (where such distinction is 

2.2.1 Device Initialization Sequence

We start with an overview of device initialization, then expand 
on the details of the device and how each step is preformed.

1. Reset the device. This is not required on initial start up.

2. The ACKNOWLEDGE status bit is set: we have noticed the device.

3. The DRIVER status bit is set: we know how to drive the device.

4. Device-specific setup, including reading the Device Feature 
  Bits, discovery of virtqueues for the device, optional MSI-X 
  setup, and reading and possibly writing the virtio 
  configuration space.

5. The subset of Device Feature Bits understood by the driver is 
  written to the device.

6. The DRIVER_OK status bit is set.

7. The device can now be used (ie. buffers added to the 

If any of these steps go irrecoverably wrong, the guest should 
set the FAILED status bit to indicate that it has given up on the 
device (it can reset the device later to restart if desired).

We now cover the fields required for general setup in detail.

2.2.2 Virtio Header

The virtio header looks as follows:

| Bits       || 32                  | 32                  | 32       | 16     | 16      | 16      | 8       | 8      |
| Read/Write || R                   | R+W                 | R+W      | R      | R+W     | R+W     | R+W     | R      |
| Purpose    || Device              | Guest               | Queue    | Queue  | Queue   | Queue   | Device  | ISR    |
|            || Features bits 0:31  | Features bits 0:31  | Address  | Size   | Select  | Notify  | Status  | Status |

If MSI-X is enabled for the device, two additional fields 
immediately follow this header:[5]

| Bits       || 16             | 16     |
| Read/Write || R+W            | R+W    |
| Purpose    || Configuration  | Queue  |
| (MSI-X)    || Vector         | Vector |

Immediately following these general headers, there may be 
device-specific headers:

| Bits       || Device Specific    |
| Read/Write || Device Specific    |
| Purpose    || Device Specific... |
|            ||                    |
+------------++--------------------+ Device Status

The Device Status field is updated by the guest to indicate its 
progress. This provides a simple low-level diagnostic: it's most 
useful to imagine them hooked up to traffic lights on the console 
indicating the status of each device.

The device can be reset by writing a 0 to this field, otherwise 
at least one bit should be set:

  ACKNOWLEDGE (1) Indicates that the guest OS has found the 
  device and recognized it as a valid virtio device.

  DRIVER (2) Indicates that the guest OS knows how to drive the 
  device. Under Linux, drivers can be loadable modules so there 
  may be a significant (or infinite) delay before setting this 

  DRIVER_OK (4) Indicates that the driver is set up and ready to 
  drive the device.

  FAILED (128) Indicates that something went wrong in the guest, 
  and it has given up on the device. This could be an internal 
  error, or the driver didn't like the device for some reason, or 
  even a fatal error during device operation. The device must be 
  reset before attempting to re-initialize. Feature Bits

The first configuration field indicates the features that the 
device supports. The bits are allocated as follows:

  0 to 23 Feature bits for the specific device type

  24 to 32 Feature bits reserved for extensions to the queue and 
  feature negotiation mechanisms

For example, feature bit 0 for a network device (i.e. Subsystem 
Device ID 1) indicates that the device supports checksumming of 

The feature bits are negotiated: the device lists all the 
features it understands in the Device Features field, and the 
guest writes the subset that it understands into the Guest 
Features field. The only way to renegotiate is to reset the 

In particular, new fields in the device configuration header are 
indicated by offering a feature bit, so the guest can check 
before accessing that part of the configuration space.

This allows for forwards and backwards compatibility: if the 
device is enhanced with a new feature bit, older guests will not 
write that feature bit back to the Guest Features field and it 
can go into backwards compatibility mode. Similarly, if a guest 
is enhanced with a feature that the device doesn't support, it 
will not see that feature bit in the Device Features field and 
can go into backwards compatibility mode (or, for poor 
implementations, set the FAILED Device Status bit). Configuration/Queue Vectors

When MSI-X capability is present and enabled in the device 
(through standard PCI configuration space) 4 bytes at byte offset 
20 are used to map configuration change and queue interrupts to 
MSI-X vectors. In this case, the ISR Status field is unused, and 
device specific configuration starts at byte offset 24 in virtio 
header structure. When MSI-X capability is not enabled, device 
specific configuration starts at byte offset 20 in virtio header.

Writing a valid MSI-X Table entry number, 0 to 0x7FF, to one of 
Configuration/Queue Vector registers, maps interrupts triggered 
by the configuration change/selected queue events respectively to 
the corresponding MSI-X vector. To disable interrupts for a 
specific event type, unmap it by writing a special NO_VECTOR 

/* Vector value used to disable MSI for queue */

#define VIRTIO_MSI_NO_VECTOR            0xffff 

Reading these registers returns vector mapped to a given event, 
or NO_VECTOR if unmapped. All queue and configuration change 
events are unmapped by default.

Note that mapping an event to vector might require allocating 
internal device resources, and might fail. Devices report such 
failures by returning the NO_VECTOR value when the relevant 
Vector field is read. After mapping an event to vector, the 
driver must verify success by reading the Vector field value: on 
success, the previously written value is returned, and on 
failure, NO_VECTOR is returned. If a mapping failure is detected, 
the driver can retry mapping with fewervectors, or disable MSI-X.

2.3 Virtqueue Configuration

As a device can have zero or more virtqueues for bulk data 
transport (for example, the network driver has two), the driver 
needs to configure them as part of the device-specific 

This is done as follows, for each virtqueue a device has:

1. Write the virtqueue index (first queue is 0) to the Queue 
  Select field.

2. Read the virtqueue size from the Queue Size field, which is 
  always a power of 2. This controls how big the virtqueue is 
  (see below). If this field is 0, the virtqueue does not exist. 

3. Allocate and zero virtqueue in contiguous physical memory, on 
  a 4096 byte alignment. Write the physical address, divided by 
  4096 to the Queue Address field.[6]

4. Optionally, if MSI-X capability is present and enabled on the 
  device, select a vector to use to request interrupts triggered 
  by virtqueue events. Write the MSI-X Table entry number 
  corresponding to this vector in Queue Vector field. Read the 
  Queue Vector field: on success, previously written value is 
  returned; on failure, NO_VECTOR value is returned.

The Queue Size field controls the total number of bytes required 
for the virtqueue according to the following formula:

	#define ALIGN(x) (((x) + 4095) & ~4095)

	static inline unsigned vring_size(unsigned int qsz)
	     return ALIGN(sizeof(struct vring_desc)*qsz + sizeof(u16)*(2 + qsz))
	          + ALIGN(sizeof(struct vring_used_elem)*qsz);

This currently wastes some space with padding, but also allows 
future extensions. The virtqueue layout structure looks like this 
(qsz is the Queue Size field, which is a variable, so this code 
won't compile):

	struct vring {
		/* The actual descriptors (16 bytes each) */
		struct vring_desc desc[qsz];
		/* A ring of available descriptor heads with free-running index. */
		struct vring_avail avail;
		// Padding to the next 4096 boundary.
		char pad[];

		// A ring of used descriptor heads with free-running index.
		struct vring_used used;

2.3.1 A Note on Virtqueue Endianness

Note that the endian of these fields and everything else in the 
virtqueue is the native endian of the guest, not little-endian as 
PCI normally is. This makes for simpler guest code, and it is 
assumed that the host already has to be deeply aware of the guest 
endian so such an “endian-aware” device is not a significant 

2.3.2 Descriptor Table

The descriptor table refers to the buffers the guest is using for 
the device. The addresses are physical addresses, and the buffers 
can be chained via the next field. Each descriptor describes a 
buffer which is read-only or write-only, but a chain of 
descriptors can contain both read-only and write-only buffers.

No descriptor chain may be more than 2^32 bytes long in total.

	struct vring_desc {
		/* Address (guest-physical). */
		u64 addr;
		/* Length. */
		u32 len;
	/* This marks a buffer as continuing via the next field. */
	#define VRING_DESC_F_NEXT   1
	/* This marks a buffer as write-only (otherwise read-only). */
	#define VRING_DESC_F_WRITE     2
	/* This means the buffer contains a list of buffer descriptors. */
		/* The flags as indicated above. */
		u16 flags;
		/* Next field if flags & NEXT */
		u16 next;

The number of descriptors in the table is specified by the Queue 
Size field for this virtqueue.

2.3.3 Indirect Descriptors

Some devices benefit by concurrently dispatching a large number 
of large requests. The VIRTIO_RING_F_INDIRECT_DESC feature can be 
used to allow this (see Appendix B: Reserved Feature Bits). To increase 
ring capacity it is possible to store a table of indirect 
descriptors anywhere in memory, and insert a descriptor in main 
virtqueue (with flags&INDIRECT on) that refers to memory buffer 
containing this indirect descriptor table; fields addr and len 
refer to the indirect table address and length in bytes, 
respectively. The indirect table layout structure looks like this 
(len is the length of the descriptor that refers to this table, 
which is a variable, so this code won't compile):

	struct indirect_descriptor_table {
		/* The actual descriptors (16 bytes each) */
		struct vring_desc desc[len / 16];

The first indirect descriptor is located at start of the indirect 
descriptor table (index 0), additional indirect descriptors are 
chained by next field. An indirect descriptor without next field 
(with flags&NEXT off) signals the end of the indirect descriptor 
table, and transfers control back to the main virtqueue. An 
indirect descriptor can not refer to another indirect descriptor 
table (flags&INDIRECT must be off). A single indirect descriptor 
table can include both read-only and write-only descriptors; 
write-only flag (flags&WRITE) in the descriptor that refers to it 
is ignored.

2.3.4 Available Ring

The available ring refers to what descriptors we are offering the
device: it refers to the head of a descriptor chain. The “flags” field
is currently 0 or 1: 1 indicating that we do not need an interrupt
when the device consumes a descriptor from the available
ring. Alternatively, the guest can ask the device to delay interrupts
until an entry with an index specified by the “ used_event” field is
written in the used ring (equivalently, until the idx field in the
used ring will reach the value used_event + 1). The method employed by
the device is controlled by the VIRTIO_RING_F_EVENT_IDX feature bit
(see Appendix B: Reserved Feature Bits). This interrupt suppression is
merely an optimization; it may not suppress interrupts entirely.

The “idx” field indicates where we would put the next descriptor 
entry (modulo the ring size). This starts at 0, and increases.

	struct vring_avail {
		u16 flags;
		u16 idx;
		u16 ring[qsz]; /* qsz is the Queue Size field read from device */
		u16 used_event;

2.3.5 Used Ring

The used ring is where the device returns buffers once it is done 
with them. The flags field can be used by the device to hint that 
no notification is necessary when the guest adds to the available 
ring. Alternatively, the “avail_event” field can be used by the 
device to hint that no notification is necessary until an entry 
with an index specified by the “avail_event” is written in the 
available ring (equivalently, until the idx field in the 
available ring will reach the value avail_event + 1). The method 
employed by the device is controlled by the guest through the 
VIRTIO_RING_F_EVENT_IDX feature bit (see Appendix B: Reserved
Feature Bits).[7]

Each entry in the ring is a pair: the head entry of the 
descriptor chain describing the buffer (this matches an entry 
placed in the available ring by the guest earlier), and the total 
of bytes written into the buffer. The latter is extremely useful 
for guests using untrusted buffers: if you do not know exactly 
how much has been written by the device, you usually have to zero 
the buffer to ensure no data leakage occurs.

	/* u32 is used here for ids for padding reasons. */
	struct vring_used_elem {
		/* Index of start of used descriptor chain. */
		u32 id;
		/* Total length of the descriptor chain which was used (written to) */
		u32 len;

	struct vring_used {
		u16 flags;
		u16 idx;
		struct vring_used_elem ring[qsz];
		u16 avail_event;

2.3.6 Helpers for Managing Virtqueues

The Linux Kernel Source code contains the definitions above and 
helper routines in a more usable form, in 
include/linux/virtio_ring.h. This was explicitly licensed by IBM 
and Red Hat under the (3-clause) BSD license so that it can be 
freely used by all other projects, and is reproduced (with slight 
variation to remove Linux assumptions) in Appendix A.

2.4 Device Operation

There are two parts to device operation: supplying new buffers to 
the device, and processing used buffers from the device. As an 
example, the virtio network device has two virtqueues: the 
transmit virtqueue and the receive virtqueue. The driver adds 
outgoing (read-only) packets to the transmit virtqueue, and then 
frees them after they are used. Similarly, incoming (write-only) 
buffers are added to the receive virtqueue, and processed after 
they are used.

2.4.1 Supplying Buffers to The Device

Actual transfer of buffers from the guest OS to the device 
operates as follows:

1. Place the buffer(s) into free descriptor(s).

  (a) If there are no free descriptors, the guest may choose to 
    notify the device even if notifications are suppressed (to 
    reduce latency).[8]

2. Place the id of the buffer in the next ring entry of the 
  available ring.

3. The steps (1) and (2) may be performed repeatedly if batching 
  is possible.

4. A memory barrier should be executed to ensure the device sees 
  the updated descriptor table and available ring before the next 

5. The available “idx” field should be increased by the number of 
  entries added to the available ring.

6. A memory barrier should be executed to ensure that we update 
  the idx field before checking for notification suppression.

7. If notifications are not suppressed, the device should be 
  notified of the new buffers.

Note that the above code does not take precautions against the 
available ring buffer wrapping around: this is not possible since 
the ring buffer is the same size as the descriptor table, so step 
(1) will prevent such a condition.

In addition, the maximum queue size is 32768 (it must be a power 
of 2 which fits in 16 bits), so the 16-bit “idx” value can always 
distinguish between a full and empty buffer.

Here is a description of each stage in more detail. Placing Buffers Into The Descriptor Table

A buffer consists of zero or more read-only physically-contiguous 
elements followed by zero or more physically-contiguous 
write-only elements (it must have at least one element). This 
algorithm maps it into the descriptor table:

1. for each buffer element, b:

  (a) Get the next free descriptor table entry, d

  (b) Set d.addr to the physical address of the start of b

  (c) Set d.len to the length of b.

  (d) If b is write-only, set d.flags to VRING_DESC_F_WRITE, 
    otherwise 0.

  (e) If there is a buffer element after this:

    i. Set d.next to the index of the next free descriptor 

    ii. Set the VRING_DESC_F_NEXT bit in d.flags.

In practice, the d.next fields are usually used to chain free 
descriptors, and a separate count kept to check there are enough 
free descriptors before beginning the mappings. Updating The Available Ring

The head of the buffer we mapped is the first d in the algorithm 
above. A naive implementation would do the following:

	avail->ring[avail->idx % qsz] = head;

However, in general we can add many descriptors before we update 
the “idx” field (at which point they become visible to the 
device), so we keep a counter of how many we've added:

	avail->ring[(avail->idx + added++) % qsz] = head; Updating The Index Field

Once the idx field of the virtqueue is updated, the device will 
be able to access the descriptor entries we've created and the 
memory they refer to. This is why a memory barrier is generally 
used before the idx update, to ensure it sees the most up-to-date 

The idx field always increments, and we let it wrap naturally at 

	avail->idx += added; Notifying The Device

Device notification occurs by writing the 16-bit virtqueue index 
of this virtqueue to the Queue Notify field of the virtio header 
in the first I/O region of the PCI device. This can be expensive, 
however, so the device can suppress such notifications if it 
doesn't need them. We have to be careful to expose the new idx 
value before checking the suppression flag: it's OK to notify 
gratuitously, but not to omit a required notification. So again, 
we use a memory barrier here before reading the flags or the 
avail_event field.

If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated, and if 
the VRING_USED_F_NOTIFY flag is not set, we go ahead and write to 
the PCI configuration space.

If the VIRTIO_F_RING_EVENT_IDX feature is negotiated, we read the 
avail_event field in the available ring structure. If the 
available index crossed_the avail_event field value since the 
last notification, we go ahead and write to the PCI configuration 
space. The avail_event field wraps naturally at 65536 as well:

	(u16)(new_idx - avail_event - 1) < (u16)(new_idx - old_idx)

2.4.2 Receiving Used Buffers From The Device

Once the device has used a buffer (read from or written to it, or 
parts of both, depending on the nature of the virtqueue and the 
device), it sends an interrupt, following an algorithm very 
similar to the algorithm used for the driver to send the device a 

1. Write the head descriptor number to the next field in the used 

2. Update the used ring idx.

3. Determine whether an interrupt is necessary:

  (a) If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated: 
    check if f the VRING_AVAIL_F_NO_INTERRUPT flag is not set in 

  (b) If the VIRTIO_F_RING_EVENT_IDX feature is negotiated: check 
    whether the used index crossed the used_event field value 
    since the last update. The used_event field wraps naturally 
    at 65536 as well:
	(u16)(new_idx - used_event - 1) < (u16)(new_idx - old_idx)

4. If an interrupt is necessary:

  (a) If MSI-X capability is disabled:

    i. Set the lower bit of the ISR Status field for the device.

    ii. Send the appropriate PCI interrupt for the device.

  (b) If MSI-X capability is enabled:

    i. Request the appropriate MSI-X interrupt message for the 
      device, Queue Vector field sets the MSI-X Table entry 

    ii. If Queue Vector field value is NO_VECTOR, no interrupt 
      message is requested for this event.

The guest interrupt handler should:

1. If MSI-X capability is disabled: read the ISR Status field, 
  which will reset it to zero. If the lower bit is zero, the 
  interrupt was not for this device. Otherwise, the guest driver 
  should look through the used rings of each virtqueue for the 
  device, to see if any progress has been made by the device 
  which requires servicing.

2. If MSI-X capability is enabled: look through the used rings of 
  each virtqueue mapped to the specific MSI-X vector for the 
  device, to see if any progress has been made by the device 
  which requires servicing.

For each ring, guest should then disable interrupts by writing 
VRING_AVAIL_F_NO_INTERRUPT flag in avail structure, if required. 
It can then process used ring entries finally enabling interrupts 
by clearing the VRING_AVAIL_F_NO_INTERRUPT flag or updating the 
EVENT_IDX field in the available structure, Guest should then 
execute a memory barrier, and then recheck the ring empty 
condition. This is necessary to handle the case where, after the 
last check and before enabling interrupts, an interrupt has been 
suppressed by the device:

	for (;;) {
		if (vq->last_seen_used != vring->used.idx) {
			if (vq->last_seen_used != vring->used.idx)

		struct vring_used_elem *e = vring.used->ring[vq->last_seen_used%vsz];

2.4.3 Dealing With Configuration Changes

Some virtio PCI devices can change the device configuration 
state, as reflected in the virtio header in the PCI configuration 
space. In this case:

1. If MSI-X capability is disabled: an interrupt is delivered and 
  the second highest bit is set in the ISR Status field to 
  indicate that the driver should re-examine the configuration 
  space.  Note that a single interrupt can indicate both that one 
  or more virtqueue has been used and that the configuration 
  space has changed: even if the config bit is set, virtqueues 
  must be scanned.

2. If MSI-X capability is enabled: an interrupt message is 
  requested. The Configuration Vector field sets the MSI-X Table 
  entry number to use. If Configuration Vector field value is 
  NO_VECTOR, no interrupt message is requested for this event.

Creating New Device Types

Various considerations are necessary when creating a new device 

  How Many Virtqueues?

It is possible that a very simple device will operate entirely 
through its configuration space, but most will need at least one 
virtqueue in which it will place requests. A device with both 
input and output (eg. console and network devices described here) 
need two queues: one which the driver fills with buffers to 
receive input, and one which the driver places buffers to 
transmit output.

  What Configuration Space Layout?

Configuration space is generally used for rarely-changing or 
initialization-time parameters.  But it is a limited resource, so 
it might be better to use a virtqueue to update configuration 
information (the network device does this for filtering, 
otherwise the table in the config space could potentially be very 

Note that this space is generally the guest's native endian, 
rather than PCI's little-endian.

  What Device Number?

Currently device numbers are assigned quite freely: a simple 
request mail to the author of this document or the Linux 
virtualization mailing list[9] will be sufficient to secure a unique one.

Meanwhile for experimental drivers, use 65535 and work backwards.

  How many MSI-X vectors?

Using the optional MSI-X capability devices can speed up 
interrupt processing by removing the need to read ISR Status 
register by guest driver (which might be an expensive operation), 
reducing interrupt sharing between devices and queues within the 
device, and handling interrupts from multiple CPUs. However, some 
systems impose a limit (which might be as low as 256) on the 
total number of MSI-X vectors that can be allocated to all 
devices. Devices and/or device drivers should take this into 
account, limiting the number of vectors used unless the device is 
expected to cause a high volume of interrupts. Devices can 
control the number of vectors used by limiting the MSI-X Table 
Size or not presenting MSI-X capability in PCI configuration 
space. Drivers can control this by mapping events to as small 
number of vectors as possible, or disabling MSI-X capability 

  Message Framing

The descriptors used for a buffer should not effect the semantics 
of the message, except for the total length of the buffer. For 
example, a network buffer consists of a 10 byte header followed 
by the network packet. Whether this is presented in the ring 
descriptor chain as (say) a 10 byte buffer and a 1514 byte 
buffer, or a single 1524 byte buffer, or even three buffers, 
should have no effect.

In particular, no implementation should use the descriptor 
boundaries to determine the size of any header in a request.[10]

  Device Improvements

Any change to configuration space, or new virtqueues, or 
behavioural changes, should be indicated by negotiation of a new 
feature bit. This establishes clarity[11] and avoids future expansion problems.

Clusters of functionality which are always implemented together 
can use a single bit, but if one feature makes sense without the 
others they should not be gratuitously grouped together to 
conserve feature bits. We can always extend the spec when the 
first person needs more than 24 feature bits for their device.

Appendix A: virtio_ring.h

/* An interface for efficient virtio implementation.
 * This header is BSD licensed so anyone can use the definitions
 * to implement compatible drivers/servers.
 * Copyright 2007, 2009, IBM Corporation
 * Copyright 2011, Red Hat, Inc
 * All rights reserved.
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of IBM nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.

/* This marks a buffer as continuing via the next field. */
#define VRING_DESC_F_NEXT       1
/* This marks a buffer as write-only (otherwise read-only). */
#define VRING_DESC_F_WRITE      2

/* The Host uses this in used->flags to advise the Guest: don't kick me
 * when you add a buffer.  It's unreliable, so it's simply an
 * optimization.  Guest will still kick if it's out of buffers. */
/* The Guest uses this in avail->flags to advise the Host: don't
 * interrupt me when you consume a buffer.  It's unreliable, so it's
 * simply an optimization.  */

/* Virtio ring descriptors: 16 bytes.
 * These can chain together via "next". */
struct vring_desc {
        /* Address (guest-physical). */
        uint64_t addr;
        /* Length. */
        uint32_t len;
        /* The flags as indicated above. */
        uint16_t flags;
        /* We chain unused descriptors via this, too */
        uint16_t next;

struct vring_avail {
        uint16_t flags;
        uint16_t idx;
        uint16_t ring[];
        uint16_t used_event;

/* u32 is used here for ids for padding reasons. */
struct vring_used_elem {
        /* Index of start of used descriptor chain. */
        uint32_t id;
        /* Total length of the descriptor chain which was written to. */
        uint32_t len;

struct vring_used {
        uint16_t flags;
        uint16_t idx;
        struct vring_used_elem ring[];
        uint16_t avail_event;

struct vring {
        unsigned int num;

        struct vring_desc *desc;
        struct vring_avail *avail;
        struct vring_used *used;

/* The standard layout for the ring is a continuous chunk of memory which
 * looks like this.  We assume num is a power of 2.
 * struct vring {
 *      // The actual descriptors (16 bytes each)
 *      struct vring_desc desc[num];
 *      // A ring of available descriptor heads with free-running index.
 *      __u16 avail_flags;
 *      __u16 avail_idx;
 *      __u16 available[num];
 *      // Padding to the next align boundary.
 *      char pad[];
 *      // A ring of used descriptor heads with free-running index.
 *      __u16 used_flags;
 *      __u16 EVENT_IDX;
 *      struct vring_used_elem used[num];
 * };
 * Note: for virtio PCI, align is 4096.
static inline void vring_init(struct vring *vr, unsigned int num, void *p,
                              unsigned long align)
        vr->num = num;
        vr->desc = p;
        vr->avail = p + num*sizeof(struct vring_desc);
        vr->used = (void *)(((unsigned long)&vr->avail->ring[num]
                              + align-1)
                            & ~(align - 1));

static inline unsigned vring_size(unsigned int num, unsigned long align)
        return ((sizeof(struct vring_desc)*num + sizeof(uint16_t)*(2+num)
                 + align - 1) & ~(align - 1))
                + sizeof(uint16_t)*3 + sizeof(struct vring_used_elem)*num;

static inline int vring_need_event(uint16_t event_idx, uint16_t new_idx, uint16_t old_idx)
         return (uint16_t)(new_idx - event_idx - 1) < (uint16_t)(new_idx - old_idx); 
#endif /* VIRTIO_RING_H */

Appendix B: Reserved Feature Bits

Currently there are five device-independent feature bits defined:

  VIRTIO_F_NOTIFY_ON_EMPTY (24) Negotiating this feature 
  indicates that the driver wants an interrupt if the device runs 
  out of available descriptors on a virtqueue, even though 
  interrupts are suppressed using the VRING_AVAIL_F_NO_INTERRUPT 
  flag or the used_event field. An example of this is the 
  networking driver: it doesn't need to know every time a packet 
  is transmitted, but it does need to free the transmitted 
  packets a finite time after they are transmitted. It can avoid 
  using a timer if the device interrupts it when all the packets 
  are transmitted.

  VIRTIO_F_RING_INDIRECT_DESC (28) Negotiating this feature indicates
  that the driver can use descriptors with the VRING_DESC_F_INDIRECT
  flag set, as described in 2.3.3 Indirect Descriptors.

  VIRTIO_F_RING_EVENT_IDX(29) This feature enables the used_event 
  and the avail_event fields. If set, it indicates that the 
  device should ignore the flags field in the available ring 
  structure. Instead, the used_event field in this structure is 
  used by guest to suppress device interrupts. Further, the 
  driver should ignore the flags field in the used ring 
  structure. Instead, the avail_event field in this structure is 
  used by the device to suppress notifications. If unset, the 
  driver should ignore the used_event field; the device should 
  ignore the avail_event field; the flags field is used

Appendix C: Network Device

The virtio network device is a virtual ethernet card, and is the 
most complex of the devices supported so far by virtio. It has 
enhanced rapidly and demonstrates clearly how support for new 
features should be added to an existing device. Empty buffers are 
placed in one virtqueue for receiving packets, and outgoing 
packets are enqueued into another for transmission in that order. 
A third command queue is used to control advanced filtering 


  Subsystem Device ID 1

  Virtqueues 0:receiveq. 1:transmitq. 2:controlq[12]

Feature bits 

  VIRTIO_NET_F_CSUM (0) Device handles packets with partial checksum

  VIRTIO_NET_F_GUEST_CSUM (1) Guest handles packets with partial checksum

  VIRTIO_NET_F_MAC (5) Device has given MAC address.

  VIRTIO_NET_F_GSO (6) (Deprecated) device handles packets with 
    any GSO type.[13] 

  VIRTIO_NET_F_GUEST_TSO4 (7) Guest can receive TSOv4.

  VIRTIO_NET_F_GUEST_TSO6 (8) Guest can receive TSOv6.

  VIRTIO_NET_F_GUEST_ECN (9) Guest can receive TSO with ECN.

  VIRTIO_NET_F_GUEST_UFO (10) Guest can receive UFO.

  VIRTIO_NET_F_HOST_TSO4 (11) Device can receive TSOv4.

  VIRTIO_NET_F_HOST_TSO6 (12) Device can receive TSOv6.

  VIRTIO_NET_F_HOST_ECN (13) Device can receive TSO with ECN.

  VIRTIO_NET_F_HOST_UFO (14) Device can receive UFO.

  VIRTIO_NET_F_MRG_RXBUF (15) Guest can merge receive buffers.

  VIRTIO_NET_F_STATUS (16) Configuration status field is 

  VIRTIO_NET_F_CTRL_VQ (17) Control channel is available.

  VIRTIO_NET_F_CTRL_RX (18) Control channel RX mode support.

  VIRTIO_NET_F_CTRL_VLAN (19) Control channel VLAN filtering.

  VIRTIO_NET_F_GUEST_ANNOUNCE(21) Guest can send gratuitous 

  Device configuration layout Two configuration fields are 
  currently defined. The mac address field always exists (though 
  is only valid if VIRTIO_NET_F_MAC is set), and the status field 
  only exists if VIRTIO_NET_F_STATUS is set. Two read-only bits 
  are currently defined for the status field: 


	struct virtio_net_config {
		u8 mac[6];
		u16 status;

Device Initialization

1. The initialization routine should identify the receive and 
  transmission virtqueues.

2. If the VIRTIO_NET_F_MAC feature bit is set, the configuration 
  space “mac” entry indicates the “physical” address of the the 
  network card, otherwise a private MAC address should be 
  assigned. All guests are expected to negotiate this feature if 
  it is set.

3. If the VIRTIO_NET_F_CTRL_VQ feature bit is negotiated, 
  identify the control virtqueue.

4. If the VIRTIO_NET_F_STATUS feature bit is negotiated, the link 
  status can be read from the bottom bit of the “status” config 
  field. Otherwise, the link should be assumed active.

5. The receive virtqueue should be filled with receive buffers. 
  This is described in detail below in “Setting Up Receive 

6. A driver can indicate that it will generate checksumless 
  packets by negotating the VIRTIO_NET_F_CSUM feature. This “
  checksum offload” is a common feature on modern network cards.

7. If that feature is negotiated[14], a driver can use TCP or UDP
  segmentation offload by negotiating the VIRTIO_NET_F_HOST_TSO4 (IPv4
  (UDP fragmentation) features. It should not send TCP packets
  requiring segmentation offload which have the Explicit Congestion
  Notification bit set, unless the VIRTIO_NET_F_HOST_ECN feature is

8. The converse features are also available: a driver can save 
  the virtual device some work by negotiating these features.[16]
   The VIRTIO_NET_F_GUEST_CSUM feature indicates that partially 
  checksummed packets can be received, and if it can do that then 
  equivalents of the features described above. See “Receiving 
  Packets” below.

Device Operation

Packets are transmitted by placing them in the transmitq, and 
buffers for incoming packets are placed in the receiveq. In each 
case, the packet itself is preceeded by a header:

	struct virtio_net_hdr {
		u8 flags;
	#define VIRTIO_NET_HDR_GSO_NONE        0
	#define VIRTIO_NET_HDR_GSO_TCPV4       1
	#define VIRTIO_NET_HDR_GSO_TCPV6       4
	#define VIRTIO_NET_HDR_GSO_ECN      0x80
		u8 gso_type;
		u16 hdr_len;
		u16 gso_size;
		u16 csum_start;
		u16 csum_offset;
	/* Only if VIRTIO_NET_F_MRG_RXBUF: */
		u16 num_buffers

The controlq is used to control device features such as 

Packet Transmission

Transmitting a single packet is simple, but varies depending on 
the different features the driver negotiated.

1. If the driver negotiated VIRTIO_NET_F_CSUM, and the packet has 
  not been fully checksummed, then the virtio_net_hdr's fields 
  are set as follows. Otherwise, the packet must be fully 
  checksummed, and flags is zero.

  • flags has the VIRTIO_NET_HDR_F_NEEDS_CSUM set,

  • csum_start is set to the offset within the packet to begin checksumming,

  • csum_offset indicates how many bytes after the csum_start the 
    new (16 bit ones' complement) checksum should be placed.[17]

2. If the driver negotiated 
  VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO, and the packet requires 
  TCP segmentation or UDP fragmentation, then the “gso_type” 
  field is set to VIRTIO_NET_HDR_GSO_TCPV4, TCPV6 or UDP. 
  (Otherwise, it is set to VIRTIO_NET_HDR_GSO_NONE). In this 
  case, packets larger than 1514 bytes can be transmitted: the 
  metadata indicates how to replicate the packet header to cut it 
  into smaller packets. The other gso fields are set:

  • hdr_len is a hint to the device as to how much of the header 
    needs to be kept to copy into each packet, usually set to the 
    length of the headers, including the transport header.[18]

  • gso_size is the maximum size of each packet beyond that 
    header (ie. MSS).

  • If the driver negotiated the VIRTIO_NET_F_HOST_ECN feature, 
    the VIRTIO_NET_HDR_GSO_ECN bit may be set in “gso_type” as 
    well, indicating that the TCP packet has the ECN bit set.[19]

3. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature, 
  the num_buffers field is set to zero.

4. The header and packet are added as one output buffer to the
  transmitq, and the device is notified of the new entry (see
  Notifying The Device).[20]

  Packet Transmission Interrupt

Often a driver will suppress transmission interrupts using the
VRING_AVAIL_F_NO_INTERRUPT flag (see 2.4.2 Receiving Used Buffers From
The Device) and check for used packets in the transmit path of following 
packets. However, it will still receive interrupts if the 
VIRTIO_F_NOTIFY_ON_EMPTY feature is negotiated, indicating that 
the transmission queue is completely emptied.

The normal behavior in this interrupt handler is to retrieve and 
new descriptors from the used ring and free the corresponding 
headers and packets.

  Setting Up Receive Buffers

It is generally a good idea to keep the receive virtqueue as 
fully populated as possible: if it runs out, network performance 
will suffer.

VIRTIO_NET_F_GUEST_UFO features are used, the Guest will need to 
accept packets of up to 65550 bytes long (the maximum size of a 
TCP or UDP packet, plus the 14 byte ethernet header), otherwise 
1514 bytes. So unless VIRTIO_NET_F_MRG_RXBUF is negotiated, every 
buffer in the receive queue needs to be at least this length [20a]

If VIRTIO_NET_F_MRG_RXBUF is negotiated, each buffer must be at 
least the size of the struct virtio_net_hdr.

  Packet Receive Interrupt

When a packet is copied into a buffer in the receiveq, the 
optimal path is to disable further interrupts for the receiveq 
(see [sub:Receiving-Used-Buffers]) and process packets until no 
more are found, then re-enable them.

Processing packet involves:

1. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature, 
  then the “num_buffers” field indicates how many descriptors 
  this packet is spread over (including this one). This allows 
  receipt of large packets without having to allocate large 
  buffers. In this case, there will be at least “num_buffers” in 
  the used ring, and they should be chained together to form a 
  single packet. The other buffers will not begin with a struct 

2. If the VIRTIO_NET_F_MRG_RXBUF feature was not negotiated, or 
  the “num_buffers” field is one, then the entire packet will be 
  contained within this buffer, immediately following the struct 

3. If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the 
  VIRTIO_NET_HDR_F_NEEDS_CSUM bit in the “flags” field may be 
  set: if so, the checksum on the packet is incomplete and the “
  csum_start” and “csum_offset” fields indicate how to calculate 
  it (see Packet Transmission point 1).

4. If the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options were 
  negotiated, then the “gso_type” may be something other than 
  VIRTIO_NET_HDR_GSO_NONE, and the “gso_size” field indicates the 
  desired MSS (see Packet Transmission point 2).

Control Virtqueue

The driver uses the control virtqueue (if VIRTIO_NET_F_VTRL_VQ is 
negotiated) to send commands to manipulate various features of 
the device which would not easily map into the configuration 

All commands are of the following form:

	struct virtio_net_ctrl {
		u8 class;
		u8 command;
		u8 command-specific-data[];
		u8 ack;

	/* ack values */
	#define VIRTIO_NET_OK     0
	#define VIRTIO_NET_ERR    1 

The class, command and command-specific-data are set by the 
driver, and the device sets the ack byte. There is little it can 
do except issue a diagnostic if the ack byte is not 

Packet Receive Filtering

If the VIRTIO_NET_F_CTRL_RX feature is negotiated, the driver can 
send control commands for promiscuous mode, multicast receiving, 
and filtering of MAC addresses.

Note that in general, these commands are best-effort: unwanted 
packets may still arrive. 

Setting Promiscuous Mode

	#define VIRTIO_NET_CTRL_RX    0

The class VIRTIO_NET_CTRL_RX has two commands: 
VIRTIO_NET_CTRL_RX_PROMISC turns promiscuous mode on and off, and 
VIRTIO_NET_CTRL_RX_ALLMULTI turns all-multicast receive on and 
off. The command-specific-data is one byte containing 0 (off) or 
1 (on).

Setting MAC Address Filtering

	struct virtio_net_ctrl_mac {
		u32 entries;
		u8 macs[entries][ETH_ALEN];

	#define VIRTIO_NET_CTRL_MAC    1
	 #define VIRTIO_NET_CTRL_MAC_TABLE_SET        0 

The device can filter incoming packets by any number of destination
MAC addresses.[21] This table is set using the class
command-specific-data is two variable length tables of 6-byte MAC
addresses. The first table contains unicast addresses, and the second
contains multicast addresses.

VLAN Filtering

If the driver negotiates the VIRTION_NET_F_CTRL_VLAN feature, it 
can control a VLAN filter table in the device.

	#define VIRTIO_NET_CTRL_VLAN       2
	 #define VIRTIO_NET_CTRL_VLAN_ADD             0
	 #define VIRTIO_NET_CTRL_VLAN_DEL             1 

command take a 16-bit VLAN id as the command-specific-data.

Gratuitous Packet Sending

If the driver negotiates the VIRTIO_NET_F_GUEST_ANNOUNCE (depends 
on VIRTIO_NET_F_CTRL_VQ), it can ask the guest to send gratuitous 
packets; this is usually done after the guest has been physically 
migrated, and needs to announce its presence on the new network 
links. (As hypervisor does not have the knowledge of guest 
network configuration (eg. tagged vlan) it is simplest to prod 
the guest in this way).

	 #define VIRTIO_NET_CTRL_ANNOUNCE_ACK             0

The Guest needs to check VIRTIO_NET_S_ANNOUNCE bit in status 
field when it notices the changes of device configuration. The 
command VIRTIO_NET_CTRL_ANNOUNCE_ACK is used to indicate that 
driver has recevied the notification and device would clear the 
VIRTIO_NET_S_ANNOUNCE bit in the status filed after it received 
this command.

Processing this notification involves:

1. Sending the gratuitous packets or marking there are pending 
  gratuitous packets to be sent and letting deferred routine to 
  send them.

2. Sending VIRTIO_NET_CTRL_ANNOUNCE_ACK command through control 

3. . 

Appendix D: Block Device

The virtio block device is a simple virtual block device (ie. 
disk). Read and write requests (and other exotic requests) are 
placed in the queue, and serviced (probably out of order) by the 
device except where noted.


  Subsystem Device ID 2

  Virtqueues 0:requestq.

  Feature bits

  VIRTIO_BLK_F_BARRIER (0) Host supports request barriers.

  VIRTIO_BLK_F_SIZE_MAX (1) Maximum size of any single segment is 
    in “size_max”.

  VIRTIO_BLK_F_SEG_MAX (2) Maximum number of segments in a 
    request is in “seg_max”.

  VIRTIO_BLK_F_GEOMETRY (4) Disk-style geometry specified in “

  VIRTIO_BLK_F_RO (5) Device is read-only.

  VIRTIO_BLK_F_BLK_SIZE (6) Block size of disk is in “blk_size”.

  VIRTIO_BLK_F_SCSI (7) Device supports scsi packet commands.

  VIRTIO_BLK_F_FLUSH (9) Cache flush command support.

  Device configuration layout The capacity of the device 
  (expressed in 512-byte sectors) is always present. The 
  availability of the others all depend on various feature bits 
  as indicated above.

	struct virtio_blk_config {
		u64 capacity;
		u32 size_max;
		u32 seg_max;
		struct virtio_blk_geometry {
			u16 cylinders;
			u8 heads;
			u8 sectors;
		} geometry;
		u32 blk_size;

Device Initialization

1. The device size should be read from the “capacity” 
  configuration field. No requests should be submitted which goes 
  beyond this limit.

2. If the VIRTIO_BLK_F_BLK_SIZE feature is negotiated, the 
  blk_size field can be read to determine the optimal sector size 
  for the driver to use. This does not effect the units used in 
  the protocol (always 512 bytes), but awareness of the correct 
  value can effect performance.

3. If the VIRTIO_BLK_F_RO feature is set by the device, any write 
  requests will fail.

Device Operation

The driver queues requests to the virtqueue, and they are used by 
the device (not necessarily in order). Each request is of form:

	struct virtio_blk_req {
		u32 type;
		u32 ioprio;
		u64 sector;
		char data[][512];
		u8 status;

If the device has VIRTIO_BLK_F_SCSI feature, it can also support 
scsi packet command requests, each of these requests is of form:

	struct virtio_scsi_pc_req {
		u32 type;
		u32 ioprio;
		u64 sector;
		char cmd[];
		char data[][512];
		u32 errors;
		u32 data_len;
		u32 sense_len;
		u32 residual;
		u8 status;

The type of the request is either a read (VIRTIO_BLK_T_IN), a write
(VIRTIO_BLK_T_OUT), a scsi packet command (VIRTIO_BLK_T_SCSI_CMD or
feature the high bit (VIRTIO_BLK_T_BARRIER) indicates that this
request acts as a barrier and that all preceeding requests must be
complete before this one, and all following requests must not be
started until this is complete. Note that a barrier does not flush
caches in the underlying backend device in host, and thus does not
serve as data consistency guarantee. Driver must use FLUSH request to
flush the host cache.

	#define VIRTIO_BLK_T_IN           0
	#define VIRTIO_BLK_T_OUT          1
	#define VIRTIO_BLK_T_SCSI_CMD     2
	#define VIRTIO_BLK_T_FLUSH        4
	#define VIRTIO_BLK_T_FLUSH_OUT    5
	#define VIRTIO_BLK_T_BARRIER	 0x80000000

The ioprio field is a hint about the relative priorities of 
requests to the device: higher numbers indicate more important 

The sector number indicates the offset (multiplied by 512) where 
the read or write is to occur. This field is unused and set to 0 
for scsi packet commands and for flush commands.

The cmd field is only present for scsi packet command requests, 
and indicates the command to perform. This field must reside in a 
single, separate read-only buffer; command length can be derived 
from the length of this buffer. 

Note that these first three (four for scsi packet commands) 
fields are always read-only: the data field is either read-only 
or write-only, depending on the request. The size of the read or 
write can be derived from the total size of the request buffers.

The sense field is only present for scsi packet command requests, 
and indicates the buffer for scsi sense data.

The data_len field is only present for scsi packet command 
requests, this field is deprecated, and should be ignored by the 
driver. Historically, devices copied data length there.

The sense_len field is only present for scsi packet command 
requests and indicates the number of bytes actually written to 
the sense buffer.

The residual field is only present for scsi packet command 
requests and indicates the residual size, calculated as data 
length - number of bytes actually transferred.

The final status byte is written by the device: either 
VIRTIO_BLK_S_OK for success, VIRTIO_BLK_S_IOERR for host or guest 
error or VIRTIO_BLK_S_UNSUPP for a request unsupported by host:

	#define VIRTIO_BLK_S_OK        0
	#define VIRTIO_BLK_S_IOERR     1
	#define VIRTIO_BLK_S_UNSUPP    2

Historically, devices assumed that the fields type, ioprio and 
sector reside in a single, separate read-only buffer; the fields 
errors, data_len, sense_len and residual reside in a single, 
separate write-only buffer; the sense field in a separate 
write-only buffer of size 96 bytes, by itself; the fields errors, 
data_len, sense_len and residual in a single write-only buffer; 
and the status field is a separate read-only buffer of size 1 
byte, by itself.

Appendix E: Console Device

The virtio console device is a simple device for data input and 
output. A device may have one or more ports. Each port has a pair 
of input and output virtqueues. Moreover, a device has a pair of 
control IO virtqueues. The control virtqueues are used to 
communicate information between the device and the driver about 
ports being opened and closed on either side of the connection, 
indication from the host about whether a particular port is a 
console port, adding new ports, port hot-plug/unplug, etc., and 
indication from the guest about whether a port or a device was 
successfully added, port open/close, etc.. For data IO, one or 
more empty buffers are placed in the receive queue for incoming 
data and outgoing characters are placed in the transmit queue.


  Subsystem Device ID 3

  Virtqueues 0:receiveq(port0). 1:transmitq(port0), 2:control 
  receiveq[24], 3:control transmitq, 4:receiveq(port1), 5:transmitq(port1), 

  Feature bits

  VIRTIO_CONSOLE_F_SIZE (0) Configuration cols and rows fields 
    are valid.

  VIRTIO_CONSOLE_F_MULTIPORT(1) Device has support for multiple 
    ports; configuration fields nr_ports and max_nr_ports are 
    valid and control virtqueues will be used.

  Device configuration layout The size of the console is supplied 
  in the configuration space if the VIRTIO_CONSOLE_F_SIZE feature 
  is set. Furthermore, if the VIRTIO_CONSOLE_F_MULTIPORT feature 
  is set, the maximum number of ports supported by the device can 
  be fetched.

	struct virtio_console_config {
		u16 cols;
		u16 rows;
		u32 max_nr_ports;

Device Initialization

1. If the VIRTIO_CONSOLE_F_SIZE feature is negotiated, the driver 
  can read the console dimensions from the configuration fields.

2. If the VIRTIO_CONSOLE_F_MULTIPORT feature is negotiated, the 
  driver can spawn multiple ports, not all of which may be 
  attached to a console. Some could be generic ports. In this 
  case, the control virtqueues are enabled and according to the 
  max_nr_ports configuration-space value, the appropriate number 
  of virtqueues are created. A control message indicating the 
  driver is ready is sent to the host. The host can then send 
  control messages for adding new ports to the device. After 
  creating and initializing each port, a 
  VIRTIO_CONSOLE_PORT_READY control message is sent to the host 
  for that port so the host can let us know of any additional 
  configuration options set for that port.

3. The receiveq for each port is populated with one or more 
  receive buffers.

Device Operation

1. For output, a buffer containing the characters is placed in 
  the port's transmitq.[25]

2. When a buffer is used in the receiveq (signalled by an 
  interrupt), the contents is the input to the port associated 
  with the virtqueue for which the notification was received.

3. If the driver negotiated the VIRTIO_CONSOLE_F_SIZE feature, a 
  configuration change interrupt may occur. The updated size can 
  be read from the configuration fields.

4. If the driver negotiated the VIRTIO_CONSOLE_F_MULTIPORT 
  feature, active ports are announced by the host using the 
  VIRTIO_CONSOLE_PORT_ADD control message. The same message is 
  used for port hot-plug as well.

5. If the host specified a port `name', a sysfs attribute is 
  created with the name filled in, so that udev rules can be 
  written that can create a symlink from the port's name to the 
  char device for port discovery by applications in the guest.

6. Changes to ports' state are effected by control messages. 
  Appropriate action is taken on the port indicated in the 
  control message. The layout of the structure of the control 
  buffer and the events associated are:

	struct virtio_console_control {
		uint32_t id;    /* Port number */
		uint16_t event; /* The kind of control event */
		uint16_t value; /* Extra information for the event */

	/* Some events for the internal messages (control packets) */
	#define VIRTIO_CONSOLE_PORT_ADD         1
	#define VIRTIO_CONSOLE_RESIZE           5
	#define VIRTIO_CONSOLE_PORT_OPEN        6
	#define VIRTIO_CONSOLE_PORT_NAME        7

Appendix F: Entropy Device

The virtio entropy device supplies high-quality randomness for 
guest use.


  Subsystem Device ID 4

  Virtqueues 0:requestq.

  Feature bits None currently defined

  Device configuration layout None currently defined.

Device Initialization

1. The virtqueue is initialized

Device Operation

When the driver requires random bytes, it places the descriptor 
of one or more buffers in the queue. It will be completely filled 
by random data by the device.

Appendix G: Memory Balloon Device

The virtio memory balloon device is a primitive device for 
managing guest memory: the device asks for a certain amount of 
memory, and the guest supplies it (or withdraws it, if the device 
has more than it asks for). This allows the guest to adapt to 
changes in allowance of underlying physical memory. If the 
feature is negotiated, the device can also be used to communicate 
guest memory statistics to the host.


  Subsystem Device ID 5

  Virtqueues 0:inflateq. 1:deflateq. 2:statsq.[26]

  Feature bits

  VIRTIO_BALLOON_F_MUST_TELL_HOST (0) Host must be told before 
    pages from the balloon are used.

  VIRTIO_BALLOON_F_STATS_VQ (1) A virtqueue for reporting guest 
    memory statistics is present.

  Device configuration layout Both fields of this configuration 
  are always available. Note that they are little endian, despite 
  convention that device fields are guest endian:

	struct virtio_balloon_config {
		u32 num_pages;
		u32 actual;

Device Initialization

1. The inflate and deflate virtqueues are identified.

2. If the VIRTIO_BALLOON_F_STATS_VQ feature bit is negotiated:

  (a) Identify the stats virtqueue.

  (b) Add one empty buffer to the stats virtqueue and notify the 

Device operation begins immediately.

Device Operation

Memory Ballooning The device is driven by the receipt of a 
configuration change interrupt.

1. The “num_pages” configuration field is examined. If this is 
  greater than the “actual” number of pages, memory must be given 
  to the balloon. If it is less than the “actual” number of 
  pages, memory may be taken back from the balloon for general 

2. To supply memory to the balloon (aka. inflate):

  (a) The driver constructs an array of addresses of unused memory
    pages. These addresses are divided by 4096[27] and the descriptor
    describing the resulting 32-bit array is added to the inflateq.

3. To remove memory from the balloon (aka. deflate):

  (a) The driver constructs an array of addresses of memory pages 
    it has previously given to the balloon, as described above. 
    This descriptor is added to the deflateq.

  (b) If the VIRTIO_BALLOON_F_MUST_TELL_HOST feature is set, the 
    guest may not use these requested pages until that descriptor 
    in the deflateq has been used by the device.

  (c) Otherwise, the guest may begin to re-use pages previously 
    given to the balloon before the device has acknowledged their 
    withdrawl. [28] 

4. In either case, once the device has completed the inflation or 
  deflation, the “actual” field of the configuration should be 
  updated to reflect the new number of pages in the balloon.[29]

Memory Statistics

The stats virtqueue is atypical because communication is driven 
by the device (not the driver). The channel becomes active at 
driver initialization time when the driver adds an empty buffer 
and notifies the device. A request for memory statistics proceeds 
as follows:

1. The device pushes the buffer onto the used ring and sends an 

2. The driver pops the used buffer and discards it.

3. The driver collects memory statistics and writes them into a 
  new buffer.

4. The driver adds the buffer to the virtqueue and notifies the 

5. The device pops the buffer (retaining it to initiate a 
  subsequent request) and consumes the statistics.

  Memory Statistics Format Each statistic consists of a 16 bit 
  tag and a 64 bit value. Both quantities are represented in the 
  native endian of the guest. All statistics are optional and the 
  driver may choose which ones to supply. To guarantee backwards 
  compatibility, unsupported statistics should be omitted.

	struct virtio_balloon_stat {
		u16 tag;
		u64 val;
	} __attribute__((packed));


  VIRTIO_BALLOON_S_SWAP_IN The amount of memory that has been 
  swapped in (in bytes).

  VIRTIO_BALLOON_S_SWAP_OUT The amount of memory that has been 
  swapped out to disk (in bytes).

  VIRTIO_BALLOON_S_MAJFLT The number of major page faults that 
  have occurred.

  VIRTIO_BALLOON_S_MINFLT The number of minor page faults that 
  have occurred.

  VIRTIO_BALLOON_S_MEMFREE The amount of memory not being used 
  for any purpose (in bytes).

  VIRTIO_BALLOON_S_MEMTOT The total amount of memory available 
  (in bytes).

Appendix I: SCSI Host Device

The virtio SCSI host device groups together one or more virtual 
logical units (such as disks), and allows communicating to them 
using the SCSI protocol. An instance of the device represents a 
SCSI host to which many targets and LUNs are attached.

The virtio SCSI device services two kinds of requests:

• command requests for a logical unit;

• task management functions related to a logical unit, target or 

The device is also able to send out notifications about added and 
removed logical units. Together, these capabilities provide a 
SCSI transport protocol that uses virtqueues as the transfer 
medium. In the transport protocol, the virtio driver acts as the 
initiator, while the virtio SCSI host provides one or more 
targets that receive and process the requests. 


  Subsystem Device ID 8

  Virtqueues 0:controlq; 1:eventq; 2..n:request queues.

  Feature bits

  VIRTIO_SCSI_F_INOUT (0) A single request can include both 
    read-only and write-only data buffers.

  VIRTIO_SCSI_F_HOTPLUG (1) The host should enable 
    hot-plug/hot-unplug of new LUNs and targets on the SCSI bus.

  Device configuration layout All fields of this configuration 
  are always available. sense_size and cdb_size are writable by 
  the guest.

	struct virtio_scsi_config {
		u32 num_queues;
		u32 seg_max;
		u32 max_sectors;
		u32 cmd_per_lun;
		u32 event_info_size;
		u32 sense_size;
		u32 cdb_size;
		u16 max_channel;
		u16 max_target;
		u32 max_lun;

  num_queues is the total number of request virtqueues exposed by 
    the device. The driver is free to use only one request queue, 
    or it can use more to achieve better performance.

  seg_max is the maximum number of segments that can be in a 
    command. A bidirectional command can include seg_max input 
    segments and seg_max output segments.

  max_sectors is a hint to the guest about the maximum transfer 
    size it should use.

  cmd_per_lun is a hint to the guest about the maximum number of 
    linked commands it should send to one LUN. The actual value 
    to be used is the minimum of cmd_per_lun and the virtqueue 

  event_info_size is the maximum size that the device will fill 
    for buffers that the driver places in the eventq. The driver 
    should always put buffers at least of this size. It is 
    written by the device depending on the set of negotated 

  sense_size is the maximum size of the sense data that the 
    device will write. The default value is written by the device 
    and will always be 96, but the driver can modify it. It is 
    restored to the default when the device is reset.

  cdb_size is the maximum size of the CDB that the driver will 
    write. The default value is written by the device and will 
    always be 32, but the driver can likewise modify it. It is 
    restored to the default when the device is reset.

  max_channel, max_target and max_lun can be used by the driver 
    as hints to constrain scanning the logical units on the 

Device Initialization

The initialization routine should first of all discover the 
device's virtqueues.

If the driver uses the eventq, it should then place at least a 
buffer in the eventq.

The driver can immediately issue requests (for example, INQUIRY 
or REPORT LUNS) or task management functions (for example, I_T 

Device Operation: request queues

The driver queues requests to an arbitrary request queue, and 
they are used by the device on that same queue. It is the 
responsibility of the driver to ensure strict request ordering 
for commands placed on different queues, because they will be 
consumed with no order constraints.

Requests have the following format: 

	struct virtio_scsi_req_cmd {
		// Read-only
		u8 lun[8];
		u64 id;
		u8 task_attr;
		u8 prio;
		u8 crn;
		char cdb[cdb_size];
		char dataout[];
		// Write-only part
		u32 sense_len;
		u32 residual;
		u16 status_qualifier;
		u8 status;
		u8 response;
		u8 sense[sense_size];
		char datain[];

	/* command-specific response values */
	#define VIRTIO_SCSI_S_OK                0
	#define VIRTIO_SCSI_S_OVERRUN           1
	#define VIRTIO_SCSI_S_ABORTED           2
	#define VIRTIO_SCSI_S_BAD_TARGET        3
	#define VIRTIO_SCSI_S_RESET             4
	#define VIRTIO_SCSI_S_BUSY              5
	#define VIRTIO_SCSI_S_FAILURE           9

	/* task_attr */
	#define VIRTIO_SCSI_S_SIMPLE            0
	#define VIRTIO_SCSI_S_ORDERED           1
	#define VIRTIO_SCSI_S_HEAD              2
	#define VIRTIO_SCSI_S_ACA               3

The lun field addresses a target and logical unit in the 
virtio-scsi device's SCSI domain. The only supported format for 
the LUN field is: first byte set to 1, second byte set to target, 
third and fourth byte representing a single level LUN structure, 
followed by four zero bytes. With this representation, a 
virtio-scsi device can serve up to 256 targets and 16384 LUNs per 

The id field is the command identifier (“tag”).

task_attr, prio and crn should be left to zero. task_attr defines 
the task attribute as in the table above, but all task attributes 
may be mapped to SIMPLE by the device; crn may also be provided 
by clients, but is generally expected to be 0. The maximum CRN 
value defined by the protocol is 255, since CRN is stored in an 
8-bit integer.

All of these fields are defined in SAM. They are always 
read-only, as are the cdb and dataout field. The cdb_size is 
taken from the configuration space.

sense and subsequent fields are always write-only. The sense_len 
field indicates the number of bytes actually written to the sense 
buffer. The residual field indicates the residual size, 
calculated as “data_length - number_of_transferred_bytes”, for 
read or write operations. For bidirectional commands, the 
number_of_transferred_bytes includes both read and written bytes. 
A residual field that is less than the size of datain means that 
the dataout field was processed entirely. A residual field that 
exceeds the size of datain means that the dataout field was 
processed partially and the datain field was not processed at 

The status byte is written by the device to be the status code as 
defined in SAM.

The response byte is written by the device to be one of the 

  VIRTIO_SCSI_S_OK when the request was completed and the status 
  byte is filled with a SCSI status code (not necessarily 

  VIRTIO_SCSI_S_OVERRUN if the content of the CDB requires 
  transferring more data than is available in the data buffers.

  VIRTIO_SCSI_S_ABORTED if the request was cancelled due to an 
  ABORT TASK or ABORT TASK SET task management function.

  VIRTIO_SCSI_S_BAD_TARGET if the request was never processed 
  because the target indicated by the lun field does not exist.

  VIRTIO_SCSI_S_RESET if the request was cancelled due to a bus 
  or device reset (including a task management function).

  VIRTIO_SCSI_S_TRANSPORT_FAILURE if the request failed due to a 
  problem in the connection between the host and the target 
  (severed link).

  VIRTIO_SCSI_S_TARGET_FAILURE if the target is suffering a 
  failure and the guest should not retry on other paths.

  VIRTIO_SCSI_S_NEXUS_FAILURE if the nexus is suffering a failure 
  but retrying on other paths might yield a different result.

  VIRTIO_SCSI_S_BUSY if the request failed but retrying on the 
  same path should work.

  VIRTIO_SCSI_S_FAILURE for other host or guest error. In 
  particular, if neither dataout nor datain is empty, and the 
  VIRTIO_SCSI_F_INOUT feature has not been negotiated, the 
  request will be immediately returned with a response equal to 

Device Operation: controlq

The controlq is used for other SCSI transport operations. 
Requests have the following format:

	struct virtio_scsi_ctrl {
		u32 type;
		u8 response;

	/* response values valid for all commands */
	#define VIRTIO_SCSI_S_OK                       0
	#define VIRTIO_SCSI_S_BAD_TARGET               3
	#define VIRTIO_SCSI_S_BUSY                     5
	#define VIRTIO_SCSI_S_TARGET_FAILURE           7
	#define VIRTIO_SCSI_S_NEXUS_FAILURE            8
	#define VIRTIO_SCSI_S_FAILURE                  9
	#define VIRTIO_SCSI_S_INCORRECT_LUN            12

The type identifies the remaining fields.

The following commands are defined:

  Task management function  
	#define VIRTIO_SCSI_T_TMF                      0

	#define VIRTIO_SCSI_T_TMF_ABORT_TASK           0
	#define VIRTIO_SCSI_T_TMF_CLEAR_ACA            2
	#define VIRTIO_SCSI_T_TMF_QUERY_TASK           6

	struct virtio_scsi_ctrl_tmf
		// Read-only part
		u32 type;
		u32 subtype;
		u8 lun[8];
		u64 id;
		// Write-only part
		u8 response;

	/* command-specific response values */

  The type is VIRTIO_SCSI_T_TMF; the subtype field defines. All 
  fields except response are filled by the driver. The subtype 
  field must always be specified and identifies the requested 
  task management function.

  Other fields may be irrelevant for the requested TMF; if so, 
  they are ignored but they should still be present. The lun 
  field is in the same format specified for request queues; the 
  single level LUN is ignored when the task management function 
  addresses a whole I_T nexus. When relevant, the value of the id 
  field is matched against the id values passed on the requestq.

  The outcome of the task management function is written by the 
  device in the response field. The command-specific response 
  values map 1-to-1 with those defined in SAM.

  Asynchronous notification query  

	#define VIRTIO_SCSI_T_AN_QUERY                    1

	struct virtio_scsi_ctrl_an {
	    // Read-only part
	    u32 type;
	    u8  lun[8];
	    u32 event_requested;
	    // Write-only part
	    u32 event_actual;
	    u8  response;


  By sending this command, the driver asks the device which 
  events the given LUN can report, as described in paragraphs 6.6 
  and A.6 of the SCSI MMC specification. The driver writes the 
  events it is interested in into the event_requested; the device 
  responds by writing the events that it supports into 

  The type is VIRTIO_SCSI_T_AN_QUERY. The lun and event_requested 
  fields are written by the driver. The event_actual and response 
  fields are written by the device.

  No command-specific values are defined for the response byte.

  Asynchronous notification subscription  
	#define VIRTIO_SCSI_T_AN_SUBSCRIBE                2

	struct virtio_scsi_ctrl_an {
		// Read-only part
		u32 type;
		u8  lun[8];
		u32 event_requested;
		// Write-only part
		u32 event_actual;
		u8  response;

  By sending this command, the driver asks the specified LUN to 
  report events for its physical interface, again as described in 
  the SCSI MMC specification. The driver writes the events it is 
  interested in into the event_requested; the device responds by 
  writing the events that it supports into event_actual.

  Event types are the same as for the asynchronous notification 
  query message.

  The type is VIRTIO_SCSI_T_AN_SUBSCRIBE. The lun and 
  event_requested fields are written by the driver. The 
  event_actual and response fields are written by the device.

  No command-specific values are defined for the response byte.

Device Operation: eventq

The eventq is used by the device to report information on logical 
units that are attached to it. The driver should always leave a 
few buffers ready in the eventq. In general, the device will not 
queue events to cope with an empty eventq, and will end up 
dropping events if it finds no buffer ready. However, when 
reporting events for many LUNs (e.g. when a whole target 
disappears), the device can throttle events to avoid dropping 
them. For this reason, placing 10-15 buffers on the event queue 
should be enough.

Buffers are placed in the eventq and filled by the device when 
interesting events occur. The buffers should be strictly 
write-only (device-filled) and the size of the buffers should be 
at least the value given in the device's configuration 

Buffers returned by the device on the eventq will be referred to 
as "events" in the rest of this section. Events have the 
following format: 

	#define VIRTIO_SCSI_T_EVENTS_MISSED   0x80000000

	struct virtio_scsi_event {
		// Write-only part
		u32 event;

If bit 31 is set in the event field, the device failed to report 
an event due to missing buffers. In this case, the driver should 
poll the logical units for unit attention conditions, and/or do 
whatever form of bus scan is appropriate for the guest operating 

Other data that the device writes to the buffer depends on the 
contents of the event field. The following events are defined:

  No event  
	#define VIRTIO_SCSI_T_NO_EVENT         0

  This event is fired in the following cases: 

  • When the device detects in the eventq a buffer that is 
    shorter than what is indicated in the configuration field, it 
    might use it immediately and put this dummy value in the 
    event field. A well-written driver will never observe this 

  • When events are dropped, the device may signal this event as 
    soon as the drivers makes a buffer available, in order to 
    request action from the driver. In this case, of course, this 
    event will be reported with the VIRTIO_SCSI_T_EVENTS_MISSED 

  Transport reset  

	struct virtio_scsi_event_reset {
		// Write-only part
		u32 event;
		u8  lun[8];
		u32 reason;

	#define VIRTIO_SCSI_EVT_RESET_HARD         0

  By sending this event, the device signals that a logical unit 
  on a target has been reset, including the case of a new device 
  appearing or disappearing on the bus.The device fills in all 
  fields. The event field is set to 
  VIRTIO_SCSI_T_TRANSPORT_RESET. The lun field addresses a 
  logical unit in the SCSI host.

  The reason value is one of the three #define values appearing 

  • VIRTIO_SCSI_EVT_RESET_REMOVED (“LUN/target removed”) is used 
    if the target or logical unit is no longer able to receive 

  • VIRTIO_SCSI_EVT_RESET_HARD (“LUN hard reset”) is used if the 
    logical unit has been reset, but is still present.

  • VIRTIO_SCSI_EVT_RESET_RESCAN (“rescan LUN/target”) is used if 
    a target or logical unit has just appeared on the device.

  The “removed” and “rescan” events, when sent for LUN 0, may 
  apply to the entire target. After receiving them the driver 
  should ask the initiator to rescan the target, in order to 
  detect the case when an entire target has appeared or 
  disappeared. These two events will never be reported unless the 
  VIRTIO_SCSI_F_HOTPLUG feature was negotiated between the host 
  and the guest.

  Events will also be reported via sense codes (this obviously 
  does not apply to newly appeared buses or targets, since the 
  application has never discovered them):

  • “LUN/target removed” maps to sense key ILLEGAL REQUEST, asc 
    0x25, ascq 0x00 (LOGICAL UNIT NOT SUPPORTED)

  • “LUN hard reset” maps to sense key UNIT ATTENTION, asc 0x29 

  • “rescan LUN/target” maps to sense key UNIT ATTENTION, asc 
    0x3f, ascq 0x0e (REPORTED LUNS DATA HAS CHANGED)

  The preferred way to detect transport reset is always to use 
  events, because sense codes are only seen by the driver when it 
  sends a SCSI command to the logical unit or target. However, in 
  case events are dropped, the initiator will still be able to 
  synchronize with the actual state of the controller if the 
  driver asks the initiator to rescan of the SCSI bus. During the 
  rescan, the initiator will be able to observe the above sense 
  codes, and it will process them as if it the driver had 
  received the equivalent event. 

  Asynchronous notification  

	struct virtio_scsi_event_an {
		// Write-only part
		u32 event;
		u8  lun[8];
		u32 reason;

  By sending this event, the device signals that an asynchronous 
  event was fired from a physical interface.

  All fields are written by the device. The event field is set to 
  VIRTIO_SCSI_T_ASYNC_NOTIFY. The lun field addresses a logical 
  unit in the SCSI host. The reason field is a subset of the 
  events that the driver has subscribed to via the "Asynchronous 
  notification subscription" command.

  When dropped events are reported, the driver should poll for 
  asynchronous events manually using SCSI commands.

Appendix X: virtio-mmio

Virtual environments without PCI support (a common situation in 
embedded devices models) might use simple memory mapped device (“
virtio-mmio”) instead of the PCI device.

The memory mapped virtio device behaviour is based on the PCI 
device specification. Therefore most of operations like device 
initialization, queues configuration and buffer transfers are 
nearly identical. Existing differences are described in the 
following sections.

Device Initialization

Instead of using the PCI IO space for virtio header, the “
virtio-mmio” device provides a set of memory mapped control 
registers, all 32 bits wide, followed by device-specific 
configuration space. The following list presents their layout:

• Offset from the device base address | Direction | Name 

• 0x000 | R | MagicValue 
 “virt” string. 

• 0x004 | R | Version 
 Device version number. Currently must be 1. 

• 0x008 | R | DeviceID 
 Virtio Subsystem Device ID (ie. 1 for network card). 

• 0x00c | R | VendorID 
 Virtio Subsystem Vendor ID. 

• 0x010 | R | HostFeatures 
 Flags representing features the device supports.
 Reading from this register returns 32 consecutive flag bits, 
  first bit depending on the last value written to 
  HostFeaturesSel register. Access to this register returns bits HostFeaturesSel*32

   to (HostFeaturesSel*32)+31, eg. feature bits 0 to 31 if 
  HostFeaturesSel is set to 0 and features bits 32 to 63 if 
  HostFeaturesSel is set to 1. Also see [sub:Feature-Bits]

• 0x014 | W | HostFeaturesSel 
 Device (Host) features word selection.
 Writing to this register selects a set of 32 device feature bits 
  accessible by reading from HostFeatures register. Device driver 
  must write a value to the HostFeaturesSel register before 
  reading from the HostFeatures register. 

• 0x020 | W | GuestFeatures 
 Flags representing device features understood and activated by 
  the driver.
 Writing to this register sets 32 consecutive flag bits, first 
  bit depending on the last value written to GuestFeaturesSel 
  register. Access to this register sets bits GuestFeaturesSel*32
  to (GuestFeaturesSel*32)+31, eg. feature bits 0 to 31 if 
  GuestFeaturesSel is set to 0 and features bits 32 to 63 if 
  GuestFeaturesSel is set to 1. Also see [sub:Feature-Bits]

• 0x024 | W | GuestFeaturesSel 
 Activated (Guest) features word selection.
 Writing to this register selects a set of 32 activated feature 
  bits accessible by writing to the GuestFeatures register. 
  Device driver must write a value to the GuestFeaturesSel 
  register before writing to the GuestFeatures register. 

• 0x028 | W | GuestPageSize 
 Guest page size.
 Device driver must write the guest page size in bytes to the 
  register during initialization, before any queues are used. 
  This value must be a power of 2 and is used by the Host to 
  calculate Guest address of the first queue page (see QueuePFN). 

• 0x030 | W | QueueSel 
 Virtual queue index (first queue is 0).
 Writing to this register selects the virtual queue that the 
  following operations on QueueNum, QueueAlign and QueuePFN apply 

• 0x034 | R | QueueNumMax 
 Maximum virtual queue size. 
 Reading from the register returns the maximum size of the queue 
  the Host is ready to process or zero (0x0) if the queue is not 
  available. This applies to the queue selected by writing to 
  QueueSel and is allowed only when QueuePFN is set to zero 
  (0x0), so when the queue is not actively used. 

• 0x038 | W | QueueNum 
 Virtual queue size.
 Queue size is a number of elements in the queue, therefore size 
  of the descriptor table and both available and used rings.
 Writing to this register notifies the Host what size of the 
  queue the Guest will use. This applies to the queue selected by 
  writing to QueueSel. 

• 0x03c | W | QueueAlign 
 Used Ring alignment in the virtual queue.
 Writing to this register notifies the Host about alignment 
  boundary of the Used Ring in bytes. This value must be a power 
  of 2 and applies to the queue selected by writing to QueueSel. 

• 0x040 | RW | QueuePFN 
 Guest physical page number of the virtual queue.
 Writing to this register notifies the host about location of the 
  virtual queue in the Guest's physical address space. This value 
  is the index number of a page starting with the queue 
  Descriptor Table. Value zero (0x0) means physical address zero 
  (0x00000000) and is illegal. When the Guest stops using the 
  queue it must write zero (0x0) to this register.
 Reading from this register returns the currently used page 
  number of the queue, therefore a value other than zero (0x0) 
  means that the queue is in use.
 Both read and write accesses apply to the queue selected by 
  writing to QueueSel. 

• 0x050 | W | QueueNotify 
 Queue notifier.
 Writing a queue index to this register notifies the Host that 
  there are new buffers to process in the queue. 

• 0x60 | R | InterruptStatus
Interrupt status.
Reading from this register returns a bit mask of interrupts 
  asserted by the device. An interrupt is asserted if the 
  corresponding bit is set, ie. equals one (1).

  – Bit 0 | Used Ring Update
	This interrupt is asserted when the Host has updated the Used 
    Ring in at least one of the active virtual queues.

  – Bit 1 | Configuration change
	This interrupt is asserted when configuration of the device has 

• 0x064 | W | InterruptACK 
 Interrupt acknowledge. 
 Writing to this register notifies the Host that the Guest 
  finished handling interrupts. Set bits in the value clear the 
  corresponding bits of the InterruptStatus register. 

• 0x070 | RW | Status 
 Device status. 
 Reading from this register returns the current device status 
 Writing non-zero values to this register sets the status flags, 
  indicating the Guest progress. Writing zero (0x0) to this 
  register triggers a device reset. 
 Also see [sub:Device-Initialization-Sequence]

• 0x100+ | RW | Config 
 Device-specific configuration space starts at an offset 0x100 
  and is accessed with byte alignment. Its meaning and size 
  depends on the device and the driver. 

Virtual queue size is a number of elements in the queue, 
therefore size of the descriptor table and both available and 
used rings.

The endianness of the registers follows the native endianness of 
the Guest. Writing to registers described as “R” and reading from 
registers described as “W” is not permitted and can cause 
undefined behavior.

The device initialization is performed as described in 2.2.1 Device
Initialization Sequence with one exception: the Guest must notify the
Host about its page size, writing the size in bytes to GuestPageSize
register before the initialization is finished.

The memory mapped virtio devices generate single interrupt only, 
therefore no special configuration is required.

Virtqueue Configuration

The virtual queue configuration is performed in a similar way to 
the one described in 2.3 Virtqueue Configuration with a few 
additional operations: 

1. Select the queue writing its index (first queue is 0) to the 
  QueueSel register. 

2. Check if the queue is not already in use: read QueuePFN 
  register, returned value should be zero (0x0). 

3. Read maximum queue size (number of elements) from the 
  QueueNumMax register. If the returned value is zero (0x0) the 
  queue is not available. 

4. Allocate and zero the queue pages in contiguous virtual 
  memory, aligning the Used Ring to an optimal boundary (usually 
  page size). Size of the allocated queue may be smaller than or 
  equal to the maximum size returned by the Host. 

5. Notify the Host about the queue size by writing the size to 
  QueueNum register. 

6. Notify the Host about the used alignment by writing its value 
  in bytes to QueueAlign register. 

7. Write the physical number of the first page of the queue to 
  the QueuePFN register. 

The queue and the device are ready to begin normal operations 

Device Operation

The memory mapped virtio device behaves in the same way as 
described in 2.4 Device Operation, with the following 

1. The device is notified about new buffers available in a queue 
  by writing the queue index to register QueueNum instead of the 
  virtio header in PCI I/O space ( Notifying The Device). 

2. The memory mapped virtio device is using single, dedicated 
  interrupt signal, which is raised when at least one of the 
  interrupts described in the InterruptStatus register 
  description is asserted. After receiving an interrupt, the 
  driver must read the InterruptStatus register to check what 
  caused the interrupt (see the register description). After the 
  interrupt is handled, the driver must acknowledge it by writing 
  a bit mask corresponding to the serviced interrupt to the 
  InterruptACK register.

[1] This lack of page-sharing implies that the implementation of the 
device (e.g. the hypervisor or host) needs full access to the 
guest memory. Communication with untrusted parties (i.e. 
inter-guest communication) requires copying.

[2] The Linux implementation further separates the PCI virtio code 
from the specific virtio drivers: these drivers are shared with 
the non-PCI implementations (currently lguest and S/390).

[3] The actual value within this range is ignored

[4] Historically, drivers have used the device before steps 5 and 6. 
This is only allowed if the driver does not use any features 
which would alter this early use of the device.

[5] ie. once you enable MSI-X on the device, the other fields move. 
If you turn it off again, they move back!

[6] The 4096 is based on the x86 page size, but it's also large 
enough to ensure that the separate parts of the virtqueue are on 
separate cache lines.

[7] These fields are kept here because this is the only part of the 
virtqueue written by the device

[8] The Linux drivers do this only for read-only buffers: for 
write-only buffers, it is assumed that the driver is merely 
trying to keep the receive buffer ring full, and no notification 
of this expected condition is necessary.

[9] https://lists.linux-foundation.org/mailman/listinfo/virtualization

[10] The current qemu device implementations mistakenly insist that 
the first descriptor cover the header in these cases exactly, so 
a cautious driver should arrange it so.

[11] Even if it does mean documenting design or implementation 

[12] Only if VIRTIO_NET_F_CTRL_VQ set

[13] It was supposed to indicate segmentation offload support, but 
upon further investigation it became clear that multiple bits 
were required.

dependent on VIRTIO_NET_F_CSUM; a dvice which offers the offload 
features must offer the checksum feature, and a driver which 
accepts the offload features must accept the checksum feature. 
Similar logic applies to the VIRTIO_NET_F_GUEST_TSO4 features 

[15] This is a common restriction in real, older network cards.

[16] For example, a network packet transported between two guests on
the same system may not require checksumming at all, nor segmentation,
if both guests are amenable.

[17] For example, consider a partially checksummed TCP (IPv4) packet. 
It will have a 14 byte ethernet header and 20 byte IP header 
followed by the TCP header (with the TCP checksum field 16 bytes 
into that header). csum_start will be 14+20 = 34 (the TCP 
checksum includes the header), and csum_offset will be 16. The 
value in the TCP checksum field should be initialized to the sum 
of the TCP pseudo header, so that replacing it by the ones' 
complement checksum of the TCP header and body will give the 
correct result.

[18] Due to various bugs in implementations, this field is not useful 
as a guarantee of the transport header size.

[19] This case is not handled by some older hardware, so is called out 
specifically in the protocol.

[20] Note that the header will be two bytes longer for the 

[20a] Obviously each one can be split across multiple descriptor 

[21] Since there are no guarentees, it can use a hash filter or
silently switch to allmulti or promiscuous mode if it is given too
many addresses.

[22] The SCSI_CMD and SCSI_CMD_OUT types are equivalent, the device 
does not distinguish between them.

[23] The FLUSH and FLUSH_OUT types are equivalent, the device does not
distinguish between them

[24] Ports 2 onwards only if VIRTIO_CONSOLE_F_MULTIPORT is set.

[25] Because this is high importance and low bandwidth, the current 
Linux implementation polls for the buffer to be used, rather than 
waiting for an interrupt, simplifying the implementation 
significantly. However, for generic serial ports with the 
O_NONBLOCK flag set, the polling limitation is relaxed and the 
consumed buffers are freed upon the next write or poll call or 
when a port is closed or hot-unplugged.

[26] Only if VIRTIO_BALLON_F_STATS_VQ set.

[27] This is historical, and independent of the guest page size

[28] In this case, deflation advice is merely a courtesy

[29] As updates to configuration space are not atomic, this field
isn't particularly reliable, but can be used to diagnose buggy guests.

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