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Subject: Re: [RFC PATCH v2 1/2] Add virtio Admin Virtqueue specification

On 7/28/2021 11:52 AM, Stefan Hajnoczi wrote:
On Tue, Jul 27, 2021 at 06:29:49PM +0300, Max Gurtovoy wrote:
On 7/27/2021 5:28 PM, Cornelia Huck wrote:
On Tue, Jul 27 2021, Stefan Hajnoczi <stefanha@redhat.com> wrote:

On Mon, Jul 26, 2021 at 07:52:53PM +0300, Max Gurtovoy wrote:
Admin virtqueues will be used to send administrative commands to
manipulate various features of the device which would not easily map
into the configuration space.

The same Admin command format will be used for all virtio devices. The
Admin command set will include 4 types of command classes:
1. The generic common class
2. The transport specific class
3. The device specific class
4. The vendor specific class

The above mechanism will enable adding various features to the virtio
specification, e.g.:
1. Format virtio-blk devices in various configurations (512B block size,
     512B + 8B T10-DIF, 4K block size, 4k + 8B T10-DIF, etc..).
2. Live migration management.
3. Encrypt/Decrypt descriptors.
4. Virtualization management.
5. Get device error logs.
6. Implement advanced vendor/device/transport specific features.
7. Run device health test.
8. More.

As virtio evolves beyond the para-virt/sw-emulated world, it's mandatory
for the specification to become flexible and allow a wider feature set.
The corrent ctrl virtq that is defined for some of the virtio devices is
device specific and wasn't designed to be a generic virtq for

Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
   admin-virtq.tex | 241 ++++++++++++++++++++++++++++++++++++++++++++++++
   content.tex     |   4 +
   2 files changed, 245 insertions(+)
   create mode 100644 admin-virtq.tex

diff --git a/admin-virtq.tex b/admin-virtq.tex
new file mode 100644
index 0000000..ccec2ca
--- /dev/null
+++ b/admin-virtq.tex
@@ -0,0 +1,241 @@
+\section{Admin Virtqueues}\label{sec:Basic Facilities of a Virtio Device / Admin Virtqueues}
+Admin virtqueues are used to send administrative commands to manipulate
+various features of the device which would not easily map into the
+configuration space.
+Use of Admin virtqueues is negotiated by the VIRTIO_F_ADMIN_VQ
+feature bit.
+Admin virtqueue index may vary among different device types.
+All commands are of the following form:
+struct virtio_admin_cmd {
+        /* Device-readable part */
+        u8 class;
+        u8 command;
+        u8 command-specific-data[];
+        /* Device-writable part */
+        u8 command-specific-result[];
+        u8 status_type : 4;
+        u8 reserved : 4;
+        u8 status;
+/* Status type values */
+#define VIRTIO_ADMIN_STATUS_TYPE_GENERIC               0
+/* Generic status values */
+#define VIRTIO_ADMIN_STATUS_GENERIC_OK                     0
+#define VIRTIO_ADMIN_STATUS_GENERIC_ERR                    1
This is very complex, and it feels like we're overengineering this.
Do you mean the status type and the status ?

+The \field{class}, \field{command} and \field{command-specific-data} are
+set by the driver, and the device sets the \field{status_type}, the
+\field{status} and  the \field{command-specific-result}, if needed.
+The virtio Admin command class codes are divided in the following form:
+/* class values that are transport, device and vendor independent */
+/* class values that are transport specific */
+/* class values that are device specific */
+/* class values that are vendor specific */
+\subsection{Admin command set}\label{sec:Basic Facilities of a Virtio Device / Admin Virtqueues / Admin command set}
+Each virtio device that advertise VIRTIO_F_ADMIN_VQ feature, MUST
"advertises the VIRTIO_F_ADMIN_VQ feature"

+support all the mandatory admin commands. A device MAY support also
+one or more optional admin commands.
+\subsubsection{Common command set}\label{sec:Basic Facilities of a Virtio Device / Admin Virtqueues / Admin command set / Common command set}
+The Common command set is a group of classes and commands within each
+of these classes which are transport, device and vendor independent.
+A mandatory class is a class that has at least one mandatory command.
+The Common command set is summarized in following table:
+Class  & Description    & M/O \\
+\hline \hline
+2-63  & reserved    & - \\
+\paragraph{Discover device class}\label{sec:Basic Facilities of a Virtio Device / Admin Virtqueues / Admin command set / Common command set / Discover device class}
+This class (opcode: 0) of commands is used to query generic device
+information. The following table describes the commands supported for
+this class:
+Command  & Description    & M/O \\
+\hline \hline
+2-255  & reserved    & - \\
+\subparagraph{Device identity command}\label{sec:Basic Facilities of a Virtio Device / Admin Virtqueues / Admin command set / Common command set / Discover device class / Device identity command}
+This mandatory command should return device identity in the following
+Bytes  & Description    & M/O \\
+\hline \hline
+03:00  & VIRTIO DEVICE ID    & M \\
+05:04  & VIRTIO TRANSPORT ID    & M \\
These fields are not defined. I wonder why they are necessary - the
driver should already have this information.
These are initial fields.

We can add also model, serial_number and more in the future.

In general, I'm a little concerned that this whole infrastructure will
increase the complexity of VIRTIO significantly with little benefit. I
do think an admin virtqueue makes sense, e.g. for migration, but would
prefer it if we focus on actual commands first instead of
infrastructure. That way it will be clear what infrastructure is needed.
admin virtq is not only for migration.

You'll be able to configure virtio device properties using user space tools
like: virtio-cli.

For example: format a block device, manage virtual function resources using
its PF, query for error logs, device health and more.
That sounds good.

In the SW world maybe all the above were redundant, but now that you have
more and more HW virtio devices the protocol should be more flexible and
HW is not special in this regard, I think this will be useful for
software too. In-band admin commands are necessary for nested
virtualization, for example. They also provide a standard admin
interface for out-of-process devices (vhost-user, etc).

Few weeks ago I've sent a concrete commands for live migration but then I
was told that new infrastructure (admin virtq) should be developed and this
is what I did in this RFC.

if you combine the 2 RFCs you can imagine what is needed here for adding
Live migration support.

But I want to add it step by step.

We need to agree on the infrastructure.

A concrete example would be good, but I think we can come up with a
bare-bones spec to start with.

- feature bit for the admin vq, as defined here
- location of the admin vq is device specific
- I think we can get away with two classes, as for feature bits (not
    device specificic and device specific); I don't think we need separate
    classes for transport or vendor specific
We need it for live migration probably. It will be a transport class.

Vendor specific is also important to allow vendors develop their special

- make the format for the request simple (command + length + payload?)
I used almost the same format as virtio net ctrl queue.
The virtio_net_ctrl packet format looks good to me, it's close to what
Cornelia's command + length + payload suggestion:

I guess I didn't understand Cornelia suggestion.

   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

I'm not sure how vendor commands will be allocated though. Will each
vendor get a unique class id to prevent collisions? If we want to
support cross-implementation migration then it may be necessary to allow
vendor command availability to change while the device is running.

vendor specific commands can collide.

Vendor A can implement class 192 to do X and Vendor B can implement class 192 to do Y.

what do you mean "support cross-implementation migration" ?

I prefer the simpler struct virtio_net_ctrl format to the more
complicated one proposed in this patch series.

This is the same besides adding status type

u8 status_type : 4;
u8 reserved : 4;

I split "u8 command-specific-data[];"
"u8 command-specific-data[];
 u8 command-specific-result[];"

to emphasize that there is some data that can be written by the device and some data written by the driver in the same command.
And this is also the case in virtio-net-ctrl, right ?

How many different (groups of) commands can we reasonably expect? Do we
need a generic discovery command, or can we get away with a feature bit
covering each new group of commands?
I can't predict the future but IMO we need a discovery command.

We have many devices and more can be added in the future.
A <u8 class, u8 command> space is 65536 bits or 8KB. I think admin
commands would not be included in VIRTIO Feature Bits but instead
reported via a separate admin command that returns up to 8KB of data:

   struct virtio_admin_report_cmds {
       /* Bitmap of available admin commands [Device->Driver]
        * bool command_present =
        *        command_bits[class * 32 + command / 8] & (command % 8);
       u8 command_bits[8192];

Yes, I divided it to multiple commands per class to cover the case we will need more than 1 bit to describe a command.

But I guess we can add it later on.

I think the above should be:

bool command_present = command_bits[class * 32 + command / 8] & (1 << (command % 8));

isn't it ?

Also what do you think about renaming <class, command> to <opcode, opmod> ?


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