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Subject: RE: [virtio-dev] [PATCH RFC] packed ring layout spec v4
Hi Michael,
Added comments inline labeled [#lga#].
BR,
-Lars
> -----Original Message-----
> From: virtio-dev@lists.oasis-open.org [mailto:virtio-dev@lists.oasis-open.org]
> On Behalf Of Michael S. Tsirkin
> Sent: 8. november 2017 13:28
> To: virtio-dev@lists.oasis-open.org
> Subject: [virtio-dev] [PATCH RFC] packed ring layout spec v4
>
> Below is an attempt at a formal write-up of the latest proposal with some
> modifications. Will reply with a pdf version as well.
>
> This is reasonably complete functionally, from spec point of view we need
> - more conformance statements
> - pseudo-code
> - discussion of memory barriers
> - rearrange existing (1.0) layout discussion to make it fit
> in a single chapter
>
> ----
>
> \section{Packed Virtqueues}\label{sec:Basic Facilities of a Virtio Device / Packed
> Virtqueues}
>
> Packed virtqueues is an alternative compact virtqueue layout using read-write
> memory, that is memory that is both read and written by both host and guest.
>
> This layout is enabled by negotiating a VIRTIO_F_PACKED_VIRTQUEUE feature.
[#lga#] A non-technical concern: By making this a feature bit, means v1.1 supports both descriptor structures in parallel: Could that not slow the adoption of the new ring structure? Another way would be to make Virtio v1.1 exclusively ring-based, and first let the device and driver negotiate the highest revision of the standard they both support, followed by negotiating the optional features within the agreed revision. Are there use-cases where the ring would under-perform relative the current structure?
>
> Packed virtqueues support up to $2^14$ queues, with up to $2^15$ entries each.
[#lga#] What limits the number of queues to 2^14?
>
> Each packed virtqueue consists of three parts:
>
> \begin{itemize}
> \item Descriptor Ring
> \item Device Event Suppression
> \item Driver Event Suppression
> \end{itemize}
>
> Where Descriptor Ring in turn consists of descriptors, and where each descriptor
> can contain the following parts:
>
> \begin{itemize}
> \item Buffer ID
> \item Buffer Address
> \item Buffer Length
> \item Flags
> \end{itemize}
>
> A buffer consists of zero or more device-readable physically-contiguous
> elements followed by zero or more physically-contiguous device-writable
> elements (each buffer has at least one element).
>
> When the driver wants to send such a buffer to the device, it writes at least one
> available descriptor describing elements of the buffer into the Descriptor Ring.
> The descriptor(s) are associated with a buffer by means of a Buffer ID stored
> within the descriptor.
[#lga#] Only a single descriptor per buffer will have a valid BufferID, right? Thus chained or indirect descriptors may/will not have a valid BufferID within them
>
> Driver then notifies the device. When the device has finished processing the
> buffer, it writes a used device descriptor including the Buffer ID into the
> Descriptor Ring (overwriting a driver descriptor previously made available), and
> sends an interrupt.
>
> Descriptor Ring is used in a circular manner: driver writes descriptors into the
> ring in order. After reaching end of ring, the next descriptor is placed at head of
> the ring. Once ring is full of driver descriptors, driver stops sending new requests
> and waits for device to start processing descriptors and to write out some used
> descriptors before making new driver descriptors available.
>
> Similarly, device reads descriptors from the ring in order and detects that a
> driver descriptor has been made available. As processing of descriptors is
> completed used descriptors are written by the device back into the ring.
>
> Note: after reading driver descriptors and starting their processing in order,
> device might complete their processing out of order. Used device descriptors
> are written in the order in which their processing is complete.
>
> Device Event suppression data structure is read-only by the device. It includes
> information for reducing the number of device interrupts to driver.
>
> Driver Event suppression data structure is write-only by the device. It includes
> information for reducing the number of driver notifications to device.
>
> \subsection{Available and Used Ring Full Counters} \label{sec:Packed Virtqueues
> / Available and Used Ring Wrap Counters} Each of the driver and the device are
> expected to maintain, internally, a single-bit ring wrap counter initialized to 1.
>
> The counter maintained by the driver is called the Available Ring Full Counter.
> Driver changes its value each time it makes available the last descriptor in the
> ring (after making the last descriptor available).
>
> The counter maintained by the device is called the Used Ring Wrap Counter.
> Device changes its value each time it uses the last descriptor in the ring (after
> marking the last descriptor used).
>
> It is easy to see that the Availablering Wrap Counter in the driver matches the
> Used Ring Wrap Counter in the device when both are processing the same
> descriptor, or when all available descriptors have been used.
[#lga#] How can a driver and a device process the same descriptor (at the same time)?
>
> To mark a descriptor as available and used, both driver and device use the
> following two flags:
> \begin{lstlisting}
> #define VIRTQ_DESC_F_AVAIL 7
> #define VIRTQ_DESC_F_USED 15
> \end{lstlisting}
>
> To mark a descriptor as available, driver sets the VIRTQ_DESC_F_AVAIL bit in
> Flags to match the internal Available Ring Wrap Counter. It also sets the
> VIRTQ_DESC_F_USED bit to match the \emph{inverse} value.
>
> To mark a descriptor as used, device sets the VIRTQ_DESC_F_USED bit in Flags
> to match the internal Used Ring Wrap Counter. It also sets the
> VIRTQ_DESC_F_AVAIL bit to match the \emph{same} value.
>
> Thus VIRTQ_DESC_F_AVAIL and VIRTQ_DESC_F_USED bits are different for an
> available descriptor and equal for a used descriptor.
>
> \subsection{Polling of available and used descriptors} \label{sec:Packed
> Virtqueues / Polling of available and used descriptors}
>
> Writes of device and driver descriptors can generally be reordered, but each side
> (driver and device) are only required to poll a single location in memory: next
> device descriptor after the one they processed previously, in circular order.
[#lga#] The poll location is not straight-forward for batch-used descriptors, and could use a little more elaboration.
>
> Sometimes device needs to only write out a single used descriptor after
> processing a batch of multiple available descriptors. As described in more detail
> below, this can happen when using descriptor chaining or with in-order use of
> descriptors. In this case, device writes out a used descriptor with buffer id of the
> last descriptor in the group.
> After processing the used descriptor, both device and driver then skip forward in
> the ring the number of the remaining descriptors in the group until processing
> (reading for the driver and writing for the device) the next used descriptor.
>
> \subsection{Write Flag}
> \label{sec:Packed Virtqueues / Write Flag}
>
> In an available descriptor, VIRTQ_DESC_F_WRITE bit within Flags is used to mark
> a descriptor as corresponding to a write-only or read-only element of a buffer.
>
> \begin{lstlisting}
> /* This marks a buffer as device write-only (otherwise device read-only). */
> #define VIRTQ_DESC_F_WRITE 2
> \end{lstlisting}
>
> In a used descriptor, this bit it used to specify whether any data has been written
> by the device into any parts of the buffer.
[#lga#] I assume "any parts of the buffer" is to handle batch-used descriptor writes, however since a used device-write-only descriptor can only contain a single length value, this can only mean starting from the address of the first device-write-only descriptor of the batch. Thus it is not "any part" but a very specific part that has must have been written. If multiple device-write-only elements are part of the buffer, they may need to be non-batch-used to correctly reflect the parts written in the buffer.
>
>
> \subsection{Buffer Address and Length}
> \label{sec:Packed Virtqueues / Buffer Address and Length}
>
> In an available descriptor, Buffer Address corresponds to the physical address of
> the buffer. The length of the buffer assumed to be physically contigious is stored
> in Buffer Length.
>
> In a used descriptor, Buffer Address is unused. Buffer Length specifies the length
> of the buffer that has been initialized (written to) by the device.
>
> Buffer length is reserved for used descriptors without the VIRTQ_DESC_F_WRITE
> flag, and is ignored by drivers.
>
> \subsection{Scatter-Gather Support}
> \label{sec:Packed Virtqueues / Scatter-Gather Support}
>
> Some drivers need an ability to supply a list of multiple buffer elements (also
> known as a scatter/gather list) with a request.
[#lga#] "list of multiple buffer elements ... with a request" is another way of saying "multiple descriptors ... for a buffer"?
> Two optional features support this: descriptor chaining and indirect descriptors.
>
> If neither feature has been negotiated, each buffer is physically-contigious,
> either read-only or write-only and is described completely by a single descriptor.
>
> While unusual (most implementations either create all lists solely using non-
> indirect descriptors, or always use a single indirect element), if both features
> have been negotiated, mixing direct and direct descriptors in a ring is valid, as
> long as each list only contains descriptors of a given type.
>
> Scatter/gather lists only apply to available descriptors. A single used descriptor
> corresponds to the whole list.
>
> The device limits the number of descriptors in a list through a bus-specific and/or
> device-specific value. If not limited, the maximum number of descriptors in a list
> is the virt queue size.
>
> \subsection{Next Flag: Descriptor Chaining} \label{sec:Packed Virtqueues / Next
> Flag: Descriptor Chaining}
>
> The VIRTIO_F_LIST_DESC feature allows driver to do this by using multiple
> descriptors, and setting the VIRTQ_DESC_F_NEXT in Flags for all but the last
> available descriptor.
>
> \begin{lstlisting}
> /* This marks a buffer as continuing. */
> #define VIRTQ_DESC_F_NEXT 1
> \end{lstlisting}
>
> Buffer ID is included in the last descriptor in the list.
>
> The driver always makes the the first descriptor in the list available after the rest
> of the list has been written out into the ring. This guarantees that the device will
> never observe a partial scatter/gather list in the ring.
>
> Device only writes out a single used descriptor for the whole list. It then skips
> forward according to the number of descriptors in the list. Driver needs to keep
> track of the size of the list corresponding to each buffer ID, to be able to skip to
> where the next used descriptor is written by the device.
>
[#lga#] Regarding this side structure for keeping track of descriptor chaining, addresses, etc. The driver needs to reference it to process used descriptors correctly. Is there a risk that the v1.0 avail/used cache misses are just traded for side structure cache misses in v1.1?
> For example, if descriptors are used in the same order in which they are made
> available, this will result in the used descriptor overwriting the first available
> descriptor in the list, the used descriptor for the next list overwriting the first
> available descriptor in the next list, etc.
>
> VIRTQ_DESC_F_NEXT is reserved in used descriptors, and should be ignored by
> drivers.
>
> \subsection{Indirect Flag: Scatter-Gather Support} \label{sec:Packed Virtqueues
> / Indirect Flag: Scatter-Gather Support}
>
> Some devices benefit by concurrently dispatching a large number of large
> requests. The VIRTIO_F_INDIRECT_DESC feature allows this. To increase ring
> capacity the driver can store (read-only by the device) table of indirect
> descriptors anywhere in memory, and insert a descriptor in main virtqueue (with
> \field{Flags}\&VIRTQ_DESC_F_INDIRECT on) that refers to a memory buffer
> containing this indirect descriptor table; \field{addr} and \field{len} refer to the
> indirect table address and length in bytes, respectively.
> \begin{lstlisting}
> /* This means the buffer contains a table of buffer descriptors. */
> #define VIRTQ_DESC_F_INDIRECT 4
> \end{lstlisting}
>
> The indirect table layout structure looks like this (\field{len} is the Buffer Length
> of the descriptor that refers to this table, which is a variable, so this code won't
> compile):
>
> \begin{lstlisting}
> struct indirect_descriptor_table {
> /* The actual descriptor structures (struct Desc each) */
> struct Desc desc[len / sizeof(struct Desc)]; }; \end{lstlisting}
>
> The first descriptor is located at start of the indirect descriptor table, additional
> indirect descriptors come immediately afterwards. \field{Flags}
> \&VIRTQ_DESC_F_WRITE is the only valid flag for descriptors in the indirect
> table. Others are reserved are ignored by the device.
> Buffer ID is also reserved and is ignored by the device.
>
> In Descriptors with VIRTQ_DESC_F_INDIRECT set VIRTQ_DESC_F_WRITE is
> reserved and is ignored by the device.
>
> \subsection{In-order use of descriptors} \label{sec:Packed Virtqueues / In-order
> use of descriptors}
>
> Some devices always use descriptors in the same order in which they have been
> made available. These devices can offer the VIRTIO_F_IN_ORDER feature. If
> negotiated, this knowledge allows devices to notify the use of a batch of buffers
> to the driver by only writing out a single used descriptor with the Buffer ID
> corresponding to the last descriptor in the batch.
>
> Device then skips forward in the ring according to the size of the the batch.
> Driver needs to look up the used Buffer ID and calculate the batch size to be able
> to advance to where the next used descriptor will be written by the device.
>
> This will result in the used descriptor overwriting the first available descriptor in
> the batch, the used descriptor for the next batch overwriting the first available
> descriptor in the next batch, etc.
>
> The skipped buffers (for which no used descriptor was written) are assumed to
> have been used (read or written) by the device completely.
>
> \subsection{Multi-buffer requests}
> \label{sec:Packed Virtqueues / Multi-descriptor batches} Some devices combine
> multiple buffers as part of processing a single request. These devices always
> makes the the first descriptor in the request available after the rest of the
> request has been written out request the ring. This guarantees that the driver
> will never observe a partial request in the ring.
>
>
> \subsection{Driver and Device Event Suppression} \label{sec:Packed Virtqueues /
> Driver and Device Event Suppression} In many systems driver and device
> notifications involve significant overhead. To mitigate this overhead, each
> virtqueue includes two identical structures used for controlling notifications
> between device and driver.
>
> Driver Event Suppression structure is read-only by the device and controls the
> events sent by the device (e.g. interrupts).
>
> Device Event Suppression structure is read-only by the driver and controls the
> events sent by the driver (e.g. IO).
>
>
> Each of these structures includes the following fields:
>
> \begin{description}
> \item [Descriptor Event Flags] Takes values:
> \begin{itemize}
> \item 00b reserved
> \item 01b enable events
> \item 11b disable events
> \item 10b enable events for a specific descriptor (as specified by Descriptor
> Event Offset/Wrap Counter).
> \end{itemize}
> \item [Descriptor Event Offset] If Event Flags set to descriptor specific event:
> offset within the ring (in units of descriptor size). Event will only trigger when
> this descriptor is made available/used respectively.
[#lga#] If it relates to a specific index in the ring, why not add a driver-notification-flag in the descriptor, such that the driver can adjust on which descriptor is wants a notification. This would eliminate the need for a separate driver notification structure. With regards to the device events, wouldn't a threshold of available descriptors or total size of available buffers be more natural than a fixed index in the ring?
> \item [Descriptor Event Wrap Counter]If Event Flags set to descriptor specific
> event: offset within the ring (in units of descriptor size). Event will only trigger
> when Ring Wrap Counter matches this value and a descriptor is made
> available/used respectively.
[#lga#] Is the idea with the "Event Wrap Counter" to reduce notifications to every other ring-wrap? How/when is this used?
> \end{description}
>
> After writing out some descriptors, both device and driver are expected to
> consult the relevant structure to find out whether interrupt should be sent. As
> this access to shared memory involves overhead for some transports, the
> following additional field is present:
>
> \begin{description}
> \item [Structure Change Event Flags] Enable/disable sending an event
> notification when the other side changes its own Event Suppression structure.
> \end{description}
>
> when enabled through this field, device and driver send an event notification
> whenever they change the driver and device event suppression structure
> respectively.
>
>
> \subsubsection{Driver notifications}
> \label{sec:Packed Virtqueues / Driver notifications} Some devices benefit from
> ability to find out the number of available descriptors in the ring, and whether to
> send interrupts to drivers without accessing ring memory:
> for efficiency or as a debugging aid.
>
> To help with these optimizations, driver notifications to the device include the
> following information:
>
> \begin{itemize}
> \item VQ number
> \item Flags - set to 00b
> \item Offset (in units of descriptor size) within the ring
> where the next available descriptor will be written \item Available Ring Wrap
> Counter \end{itemize}
>
> Whenever driver notifies device about a Device Event Suppression Structure
> change (if enabled through Structure Change Event Flags in Driver Event
> Suppression Structure), it sends a copy of the up-to-date Event Suppression
> Structure:
>
> \begin{itemize}
> \item VQ number
> \item Descriptor Event Flags
> \item Descriptor Event Offset
> \item Descriptor Event Wrap Counter
> \end{itemize}
>
>
> \subsubsection{Structure Size and Alignment} \label{sec:Packed Virtqueues /
> Structure Size and Alignment}
>
> Each part of the virtqueue is physically-contiguous in guest memory, and has
> different alignment requirements.
>
> The memory aligment and size requirements, in bytes, of each part of the
> virtqueue are summarized in the following table:
>
> \begin{tabular}{|l|l|l|}
> \hline
> Virtqueue Part & Alignment & Size \\
> \hline \hline
> Descriptor Ring & 16 & $16 * $(Queue Size) \\
> \hline
> Device Event Suppression & 4 & 4 \\
> \hline
> Driver Event Suppression & 4 & 4 \\
> \hline
> \end{tabular}
>
> The Alignment column gives the minimum alignment for each part of the
> virtqueue.
>
> The Size column gives the total number of bytes for each part of the virtqueue.
>
> Queue Size corresponds to the maximum number of descriptors in the
> virtqueue\footnote{For example, if Queue Size is 4 then at most 4 buffers can be
> queued at any given time.}. Queue Size value does not have to be a power of 2
> unless enforced by the transport.
>
> \drivernormative{\subsection}{Virtqueues}{Basic Facilities of a Virtio Device /
> Packed Virtqueues} The driver MUST ensure that the physical address of the first
> byte of each virtqueue part is a multiple of the specified alignment value in the
> above table.
>
> \devicenormative{\subsection}{Virtqueues}{Basic Facilities of a Virtio Device /
> Packed Virtqueues} The device MUST start processing driver descriptors in the
> order in which they appear in the ring.
> The device MUST start writing device descriptors into the ring in the order in
> which they complete.
> Device MAY reorder descriptor writes once they are started.
>
> \subsection{The Virtqueue Descriptor Format}\label{sec:Basic Facilities of a
> Virtio Device / Packed Virtqueues / The Virtqueue Descriptor Format}
>
> The available descriptor refers to the buffers the driver is sending to the device.
> \field{addr} is a physical address, and the descriptor is identified with a buffer
> using the \field{id} field.
>
> \begin{lstlisting}
> struct virtq_desc {
> /* Buffer Address. */
> le64 addr;
> /* Buffer Length. */
> le32 len;
> /* Buffer ID. */
> le16 id;
> /* The flags depending on descriptor type. */
> le16 flags;
> };
> \end{lstlisting}
>
> The descriptor ring is zero-initialized.
>
> \subsection{Event Suppression Structure Format}\label{sec:Basic Facilities of a
> Virtio Device / Packed Virtqueues / Event Suppression Structure Format}
>
> The following structure is used to reduce the number of notifications sent
> between driver and device.
>
> \begin{lstlisting}
> __le16 desc_event_off : 15; /* Descriptor Event Offset */
> int desc_event_wrap : 1; /* Descriptor Event Wrap Counter */
> __le16 desc_event_flags : 2; /* Descriptor Event Flags */
> __le16 structure_change_flags : 1; /* Structure Change Event Flags */
> \end{lstlisting}
>
> \subsection{Driver Notification Format}\label{sec:Basic Facilities of a Virtio
> Device / Packed Virtqueues / Driver Notification Format}
>
> The following structure is used to notify device of available descriptors and of
> event suppression structure changes:
>
> \begin{lstlisting}
> __le16 vqn : 14;
> __le16 desc_event_flags : 2;
> __le16 desc_event_off : 15;
> int desc_event_wrap : 1;
> \end{lstlisting}
>
> \devicenormative{\subsubsection}{The Virtqueue Descriptor Table}{Basic
> Facilities of a Virtio Device / Virtqueues / The Virtqueue Descriptor Table} A
> device MUST NOT write to a device-readable buffer, and a device SHOULD NOT
> read a device-writable buffer.
> A device MUST NOT use a descriptor unless it observes VIRTQ_DESC_F_AVAIL bit
> in its \field{flags} being changed.
> A device MUST NOT change a descriptor after changing it's
> VIRTQ_DESC_F_USED bit in its \field{flags}.
>
> \drivernormative{\subsubsection}{The Virtqueue Descriptor Table}{Basic
> Facilities of a Virtio Device / Virtqueues / The Virtqueue Descriptor Table} A
> driver MUST NOT change a descriptor unless it observes VIRTQ_DESC_F_USED
> bit in its \field{flags} being changed.
> A driver MUST NOT change a descriptor after changing VIRTQ_DESC_F_USED bit
> in its \field{flags}.
>
> \drivernormative{\paragraph}{Scatter-Gather Support}{Basic Facilities of a Virtio
> Device / Packed Virtqueues / Scatter-Gather Support} The driver MUST NOT set
> the DESC_F_LIST_NEXT flag unless the VIRTIO_F_LIST_DESC feature was
> negotiated.
>
> A driver MUST NOT create a descriptor list longer than allowed by the device.
>
> A driver MUST NOT create a descriptor list longer than the Queue Size.
>
> This implies that loops in the descriptor list are forbidden!
>
> The driver MUST place any device-writable descriptor elements after any device-
> readable descriptor elements.
>
> A driver MUST NOT depend on the device to use more descriptors to be able to
> write out all descriptors in a list. A driver MUST make sure there's enough space
> in the ring for the whole list before making any of the descriptors available to
> the device.
>
> A driver MUST NOT make the first descriptor in the list available before
> initializing the rest of the descriptors.
>
> \devicenormative{\paragraph}{Scatter-Gather Support}{Basic Facilities of a
> Virtio Device / Packed Virtqueues / Scatter-Gather Support} The device MUST
> use descriptors in a list chained by the VIRTQ_DESC_F_NEXT flag in the same
> order that they were made available by the driver.
[#lga#] Ambiguity with regards to "order ... made available". It is not in the order the driver set the "available" flag, since the first descriptor must have its flag set last (per previous section). More correct to say: "in ring order"?
>
> The device MAY limit the number of buffers it will allow in a list.
>
> \drivernormative{\paragraph}{Indirect Descriptors}{Basic Facilities of a Virtio
> Device / Virtqueues / The Virtqueue Descriptor Table / Indirect Descriptors} The
> driver MUST NOT set the DESC_F_INDIRECT flag unless the
> VIRTIO_F_INDIRECT_DESC feature was negotiated. The driver MUST NOT
> set any flags except DESC_F_WRITE within an indirect descriptor.
>
> A driver MUST NOT create a descriptor chain longer than allowed by the device.
>
> A driver MUST NOT write direct descriptors with DESC_F_INDIRECT set in a
> scatter-gather list linked by VIRTQ_DESC_F_NEXT.
> \field{flags}.
>
[#lga#] Is batch-used possible for an indirect buffer, by setting the WRITE flag and length in the used ring descriptor? That way the device does not have to write to the indirect descriptors at all (and the driver not read them). If the WRITE-flag in the used ring descriptor is not set, and the indirect descriptor list contains device-writable elements, then the device must update the indirect descriptors and the driver must read them. Can a device do batch-used within an indirect descriptor-list?
>
>
>
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