Re: [PATCH v3] introduce VFIO-over-socket protocol specificaion

[Nikos Dragazis]

Hi Thanos,

I had a quick look on the spec. Leaving some comments inline.

On 17/7/20 2:20 μ.μ., Thanos Makatos wrote:

> This patch introduces the VFIO-over-socket protocol specification, which
> is designed to allow devices to be emulated outside QEMU, in a separate
> process. VFIO-over-socket reuses the existing VFIO defines, structs and
> concepts.
>
> It has been earlier discussed as an RFC in:
> "RFC: use VFIO over a UNIX domain socket to implement device offloading"
>
> Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
> Signed-off-by: Thanos Makatos <thanos.makatos@nutanix.com>
>
> ---
>
> Changed since v1:
>    * fix coding style issues
>    * update MAINTAINERS for VFIO-over-socket
>    * add vfio-over-socket to ToC
>
> Changed since v2:
>    * fix whitespace
>
> Regarding the build failure, I have not been able to reproduce it locally
> using the docker image on my Debian 10.4 machine.
> ---
>   MAINTAINERS                     |    6 +
>   docs/devel/index.rst            |    1 +
>   docs/devel/vfio-over-socket.rst | 1135 +++++++++++++++++++++++++++++++++++++++
>   3 files changed, 1142 insertions(+)
>   create mode 100644 docs/devel/vfio-over-socket.rst
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 030faf0..bb81590 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -1732,6 +1732,12 @@ F: hw/vfio/ap.c
>   F: docs/system/s390x/vfio-ap.rst
>   L: qemu-s390x@nongnu.org
>   
> +VFIO-over-socket
> +M: John G Johnson <john.g.johnson@oracle.com>
> +M: Thanos Makatos <thanos.makatos@nutanix.com>
> +S: Supported
> +F: docs/devel/vfio-over-socket.rst
> +
>   vhost
>   M: Michael S. Tsirkin <mst@redhat.com>
>   S: Supported
> diff --git a/docs/devel/index.rst b/docs/devel/index.rst
> index ae6eac7..0439460 100644
> --- a/docs/devel/index.rst
> +++ b/docs/devel/index.rst
> @@ -30,3 +30,4 @@ Contents:
>      reset
>      s390-dasd-ipl
>      clocks
> +   vfio-over-socket
> diff --git a/docs/devel/vfio-over-socket.rst b/docs/devel/vfio-over-socket.rst
> new file mode 100644
> index 0000000..b474f23
> --- /dev/null
> +++ b/docs/devel/vfio-over-socket.rst
> @@ -0,0 +1,1135 @@
> +***************************************
> +VFIO-over-socket Protocol Specification
> +***************************************
> +
> +Version 0.1
> +
> +Introduction
> +============
> +VFIO-over-socket, also known as vfio-user, is a protocol that allows a device

I think there is no point in having two names for the same protocol,
"vfio-over-socket" and "vfio-user".

> +to be virtualized in a separate process outside of QEMU. VFIO-over-socket
> +devices consist of a generic VFIO device type, living inside QEMU, which we
> +call the client, and the core device implementation, living outside QEMU, which
> +we call the server. VFIO-over-socket can be the main transport mechanism for
> +multi-process QEMU, however it can be used by other applications offering
> +device virtualization. Explaining the advantages of a
> +disaggregated/multi-process QEMU, and device virtualization outside QEMU in
> +general, is beyond the scope of this document.
> +
> +This document focuses on specifying the VFIO-over-socket protocol. VFIO has
> +been chosen for the following reasons:
> +
> +1) It is a mature and stable API, backed by an extensively used framework.
> +2) The existing VFIO client implementation (qemu/hw/vfio/) can be largely
> +   reused.
> +
> +In a proof of concept implementation it has been demonstrated that using VFIO
> +over a UNIX domain socket is a viable option. VFIO-over-socket is designed with
> +QEMU in mind, however it could be used by other client applications. The
> +VFIO-over-socket protocol does not require that QEMU's VFIO client
> +implementation is used in QEMU. None of the VFIO kernel modules are required
> +for supporting the protocol, neither in the client nor the server, only the
> +source header files are used.
> +
> +The main idea is to allow a virtual device to function in a separate process in
> +the same host over a UNIX domain socket. A UNIX domain socket (AF_UNIX) is
> +chosen because we can trivially send file descriptors over it, which in turn
> +allows:
> +
> +* Sharing of guest memory for DMA with the virtual device process.
> +* Sharing of virtual device memory with the guest for fast MMIO.
> +* Efficient sharing of eventfd's for triggering interrupts.
> +
> +However, other socket types could be used which allows the virtual device
> +process to run in a separate guest in the same host (AF_VSOCK) or remotely
> +(AF_INET). Theoretically the underlying transport doesn't necessarily have to
> +be a socket, however we don't examine such alternatives. In this document we
> +focus on using a UNIX domain socket and introduce basic support for the other
> +two types of sockets without considering performance implications.
> +
> +This document does not yet describe any internal details of the server-side
> +implementation, however QEMU's VFIO client implementation will have to be
> +adapted according to this protocol in order to support VFIO-over-socket virtual
> +devices.
> +
> +VFIO
> +====
> +VFIO is a framework that allows a physical device to be securely passed through
> +to a user space process; the kernel does not drive the device at all.

I would remove the last part: "the kernel does not drive the device at
all". Isn't that quite inaccurate? The kernel does drive the device with
the vfio driver. The user space driver needs the vfio driver in order to
do certain things like, for example, write on a port or receive
notifications for device interrupts.

> +Typically, the user space process is a VM and the device is passed through to
> +it in order to achieve high performance. VFIO provides an API and the required
> +functionality in the kernel. QEMU has adopted VFIO to allow a guest virtual
> +machine to directly access physical devices, instead of emulating them in

Maybe s/guest virtual machine/guest ?

> +software

Missing dot here.

> +
> +VFIO-over-socket reuses the core VFIO concepts defined in its API, but
> +implements them as messages to be sent over a UNIX-domain socket. It does not

s/UNIX-domain/UNIX domain (just to have the same name everywhere)

> +change the kernel-based VFIO in any way, in fact none of the VFIO kernel
> +modules need to be loaded to use VFIO-over-socket. It is also possible for QEMU
> +to concurrently use the current kernel-based VFIO for one guest device, and use
> +VFIO-over-socket for another device in the same guest.
> +
> +VFIO Device Model
> +-----------------
> +A device under VFIO presents a standard VFIO model to the user process. Many
> +of the VFIO operations in the existing kernel model use the ioctl() system
> +call, and references to the existing model are called the ioctl()
> +implementation in this document.
> +
> +The following sections describe the set of messages that implement the VFIO
> +device model over a UNIX domain socket. In many cases, the messages are direct
> +translations of data structures used in the ioctl() implementation. Messages
> +derived from ioctl()s will have a name derived from the ioctl() command name.
> +E.g., the VFIO_GET_INFO ioctl() command becomes a VFIO_USER_GET_INFO message.
> +The purpose for this reuse is to share as much code as feasible with the

s/for/of

> +ioctl() implementation.
> +
> +Client and Server
> +^^^^^^^^^^^^^^^^^
> +The socket connects two processes together: a client process and a server
> +process. In the context of this document, the client process is the process
> +emulating a guest virtual machine, such as QEMU. The server process is a
> +process that provides device emulation.
> +
> +Connection Initiation
> +^^^^^^^^^^^^^^^^^^^^^
> +After the client connects to the server, the initial server message is
> +VFIO_USER_VERSION to propose a protocol version and set of capabilities to
> +apply to the session. The client replies with a compatible version and set of
> +capabilities it will support, or closes the connection if it cannot support the
> +advertised version.
> +
> +Guest Memory Configuration
> +^^^^^^^^^^^^^^^^^^^^^^^^^^
> +The client uses VFIO_USER_DMA_MAP and VFIO_USER_DMA_UNMAP messages to inform
> +the server of the valid guest DMA ranges that the server can access on behalf
> +of a device. Guest memory may be accessed by the server via VFIO_USER_DMA_READ
> +and VFIO_USER_DMA_WRITE messages over the socket.
> +
> +An optimization for server access to guest memory is for the client to provide
> +file descriptors the server can mmap() to directly access guest memory. Note
> +that mmap() privileges cannot be revoked by the client, therefore file
> +descriptors should only be exported in environments where the client trusts the
> +server not to corrupt guest memory.
> +
> +Device Information
> +^^^^^^^^^^^^^^^^^^
> +The client uses a VFIO_USER_DEVICE_GET_INFO message to query the server for
> +information about the device. This information includes:
> +
> +* The device type and capabilities,
> +* the number of memory regions, and
> +* the device presents to the guest the number of interrupt types the device
> +  supports.
> +
> +Region Information
> +^^^^^^^^^^^^^^^^^^
> +The client uses VFIO_USER_DEVICE_GET_REGION_INFO messages to query the server
> +for information about the device's memory regions. This information describes:
> +
> +* Read and write permissions, whether it can be memory mapped, and whether it
> +  supports additional capabilities.
> +* Region index, size, and offset.
> +
> +When a region can be mapped by the client, the server provides a file
> +descriptor which the client can mmap(). The server is responsible for polling
> +for client updates to memory mapped regions.
> +
> +Region Capabilities
> +"""""""""""""""""""
> +Some regions have additional capabilities that cannot be described adequately
> +by the region info data structure. These capabilities are returned in the
> +region info reply in a list similar to PCI capabilities in a PCI device's
> +configuration space.
> +
> +Sparse Regions
> +""""""""""""""
> +A region can be memory-mappable in whole or in part. When only a subset of a
> +region can be mapped by the client, a VFIO_REGION_INFO_CAP_SPARSE_MMAP
> +capability is included in the region info reply. This capability describes
> +which portions can be mapped by the client.
> +
> +For example, in a virtual NVMe controller, sparse regions can be used so that
> +accesses to the NVMe registers (found in the beginning of BAR0) are trapped (an
> +infrequent an event), while allowing direct access to the doorbells (an

s/an event/event

> +extremely frequent event as every I/O submission requires a write to BAR0),
> +found right after the NVMe registers in BAR0.
> +
> +Interrupts
> +^^^^^^^^^^
> +The client uses VFIO_USER_DEVICE_GET_IRQ_INFO messages to query the server for
> +the device's interrupt types. The interrupt types are specific to the bus the
> +device is attached to, and the client is expected to know the capabilities of
> +each interrupt type. The server can signal an interrupt either with
> +VFIO_USER_VM_INTERRUPT messages over the socket, or can directly inject
> +interrupts into the guest via an event file descriptor. The client configures
> +how the server signals an interrupt with VFIO_USER_SET_IRQS messages.
> +
> +Device Read and Write
> +^^^^^^^^^^^^^^^^^^^^^
> +When the guest executes load or store operations to device memory, the client
> +forwards these operations to the server with VFIO_USER_REGION_READ or
> +VFIO_USER_REGION_WRITE messages. The server will reply with data from the
> +device on read operations or an acknowledgement on write operations.
> +
> +DMA
> +^^^
> +When a device performs DMA accesses to guest memory, the server will forward
> +them to the client with VFIO_USER_DMA_READ and VFIO_USER_DMA_WRITE messages.
> +These messages can only be used to access guest memory the client has
> +configured into the server.
> +
> +Protocol Specification
> +======================
> +To distinguish from the base VFIO symbols, all VFIO-over-socket symbols are
> +prefixed with vfio_user or VFIO_USER. In revision 0.1, all data is in the
> +little-endian format, although this may be relaxed in future revision in cases
> +where the client and server are both big-endian. The messages are formatted
> +for seamless reuse of the native VFIO structs. A server can serve:
> +
> +1) multiple clients, and/or
> +2) multiple virtual devices, belonging to one or more clients.
> +
> +Therefore each message requires a header that uniquely identifies the virtual
> +device. It is a server-side implementation detail whether a single server
> +handles multiple virtual devices from the same or multiple guests.
> +
> +Socket
> +------
> +A single UNIX domain socket is assumed to be used for each device. The location

Is it correct for a spec to assume things?

> +of the socket is implementation-specific. Multiplexing clients, devices, and
> +servers over the same socket is not supported in this version of the protocol,
> +but a device ID field exists in the message header so that a future support can
> +be added without a major version change.
> +
> +Authentication
> +--------------
> +For AF_UNIX, we rely on OS mandatory access controls on the socket files,
> +therefore it is up to the management layer to set up the socket as required.
> +Socket types than span guests or hosts will require a proper authentication
> +mechanism. Defining that mechanism is deferred to a future version of the
> +protocol.
> +
> +Request Concurrency
> +-------------------
> +There can be multiple outstanding requests per virtual device, e.g. a
> +frame buffer where the guest does multiple stores to the virtual device. The
> +server can execute and reorder non-conflicting requests in parallel, depending
> +on the device semantics.
> +
> +Socket Disconnection Behavior
> +-----------------------------
> +The server and the client can disconnect from each other, either intentionally
> +or unexpectedly. Both the client and the server need to know how to handle such
> +events.
> +
> +Server Disconnection
> +^^^^^^^^^^^^^^^^^^^^
> +A server disconnecting from the client may indicate that:
> +
> +1) A virtual device has been restarted, either intentionally (e.g. because of a
> +device update) or unintentionally (e.g. because of a crash). In any case, the
> +virtual device will come back so the client should not do anything (e.g. simply
> +reconnect and retry failed operations).
> +

Indentation issue ^^ (also remove the space, there are no spaces between the
elements in the other numbered lists).

> +2) A virtual device has been shut down with no intention to be restarted.
> +
> +It is impossible for the client to know whether or not a failure is
> +intermittent or innocuous and should be retried, therefore the client should
> +attempt to reconnect to the socket. Since an intentional server restart (e.g.
> +due to an upgrade) might take some time, a reasonable timeout should be used.
> +In cases where the disconnection is expected (e.g. the guest shutting down), no
> +new requests will be sent anyway so this situation doesn't pose a problem. The
> +control stack will clean up accordingly.
> +
> +Parametrizing this behaviour by having the virtual device advertise a
> +reasonable reconnect is deferred to a future version of the protocol.
> +
> +Client Disconnection
> +^^^^^^^^^^^^^^^^^^^^
> +The client disconnecting from the server primarily means that the QEMU process
> +has exited. Currently this means that the guest is shut down so the device is
> +no longer needed therefore the server can automatically exit. However, there
> +can be cases where a client disconnect should not result in a server exit:

s/disconnect/disconnection

> +
> +1) A single server serving multiple clients.
> +2) A multi-process QEMU upgrading itself step by step, which isn't yet
> +   implemented.
> +
> +Therefore in order for the protocol to be forward compatible the server should
> +take no action when the client disconnects. If anything happens to the client
> +process the control stack will know about it and can clean up resources
> +accordingly.
> +
> +Request Retry and Response Timeout
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +QEMU's VFIO retries certain operations if they fail. While this makes sense for
> +real HW, we don't know for sure whether it makes sense for virtual devices. A
> +failed request is a request that has been successfully sent and has been
> +responded to with an error code. Failure to send the request in the first place
> +(e.g. because the socket is disconnected) is a different type of error examined
> +earlier in the disconnect section.
> +
> +Defining a retry and timeout scheme if deferred to a future version of the

s/if/is

> +protocol.
> +
> +Commands
> +--------
> +The following table lists the VFIO message command IDs, and whether the
> +message request is sent from the client or the server.
> +
> ++----------------------------------+---------+-------------------+
> +| Name                             | Command | Request Direction |
> ++==================================+=========+===================+
> +| VFIO_USER_VERSION                | 1       | server → client   |
> ++----------------------------------+---------+-------------------+
> +| VFIO_USER_DMA_MAP                | 2       | client → server   |
> ++----------------------------------+---------+-------------------+
> +| VFIO_USER_DMA_UNMAP              | 3       | client → server   |
> ++----------------------------------+---------+-------------------+
> +| VFIO_USER_DEVICE_GET_INFO        | 4       | client → server   |
> ++----------------------------------+---------+-------------------+
> +| VFIO_USER_DEVICE_GET_REGION_INFO | 5       | client → server   |
> ++----------------------------------+---------+-------------------+
> +| VFIO_USER_DEVICE_GET_IRQ_INFO    | 6       | client → server   |
> ++----------------------------------+---------+-------------------+
> +| VFIO_USER_DEVICE_SET_IRQS        | 7       | client → server   |
> ++----------------------------------+---------+-------------------+
> +| VFIO_USER_REGION_READ            | 8       | client → server   |
> ++----------------------------------+---------+-------------------+
> +| VFIO_USER_REGION_WRITE           | 9       | client → server   |
> ++----------------------------------+---------+-------------------+
> +| VFIO_USER_DMA_READ               | 10      | server → client   |
> ++----------------------------------+---------+-------------------+
> +| VFIO_USER_DMA_READ               | 11      | server → client   |

Isn't that VFIO_USER_DMA_WRITE?

> ++----------------------------------+---------+-------------------+
> +| VFIO_USER_VM_INTERRUPT           | 12      | server → client   |
> ++----------------------------------+---------+-------------------+
> +| VFIO_DEVICE_RESET                | 13      | client → server   |
> ++----------------------------------+---------+-------------------+
> +
> +Header
> +------
> +All messages are preceded by a 16 byte header that contains basic information
> +about the message. The header is followed by message-specific data described
> +in the sections below.
> +
> ++----------------+--------+-------------+
> +| Name           | Offset | Size        |
> ++================+========+=============+
> +| Device ID      | 0      | 2           |
> ++----------------+--------+-------------+
> +| Message ID     | 2      | 2           |
> ++----------------+--------+-------------+
> +| Command        | 4      | 4           |
> ++----------------+--------+-------------+
> +| Message size   | 8      | 4           |
> ++----------------+--------+-------------+
> +| Flags          | 12     | 4           |
> ++----------------+--------+-------------+
> +|                | +-----+------------+ |
> +|                | | Bit | Definition | |
> +|                | +=====+============+ |
> +|                | | 0   | Reply      | |
> +|                | +-----+------------+ |
> +|                | | 1   | No_reply   | |
> +|                | +-----+------------+ |
> ++----------------+--------+-------------+
> +| <message data> | 16     | variable    |
> ++----------------+--------+-------------+
> +
> +* Device ID identifies the destination device of the message. This field is
> +  reserved when the server only supports one device per socket.
> +* Message ID identifies the message, and is used in the message 
> acknowledgement.
> +* Command specifies the command to be executed, listed in the Command Table.
> +* Message size contains the size of the entire message, including the header.
> +* Flags contains attributes of the message:
> +
> +  * The reply bit differentiates request messages from reply messages. A reply
> +    message acknowledges a previous request with the same message ID.
> +  * No_reply indicates that no reply is needed for this request. This is
> +    commonly used when multiple requests are sent, and only the last needs
> +    acknowledgement.
> +
> +VFIO_USER_VERSION
> +-----------------
> +
> +Message format
> +^^^^^^^^^^^^^^
> +
> ++--------------+------------------------+
> +| Name         | Value                  |
> ++==============+========================+
> +| Device ID    | 0                      |
> ++--------------+------------------------+
> +| Message ID   | <ID>                   |
> ++--------------+------------------------+
> +| Command      | 1                      |
> ++--------------+------------------------+
> +| Message size | 16 + version length    |
> ++--------------+------------------------+
> +| Flags        | Reply bit set in reply |
> ++--------------+------------------------+
> +| Version      | JSON byte array        |
> ++--------------+------------------------+
> +
> +This is the initial message sent by the server after the socket connection is
> +established. The version is in JSON format, and the following objects must be
> +included:
> +
> ++--------------+--------+---------------------------------------------------+
> +| Name         | Type   | Description                                       |
> ++==============+========+===================================================+
> +| version      | object | {“major”: <number>, “minor”: <number>}            |
> +|              |        | Version supported by the sender, e.g. “0.1”.      |
> ++--------------+--------+---------------------------------------------------+
> +| type         | string | Fixed to “vfio-user”.                             |
> ++--------------+--------+---------------------------------------------------+
> +| capabilities | array  | Reserved. Can be omitted for v0.1, otherwise must |
> +|              |        | be empty.                                         |
> ++--------------+--------+---------------------------------------------------+
> +
> +Versioning and Feature Support
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +Upon accepting a connection, the server must send a VFIO_USER_VERSION message
> +proposing a protocol version and a set of capabilities. The client compares
> +these with the versions and capabilities it supports and sends a
> +VFIO_USER_VERSION reply according to the following rules.
> +
> +* The major version in the reply must be the same as proposed. If the client
> +  does not support the proposed major, it closes the connection.
> +* The minor version in the reply must be equal to or less than the minor
> +  version proposed.
> +* The capability list must be a subset of those proposed. If the client
> +  requires a capability the server did not include, it closes the connection.
> +* If type is not “vfio-user”, the client closes the connection.
> +
> +The protocol major version will only change when incompatible protocol changes
> +are made, such as changing the message format. The minor version may change
> +when compatible changes are made, such as adding new messages or capabilities,
> +Both the client and server must support all minor versions less than the
> +maximum minor version it supports. E.g., an implementation that supports
> +version 1.3 must also support 1.0 through 1.2.
> +
> +VFIO_USER_DMA_MAP
> +-----------------
> +
> +VFIO_USER_DMA_UNMAP
> +-------------------
> +
> +Message Format
> +^^^^^^^^^^^^^^
> +
> ++--------------+------------------------+
> +| Name         | Value                  |
> ++==============+========================+
> +| Device ID    | 0                      |
> ++--------------+------------------------+
> +| Message ID   | <ID>                   |
> ++--------------+------------------------+
> +| Command      | MAP=2, UNMAP=3         |
> ++--------------+------------------------+
> +| Message size | 16 + table size        |
> ++--------------+------------------------+
> +| Flags        | Reply bit set in reply |
> ++--------------+------------------------+
> +| Table        | array of table entries |
> ++--------------+------------------------+
> +
> +This message is sent by the client to the server to inform it of the guest
> +memory regions the device can access. It must be sent before the device can
> +perform any DMA to the guest. It is normally sent directly after the version
> +handshake is completed, but may also occur when memory is added or subtracted
> +in the guest.
> +
> +The table is an array of the following structure. This structure is 32 bytes
> +in size, so the message size will be 16 + (# of table entries * 32). If a
> +region being added can be directly mapped by the server, an array of file
> +descriptors will be sent as part of the message meta-data. Each region entry
> +will have a corresponding file descriptor. On AF_UNIX sockets, the file
> +descriptors will be passed as SCM_RIGHTS type ancillary data.
> +
> +Table entry format
> +^^^^^^^^^^^^^^^^^^
> +
> ++-------------+--------+-------------+
> +| Name        | Offset | Size        |
> ++=============+========+=============+
> +| Address     | 0      | 8           |
> ++-------------+--------+-------------+
> +| Size        | 8      | 8           |
> ++-------------+--------+-------------+
> +| Offset      | 16     | 8           |
> ++-------------+--------+-------------+
> +| Protections | 24     | 4           |
> ++-------------+--------+-------------+
> +| Flags       | 28     | 4           |
> ++-------------+--------+-------------+
> +|             | +-----+------------+ |
> +|             | | Bit | Definition | |
> +|             | +=====+============+ |
> +|             | | 0   | Mappable   | |
> +|             | +-----+------------+ |
> ++-------------+--------+-------------+
> +
> +* Address is the base DMA address of the region.
> +* Size is the size of the region.
> +* Offset is the file offset of the region with respect to the associated file
> +  descriptor.
> +* Protections are the region's protection attributes as encoded in
> +  ``<sys/mman.h>``.
> +* Flags contain the following region attributes:
> +
> +  * Mappable indicate the region can be mapped via the mmap() system call using
> +    the file descriptor provided in the message meta-data.
> +
> +VFIO_USER_DEVICE_GET_INFO
> +-------------------------
> +
> +Message format
> +^^^^^^^^^^^^^^
> +
> ++--------------+----------------------------+
> +| Name         | Value                      |
> ++==============+============================+
> +| Device ID    | <ID>                       |
> ++--------------+----------------------------+
> +| Message ID   | <ID>                       |
> ++--------------+----------------------------+
> +| Command      | 4                          |
> ++--------------+----------------------------+
> +| Message size | 16 in request, 32 in reply |
> ++--------------+----------------------------+
> +| Flags        | Reply bit set in reply     |
> ++--------------+----------------------------+
> +| Device info  | VFIO device info           |
> ++--------------+----------------------------+
> +
> +This message is sent by the client to the server to query for basic information
> +about the device. Only the message header is needed in the request message.
> +The VFIO device info structure is defined in ``<sys/vfio.h>`` (``struct
> +vfio_device_info``).
> +
> +VFIO device info format
> +^^^^^^^^^^^^^^^^^^^^^^^
> +
> ++-------------+--------+--------------------------+
> +| Name        | Offset | Size                     |
> ++=============+========+==========================+
> +| argsz       | 16     | 4                        |
> ++-------------+--------+--------------------------+
> +| flags       | 20     | 4                        |
> ++-------------+--------+--------------------------+
> +|             | +-----+-------------------------+ |
> +|             | | Bit | Definition              | |
> +|             | +=====+=========================+ |
> +|             | | 0   | VFIO_DEVICE_FLAGS_RESET | |
> +|             | +-----+-------------------------+ |
> +|             | | 1   | VFIO_DEVICE_FLAGS_PCI   | |
> +|             | +-----+-------------------------+ |
> ++-------------+--------+--------------------------+
> +| num_regions | 24     | 4                        |
> ++-------------+--------+--------------------------+
> +| num_irqs    | 28     | 4                        |
> ++-------------+--------+--------------------------+
> +
> +* argz is reserved in vfio-user, it is only used in the ioctl() VFIO
> +  implementation.
> +* flags contains the following device attributes.
> +
> +  * VFIO_DEVICE_FLAGS_RESET indicates the device supports the
> +    VFIO_USER_DEVICE_RESET message.
> +  * VFIO_DEVICE_FLAGS_PCI indicates the device is a PCI device.
> +
> +* num_regions is the number of memory regions the device exposes.
> +* num_irqs is the number of distinct interrupt types the device supports.
> +
> +This version of the protocol only supports PCI devices. Additional devices may
> +be supported in future versions.
> +
> +VFIO_USER_DEVICE_GET_REGION_INFO
> +--------------------------------
> +
> +Message format
> +^^^^^^^^^^^^^^
> +
> ++--------------+------------------+
> +| Name         | Value            |
> ++==============+==================+
> +| Device ID    | <ID>             |
> ++--------------+------------------+
> +| Message ID   | <ID>             |
> ++--------------+------------------+
> +| Command      | 5                |
> ++--------------+------------------+
> +| Message size | 48 + any caps    |
> ++--------------+------------------+
> +| Flags Reply  | bit set in reply |
> ++--------------+------------------+
> +| Region info  | VFIO region info |
> ++--------------+------------------+
> +
> +This message is sent by the client to the server to query for information about
> +device memory regions. The VFIO region info structure is defined in
> +``<sys/vfio.h>`` (``struct vfio_region_info``).
> +
> +VFIO region info format
> +^^^^^^^^^^^^^^^^^^^^^^^
> +
> ++------------+--------+------------------------------+
> +| Name       | Offset | Size                         |
> ++============+========+==============================+
> +| argsz      | 16     | 4                            |
> ++------------+--------+------------------------------+
> +| flags      | 20     | 4                            |
> ++------------+--------+------------------------------+
> +|            | +-----+-----------------------------+ |
> +|            | | Bit | Definition                  | |
> +|            | +=====+=============================+ |
> +|            | | 0   | VFIO_REGION_INFO_FLAG_READ  | |
> +|            | +-----+-----------------------------+ |
> +|            | | 1   | VFIO_REGION_INFO_FLAG_WRITE | |
> +|            | +-----+-----------------------------+ |
> +|            | | 2   | VFIO_REGION_INFO_FLAG_MMAP  | |
> +|            | +-----+-----------------------------+ |
> +|            | | 3   | VFIO_REGION_INFO_FLAG_CAPS  | |
> +|            | +-----+-----------------------------+ |
> ++------------+--------+------------------------------+
> +| index      | 24     | 4                            |
> ++------------+--------+------------------------------+
> +| cap_offset | 28     | 4                            |
> ++------------+--------+------------------------------+
> +| size       | 32     | 8                            |
> ++------------+--------+------------------------------+
> +| offset     | 40     | 8                            |
> ++------------+--------+------------------------------+
> +
> +* argz is reserved in vfio-user, it is only used in the ioctl() VFIO
> +  implementation.
> +* flags are attributes of the region:
> +
> +  * VFIO_REGION_INFO_FLAG_READ allows client read access to the region.
> +  * VFIO_REGION_INFO_FLAG_WRITE allows client write access region.

s/region/to the region

> +  * VFIO_REGION_INFO_FLAG_MMAP specifies the client can mmap() the region. When
> +    this flag is set, the reply will include a file descriptor in its 
> meta-data.
> +    On AF_UNIX sockets, the file descriptors will be passed as SCM_RIGHTS type
> +    ancillary data.
> +  * VFIO_REGION_INFO_FLAG_CAPS indicates additional capabilities found in the
> +    reply.
> +
> +* index is the index of memory region being queried, it is the only field that
> +  is required to be set in the request message.
> +* cap_offset describes where additional region capabilities can be found.
> +  cap_offset is relative to the beginning of the VFIO region info structure.
> +  The data structure it points is a VFIO cap header defined in 
> ``<sys/vfio.h>``.
> +* size is the size of the region.
> +* offset is the offset given to the mmap() system call for regions with the
> +  MMAP attribute. It is also used as the base offset when mapping a VFIO
> +  sparse mmap area, described below.
> +
> +VFIO Region capabilities
> +^^^^^^^^^^^^^^^^^^^^^^^^
> +The VFIO region information can also include a capabilities list. This list is
> +similar to a PCI capability list - each entry has a common header that
> +identifies a capability and where the next capability in the list can be found.
> +The VFIO capability header format is defined in ``<sys/vfio.h>`` (``struct
> +vfio_info_cap_header``).
> +
> +VFIO cap header format
> +^^^^^^^^^^^^^^^^^^^^^^
> +
> ++---------+--------+------+
> +| Name    | Offset | Size |
> ++=========+========+======+
> +| id      | 0      | 2    |
> ++---------+--------+------+
> +| version | 2      | 2    |
> ++---------+--------+------+
> +| next    | 4      | 4    |
> ++---------+--------+------+
> +
> +* id is the capability identity.
> +* version is a capability-specific version number.
> +* next specifies the offset of the next capability in the capability list. It
> +  is relative to the beginning of the VFIO region info structure.
> +
> +VFIO sparse mmap
> +^^^^^^^^^^^^^^^^
> +
> ++------------------+----------------------------------+
> +| Name             | Value                            |
> ++==================+==================================+
> +| id               | VFIO_REGION_INFO_CAP_SPARSE_MMAP |
> ++------------------+----------------------------------+
> +| version          | 0x1                              |
> ++------------------+----------------------------------+
> +| next             | <next>                           |
> ++------------------+----------------------------------+
> +| sparse mmap info | VFIO region info sparse mmap     |
> ++------------------+----------------------------------+
> +
> +The only capability supported in this version of the protocol is for sparse
> +mmap. This capability is defined when only a subrange of the region supports
> +direct access by the client via mmap(). The VFIO sparse mmap area is defined in
> +``<sys/vfio.h>`` (``struct vfio_region_sparse_mmap_area``).
> +
> +VFIO region info cap sparse mmap
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> ++----------+--------+------+
> +| Name     | Offset | Size |
> ++==========+========+======+
> +| nr_areas | 0      | 4    |
> ++----------+--------+------+
> +| reserved | 4      | 4    |
> ++----------+--------+------+
> +| offset   | 8      | 8    |
> ++----------+--------+------+
> +| size     | 16     | 9    |
> ++----------+--------+------+
> +| ...      |        |      |
> ++----------+--------+------+
> +
> +* nr_areas is the number of sparse mmap areas in the region.
> +* offset and size describe a single area that can be mapped by the client.
> +  There will be nr_areas pairs of offset and size. The offset will be added to
> +  the base offset given in the VFIO_USER_DEVICE_GET_REGION_INFO to form the
> +  offset argument of the subsequent mmap() call.
> +
> +The VFIO sparse mmap area is defined in ``<sys/vfio.h>`` (``struct
> +vfio_region_info_cap_sparse_mmap``).
> +
> +VFIO_USER_DEVICE_GET_IRQ_INFO
> +-----------------------------
> +
> +Message format
> +^^^^^^^^^^^^^^
> +
> ++--------------+------------------------+
> +| Name         | Value                  |
> ++==============+========================+
> +| Device ID    | <ID>                   |
> ++--------------+------------------------+
> +| Message ID   | <ID>                   |
> ++--------------+------------------------+
> +| Command      | 6                      |
> ++--------------+------------------------+
> +| Message size | 32                     |
> ++--------------+------------------------+
> +| Flags        | Reply bit set in reply |
> ++--------------+------------------------+
> +| IRQ info     | VFIO IRQ info          |
> ++--------------+------------------------+
> +
> +This message is sent by the client to the server to query for information about
> +device interrupt types. The VFIO IRQ info structure is defined in
> +``<sys/vfio.h>`` (``struct vfio_irq_info``).
> +
> +VFIO IRQ info format
> +^^^^^^^^^^^^^^^^^^^^
> +
> ++-------+--------+---------------------------+
> +| Name  | Offset | Size                      |
> ++=======+========+===========================+
> +| argsz | 16     | 4                         |
> ++-------+--------+---------------------------+
> +| flags | 20     | 4                         |
> ++-------+--------+---------------------------+
> +|       | +-----+--------------------------+ |
> +|       | | Bit | Definition               | |
> +|       | +=====+==========================+ |
> +|       | | 0   | VFIO_IRQ_INFO_EVENTFD    | |
> +|       | +-----+--------------------------+ |
> +|       | | 1   | VFIO_IRQ_INFO_MASKABLE   | |
> +|       | +-----+--------------------------+ |
> +|       | | 2   | VFIO_IRQ_INFO_AUTOMASKED | |
> +|       | +-----+--------------------------+ |
> +|       | | 3   | VFIO_IRQ_INFO_NORESIZE   | |
> +|       | +-----+--------------------------+ |
> ++-------+--------+---------------------------+
> +| index | 24     | 4                         |
> ++-------+--------+---------------------------+
> +| count | 28     | 4                         |
> ++-------+--------+---------------------------+
> +
> +* argz is reserved in vfio-user, it is only used in the ioctl() VFIO
> +  implementation.
> +* flags defines IRQ attributes:
> +
> +  * VFIO_IRQ_INFO_EVENTFD indicates the IRQ type can support server eventfd
> +    signalling.
> +  * VFIO_IRQ_INFO_MASKABLE indicates that the IRQ type supports the MASK and
> +    UNMASK actions in a VFIO_USER_DEVICE_SET_IRQS message.
> +  * VFIO_IRQ_INFO_AUTOMASKED indicates the IRQ type masks itself after being
> +    triggered, and the client must send an UNMASK action to receive new
> +    interrupts.
> +  * VFIO_IRQ_INFO_NORESIZE indicates VFIO_USER_SET_IRQS operations setup
> +    interrupts as a set, and new subindexes cannot be enabled without disabling
> +    the entire type.
> +
> +* index is the index of IRQ type being queried, it is the only field that is
> +  required to be set in the request message.
> +* count describes the number of interrupts of the queried type.
> +
> +VFIO_USER_DEVICE_SET_IRQS
> +-------------------------
> +
> +Message format
> +^^^^^^^^^^^^^^
> +
> ++--------------+------------------------+
> +| Name         | Value                  |
> +| Device ID    | <ID>                   |
> ++--------------+------------------------+
> +| Message ID   | <ID>                   |
> ++--------------+------------------------+
> +| Command      | 7                      |
> ++--------------+------------------------+
> +| Message size | 36 + any data          |
> ++--------------+------------------------+
> +| Flags        | Reply bit set in reply |
> ++--------------+------------------------+
> +| IRQ set      | VFIO IRQ set           |
> ++--------------+------------------------+
> +
> +This message is sent by the client to the server to set actions for device
> +interrupt types. The VFIO IRQ set structure is defined in ``<sys/vfio.h>``
> +(``struct vfio_irq_set``).
> +
> +VFIO IRQ info format
> +^^^^^^^^^^^^^^^^^^^^
> +
> ++-------+--------+------------------------------+
> +| Name  | Offset | Size                         |
> ++=======+========+==============================+
> +| argsz | 6      | 4                            |
> ++-------+--------+------------------------------+
> +| flags | 20     | 4                            |
> ++-------+--------+------------------------------+
> +|       | +-----+-----------------------------+ |
> +|       | | Bit | Definition                  | |
> +|       | +=====+=============================+ |
> +|       | | 0   | VFIO_IRQ_SET_DATA_NONE      | |
> +|       | +-----+-----------------------------+ |
> +|       | | 1   | VFIO_IRQ_SET_DATA_BOOL      | |
> +|       | +-----+-----------------------------+ |
> +|       | | 2   | VFIO_IRQ_SET_DATA_EVENTFD   | |
> +|       | +-----+-----------------------------+ |
> +|       | | 3   | VFIO_IRQ_SET_ACTION_MASK    | |
> +|       | +-----+-----------------------------+ |
> +|       | | 4   | VFIO_IRQ_SET_ACTION_UNMASK  | |
> +|       | +-----+-----------------------------+ |
> +|       | | 5   | VFIO_IRQ_SET_ACTION_TRIGGER | |
> +|       | +-----+-----------------------------+ |
> ++-------+--------+------------------------------+
> +| index | 24     | 4                            |
> ++-------+--------+------------------------------+
> +| start | 28     | 4                            |
> ++-------+--------+------------------------------+
> +| count | 32     | 4                            |
> ++-------+--------+------------------------------+
> +| data  | 36     | variable                     |
> ++-------+--------+------------------------------+
> +
> +* argz is reserved in vfio-user, it is only used in the ioctl() VFIO
> +  implementation.
> +* flags defines the action performed on the interrupt range. The DATA flags
> +  describe the data field sent in the message; the ACTION flags describe the
> +  action to be performed. The flags are mutually exclusive for both sets.
> +
> +  * VFIO_IRQ_SET_DATA_NONE indicates there is no data field in the request. The
> +    action is performed unconditionally.
> +  * VFIO_IRQ_SET_DATA_BOOL indicates the data field is an array of boolean
> +    bytes. The action is performed if the corresponding boolean is true.
> +  * VFIO_IRQ_SET_DATA_EVENTFD indicates an array of event file descriptors was
> +    sent in the message meta-data. These descriptors will be signalled when the
> +    action defined by the action flags occurs. In AF_UNIX sockets, the
> +    descriptors are sent as SCM_RIGHTS type ancillary data.
> +  * VFIO_IRQ_SET_ACTION_MASK indicates a masking event. It can be used with
> +    VFIO_IRQ_SET_DATA_BOOL or VFIO_IRQ_SET_DATA_NONE to mask an interrupt, or
> +    with VFIO_IRQ_SET_DATA_EVENTFD to generate an event when the guest masks
> +    the interrupt.
> +  * VFIO_IRQ_SET_ACTION_UNMASK indicates an unmasking event. It can be used
> +    with VFIO_IRQ_SET_DATA_BOOL or VFIO_IRQ_SET_DATA_NONE to unmask an
> +    interrupt, or with VFIO_IRQ_SET_DATA_EVENTFD to generate an event when the
> +    guest unmasks the interrupt.
> +  * VFIO_IRQ_SET_ACTION_TRIGGER indicates a triggering event. It can be used
> +    with VFIO_IRQ_SET_DATA_BOOL or VFIO_IRQ_SET_DATA_NONE to trigger an
> +    interrupt, or with VFIO_IRQ_SET_DATA_EVENTFD to generate an event when the
> +    guest triggers the interrupt.
> +
> +* index is the index of IRQ type being setup.
> +* start is the start of the subindex being set.
> +* count describes the number of sub-indexes being set. As a special case, a
> +  count of 0 with data flags of VFIO_IRQ_SET_DATA_NONE disables all interrupts
> +  of the index data is an optional field included when the
> +  VFIO_IRQ_SET_DATA_BOOL flag is present. It contains an array of booleans
> +  that specify whether the action is to be performed on the corresponding
> +  index. It's used when the action is only performed on a subset of the range
> +  specified.
> +
> +Not all interrupt types support every combination of data and action flags.
> +The client must know the capabilities of the device and IRQ index before it
> +sends a VFIO_USER_DEVICE_SET_IRQ message.
> +
> +Read and Write Operations
> +-------------------------
> +
> +Not all I/O operations between the client and server can be done via direct
> +access of memory mapped with an mmap() call. In these cases, the client and
> +server use messages sent over the socket. It is expected that these operations
> +will have lower performance than direct access.
> +
> +The client can access device memory with VFIO_USER_REGION_READ and
> +VFIO_USER_REGION_WRITE requests. These share a common data structure that
> +appears after the 16 byte message header.
> +
> +REGION Read/Write Data
> +^^^^^^^^^^^^^^^^^^^^^^
> +
> ++--------+--------+----------+
> +| Name   | Offset | Size     |
> ++========+========+==========+
> +| Offset | 16     | 8        |
> ++--------+--------+----------+
> +| Region | 24     | 4        |
> ++--------+--------+----------+
> +| Count  | 28     | 4        |
> ++--------+--------+----------+
> +| Data   | 32     | variable |
> ++--------+--------+----------+
> +
> +* Offset into the region being accessed.
> +* Region is the index of the region being accessed.
> +* Count is the size of the data to be transferred.
> +* Data is the data to be read or written.
> +
> +The server can access guest memory with VFIO_USER_DMA_READ and
> +VFIO_USER_DMA_WRITE messages. These also share a common data structure that
> +appears after the 16 byte message header.
> +
> +DMA Read/Write Data
> +^^^^^^^^^^^^^^^^^^^
> +
> ++---------+--------+----------+
> +| Name    | Offset | Size     |
> ++=========+========+==========+
> +| Address | 16     | 8        |
> ++---------+--------+----------+
> +| Count   | 24     | 4        |
> ++---------+--------+----------+
> +| Data    | 28     | variable |
> ++---------+--------+----------+
> +
> +* Address is the area of guest memory being accessed. This address must have
> +  been exported to the server with a VFIO_USER_DMA_MAP message.
> +* Count is the size of the data to be transferred.
> +* Data is the data to be read or written.
> +
> +Address and count can also be accessed as ``struct iovec`` from 
> ``<sys/uio.h>``.
> +
> +VFIO_USER_REGION_READ
> +---------------------
> +
> +Message format
> +^^^^^^^^^^^^^^
> +
> ++--------------+------------------------+
> +| Name         | Value                  |
> ++==============+========================+
> +| Device ID    | <ID>                   |
> ++--------------+------------------------+
> +| Message ID   | <ID>                   |
> ++--------------+------------------------+
> +| Command      | 8                      |
> ++--------------+------------------------+
> +| Message size | 32 + data size         |
> ++--------------+------------------------+
> +| Flags Reply  | bit set in reply       |
> ++--------------+------------------------+
> +| Read info    | REGION read/write data |
> ++--------------+------------------------+
> +
> +This request is sent from the client to the server to read from device memory.
> +In the request messages, there will be no data, and the count field will be the
> +amount of data to be read. The reply will include the data read, and its count
> +field will be the amount of data read.
> +
> +VFIO_USER_REGION_WRITE
> +----------------------
> +
> +Message format
> +^^^^^^^^^^^^^^
> +
> ++--------------+------------------------+
> +| Name         | Value                  |
> ++==============+========================+
> +| Device ID    | <ID>                   |
> ++--------------+------------------------+
> +| Message ID   | <ID>                   |
> ++--------------+------------------------+
> +| Command      | 9                      |
> ++--------------+------------------------+
> +| Message size | 32 + data size         |
> ++--------------+------------------------+
> +| Flags        | Reply bit set in reply |
> ++--------------+------------------------+
> +| Write info   | REGION read write data |
> ++--------------+------------------------+
> +
> +This request is sent from the client to the server to write to device memory.
> +The request message will contain the data to be written, and its count field
> +will contain the amount of write data. The count field in the reply will be
> +zero.
> +
> +VFIO_USER_DMA_READ
> +------------------
> +
> +Message format
> +^^^^^^^^^^^^^^
> +
> ++--------------+---------------------+
> +| Name         | Value               |
> ++==============+=====================+
> +| Device ID    | <ID>                |
> ++--------------+---------------------+
> +| Message ID   | <ID>                |
> ++--------------+---------------------+
> +| Command      | 10                  |
> ++--------------+---------------------+
> +| Message size | 28 + data size      |
> ++--------------+---------------------+
> +| Flags Reply  | bit set in reply    |
> ++--------------+---------------------+
> +| DMA info     | DMA read/write data |
> ++--------------+---------------------+
> +
> +This request is sent from the server to the client to read from guest memory.
> +In the request messages, there will be no data, and the count field will be the
> +amount of data to be read. The reply will include the data read, and its count
> +field will be the amount of data read.
> +
> +VFIO_USER_DMA_WRITE
> +-------------------
> +
> +Message format
> +^^^^^^^^^^^^^^
> +
> ++--------------+------------------------+
> +| Name         | Value                  |
> ++==============+========================+
> +| Device ID    | <ID>                   |
> ++--------------+------------------------+
> +| Message ID   | <ID>                   |
> ++--------------+------------------------+
> +| Command      | 11                     |
> ++--------------+------------------------+
> +| Message size | 28 + data size         |
> ++--------------+------------------------+
> +| Flags        | Reply bit set in reply |
> ++--------------+------------------------+
> +| DMA info     | DMA read/write data    |
> ++--------------+------------------------+
> +
> +This request is sent from the server to the client to write to guest memory.
> +The request message will contain the data to be written, and its count field
> +will contain the amount of write data. The count field in the reply will be
> +zero.
> +
> +VFIO_USER_VM_INTERRUPT
> +----------------------
> +
> +Message format
> +^^^^^^^^^^^^^^
> +
> ++----------------+------------------------+
> +| Name           | Value                  |
> ++================+========================+
> +| Device ID      | <ID>                   |
> ++----------------+------------------------+
> +| Message ID     | <ID>                   |
> ++----------------+------------------------+
> +| Command        | 12                     |
> ++----------------+------------------------+
> +| Message size   | 24                     |
> ++----------------+------------------------+
> +| Flags          | Reply bit set in reply |
> ++----------------+------------------------+
> +| Interrupt info | <interrupt>            |
> ++----------------+------------------------+
> +
> +This request is sent from the server to the client to signal the device has
> +raised an interrupt.
> +
> +Interrupt info format
> +^^^^^^^^^^^^^^^^^^^^^
> +
> ++----------+--------+------+
> +| Name     | Offset | Size |
> ++==========+========+======+
> +| Index    | 16     | 4    |
> ++----------+--------+------+
> +| Subindex | 20     | 4    |
> ++----------+--------+------+
> +
> +* Index is the interrupt index; it is the same value used in 
> VFIO_USER_SET_IRQS.
> +* Subindex is relative to the index, e.g., the vector number used in PCI MSI/X
> +  type interrupts.
> +
> +VFIO_USER_DEVICE_RESET
> +----------------------
> +
> +Message format
> +^^^^^^^^^^^^^^
> +
> ++--------------+------------------------+
> +| Name         | Value                  |
> ++==============+========================+
> +| Device ID    | <ID>                   |
> ++--------------+------------------------+
> +| Message ID   | <ID>                   |
> ++--------------+------------------------+
> +| Command      | 13                     |
> ++--------------+------------------------+
> +| Message size | 16                     |
> ++--------------+------------------------+
> +| Flags        | Reply bit set in reply |
> ++--------------+------------------------+
> +
> +This request is sent from the client to the server to reset the device.
> +
> +Appendices
> +==========
> +
> +Unused VFIO ioctl() commands
> +----------------------------
> +
> +The following commands must be handled by the client and not sent to the 
> server:
> +
> +* VFIO_GET_API_VERSION
> +* VFIO_CHECK_EXTENSION
> +* VFIO_SET_IOMMU
> +* VFIO_GROUP_GET_STATUS
> +* VFIO_GROUP_SET_CONTAINER
> +* VFIO_GROUP_UNSET_CONTAINER
> +* VFIO_GROUP_GET_DEVICE_FD
> +* VFIO_IOMMU_GET_INFO
> +
> +However, once support for live migration for VFIO devices is finalized some
> +of the above commands might have to be handled by the client. This will be
> +addressed in a future protocol version.
> +
> +Live Migration
> +--------------
> +Currently live migration is not supported for devices passed through via VFIO,
> +therefore it is not supported for VFIO-over-socket, either. This is being
> +actively worked on in the "Add migration support for VFIO devices" (v25) patch
> +series.
> +
> +VFIO groups and containers
> +^^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +The current VFIO implementation includes group and container idioms that
> +describe how a device relates to the host IOMMU. In the VFIO over socket
> +implementation, the IOMMU is implemented in SW by the client, and isn't visible
> +to the server. The simplest idea is for the client is to put each device into


s/is for/for

> +its own group and container.