diff mbox series

[v6] introduce vfio-user protocol specification

Message ID 20201109180001.177665-1-thanos.makatos@nutanix.com
State New
Headers show
Series [v6] introduce vfio-user protocol specification | expand

Commit Message

Thanos Makatos Nov. 9, 2020, 6 p.m. UTC
This patch introduces the vfio-user protocol specification (formerly
known as VFIO-over-socket), which is designed to allow devices to be
emulated outside QEMU, in a separate process. vfio-user 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
oading"

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

Changed since v3:
  * rename protocol to vfio-user
  * add table of contents
  * fix Unicode problems
  * fix typos and various reStructuredText issues
  * various stylistic improvements
  * add backend program conventions
  * rewrite part of intro, drop QEMU-specific stuff
  * drop QEMU-specific paragraph about implementation
  * explain that passing of FDs isn't necessary
  * minor improvements in the VFIO section
  * various text substitutions for the sake of consistency
  * drop paragraph about client and server, already explained in
  * intro
  * drop device ID
  * drop type from version
  * elaborate on request concurrency
  * convert some inessential paragraphs into notes
  * explain why some existing VFIO defines cannot be reused
  * explain how to make changes to the protocol
  * improve text of DMA map
  * reword comment about existing VFIO commands
  * add reference to Version section
  * reset device on disconnection
  * reword live migration section
  * replace sys/vfio.h with linux/vfio.h
  * drop reference to iovec
  * use argz the same way it is used in VFIO
  * add type field in header for clarity

Changed since v4:
  * introduce support for live migration as defined in
  * include/uapi/linux/vfio.h
  * introduce 'max_fds' and 'migration' capabilities:
  * remove 'index' from VFIO_USER_DEVICE_GET_IRQ_INFO
  * fix minor typos and reworded some text for clarity

Changed since v5:
  * fix minor typos
  * separate VFIO_USER_DMA_MAP and VFIO_USER_DMA_UNMAP
  * clarify meaning of VFIO bitmap size field
  * move version major/minor outside JSON
  * client proposes version first
  * make Errno optional in message header
  * clarification about message ID uniqueness
  * clarify that server->client request can appear in between
    client->server request/reply

You can focus on v5 to v6 changes by cloning my fork
(https://github.com/tmakatos/qemu) and doing:

        git diff refs/tags/vfio-user/v5 refs/heads/vfio-user/v6

There's still an outstanding question to asnwer regarding why it
DMA_*MAP instead of *MAP_DMA, however I believe it can wait until the
next revisition.
---
 MAINTAINERS              |    6 +
 docs/devel/index.rst     |    1 +
 docs/devel/vfio-user.rst | 1641 ++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 1648 insertions(+)
 create mode 100644 docs/devel/vfio-user.rst

Comments

Thanos Makatos Nov. 13, 2020, 9:22 a.m. UTC | #1
> +Version Data Format
> +^^^^^^^^^^^^^^^^^^^
> +
> +The version data is an optional JSON byte array with the following format:
> +
> ++--------------+------------------+-------------------------------------------+
> +| Name         | Type             | Description                               |
> ++==============+==================+======================
> =====================+
> +| capabilities | collection of    | Contains common capabilities that the     |
> +|              | name/value pairs | sender supports. Optional.                |
> ++--------------+------------------+-------------------------------------------+
> +
> +Capabilities:
> +
> ++---------------+------------------+------------------------------------------+
> +| Name          | Type             | Description                              |
> ++===============+==================+=====================
> =====================+
> +| ``max_fds``   | number           | Maximum number of file descriptors that  |
> +|               |                  | can be received by the sender. Optional. |
> +|               |                  | If not specified then the receiver must  |
> +|               |                  | assume ``max_fds=1``.                    |
> ++---------------+------------------+------------------------------------------+
> +| ``migration`` | collection of    | Migration capability parameters. If      |
> +|               | name/value pairs | missing then migration is not supported  |
> +|               |                  | by the sender.                           |
> ++---------------+------------------+------------------------------------------+

Can we introduce the following parameters that can help simplify the
server implementation?

* max_dma_regions: maximum number of DMA regions per VFIO_USER_DMA_MAP/UNMAP
                   message
				   
* max_dirty_bitmaps: maximum number of dirty bitmaps requested per
                     VFIO_USER_DIRTY_PAGES message
					 
If these parameters are missing then they can be unlimited. The vfio-user client
will have to break a single operation into multiple messages.


> +VFIO_USER_DEVICE_GET_REGION_INFO
> +--------------------------------
> +
> +Message format
> +^^^^^^^^^^^^^^
> +
> ++--------------+------------------------+
> +| Name         | Value                  |
> ++==============+========================+
> +| Message ID   | <ID>                   |
> ++--------------+------------------------+
> +| Command      | 5                      |
> ++--------------+------------------------+
> +| Message size | 48 + any caps          |
> ++--------------+------------------------+
> +| Flags        | Reply bit set in reply |
> ++--------------+------------------------+
> +| Error        | 0/errno                |
> ++--------------+------------------------+
> +| Region info  | VFIO region info       |
> ++--------------+------------------------+
> +
> +This command 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 ``<linux/vfio.h>`` (``struct vfio_region_info``). Since the client
> +does not know the size of the capabilities, the size of the reply it should
> +expect is 48 plus any capabilities whose size is indicated in the size field of
> +the reply header.
> +
> +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                            |
> ++------------+--------+------------------------------+
> +
> +* *argsz* is the size of the VFIO region info structure plus the
> +  size of any region capabilities returned.
> +* *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 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 command 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
> +  ``<linux/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.

In the command message we only use vfio_region_info.index. VFIO passes the
entire struct since it gets filled in, however this doesn't make sense in
vfio-user. Shall we drop sending the entire struct vfio_region_info and only
send the region index?
diff mbox series

Patch

diff --git a/MAINTAINERS b/MAINTAINERS
index 7e442b5247..3611f9e365 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1754,6 +1754,12 @@  F: hw/vfio/ap.c
 F: docs/system/s390x/vfio-ap.rst
 L: qemu-s390x@nongnu.org
 
+vfio-user
+M: John G Johnson <john.g.johnson@oracle.com>
+M: Thanos Makatos <thanos.makatos@nutanix.com>
+S: Supported
+F: docs/devel/vfio-user.rst
+
 vhost
 M: Michael S. Tsirkin <mst@redhat.com>
 S: Supported
diff --git a/docs/devel/index.rst b/docs/devel/index.rst
index 77baae5c77..7c7740a096 100644
--- a/docs/devel/index.rst
+++ b/docs/devel/index.rst
@@ -34,3 +34,4 @@  Contents:
    clocks
    qom
    block-coroutine-wrapper
+   vfio-user
diff --git a/docs/devel/vfio-user.rst b/docs/devel/vfio-user.rst
new file mode 100644
index 0000000000..4f4df6ebb5
--- /dev/null
+++ b/docs/devel/vfio-user.rst
@@ -0,0 +1,1641 @@ 
+.. include:: <isonum.txt>
+
+********************************
+vfio-user Protocol Specification
+********************************
+
+------------
+Version_ 0.1
+------------
+
+.. contents:: Table of Contents
+
+Introduction
+============
+vfio-user is a protocol that allows a device to be emulated in a separate
+process outside of a Virtual Machine Monitor (VMM). vfio-user devices consist
+of a generic VFIO device type, living inside the VMM, which we call the client,
+and the core device implementation, living outside the VMM, which we call the
+server.
+
+The `Linux VFIO ioctl interface <https://www.kernel.org/doc/html/latest/driver-api/vfio.html>`_
+been chosen as the base for this protocol for the following reasons:
+
+1) It is a mature and stable API, backed by an extensively used framework.
+2) The existing VFIO client implementation in QEMU (qemu/hw/vfio/) can be
+   largely reused.
+
+.. Note::
+   In a proof of concept implementation it has been demonstrated that using VFIO
+   over a UNIX domain socket is a viable option. vfio-user is designed with
+   QEMU in mind, however it could be used by other client applications. The
+   vfio-user 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 file descriptors can be trivially sent over it, which in turn
+allows:
+
+* Sharing of client memory for DMA with the server.
+* Sharing of server memory with the client for fast MMIO.
+* Efficient sharing of eventfd's for triggering interrupts.
+
+Other socket types could be used which allow the server to run in a separate
+guest in the same host (AF_VSOCK) or remotely (AF_INET). Theoretically the
+underlying transport does not necessarily have to be a socket, however we do
+not examine such alternatives. In this protocol version we focus on using a
+UNIX domain socket and introduce basic support for the other two types of
+sockets without considering performance implications.
+
+While passing of file descriptors is desirable for performance reasons, it is
+not necessary neither for the client nor for the server to support it in order
+to implement the protocol. There is always an in-band, message-passing fall
+back mechanism.
+
+VFIO
+====
+VFIO is a framework that allows a physical device to be securely passed through
+to a user space process; the device-specific kernel driver does not drive the
+device at all.  Typically, the user space process is a VMM 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 to directly access physical devices, instead of emulating them in
+software.
+
+vfio-user reuses the core VFIO concepts defined in its API, but implements them
+as messages to be sent over a socket. It does not change the kernel-based VFIO
+in any way, in fact none of the VFIO kernel modules need to be loaded to use
+vfio-user. It is also possible for the client to concurrently use the current
+kernel-based VFIO for one device, and vfio-user for another device.
+
+VFIO Device Model
+-----------------
+A device under VFIO presents a standard interface to the user process. Many of
+the VFIO operations in the existing interface use the ioctl() system call, and
+references to the existing interface are called the ioctl() implementation in
+this document.
+
+The following sections describe the set of messages that implement the VFIO
+interface over a 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 of this reuse is to share as much code as feasible with the ioctl()
+implementation.
+
+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 supports, or closes the connection if it cannot support the
+advertised version.
+
+DMA Memory Configuration
+^^^^^^^^^^^^^^^^^^^^^^^^
+The client uses VFIO_USER_DMA_MAP and VFIO_USER_DMA_UNMAP messages to inform
+the server of the valid DMA ranges that the server can access on behalf
+of a device. DMA 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 client memory is for the client to provide
+file descriptors the server can mmap() to directly access client 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 whether it supports reset (``VFIO_DEVICE_FLAGS_``),
+* the number of device regions, and
+* the device presents to the client 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 (``VFIO_REGION_INFO_CAP_``).
+* 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.
+
+.. Note::
+   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 event), while allowing direct access to the doorbells
+   (an extremely frequent event as every I/O submission requires a write to
+   BAR0), found right after the NVMe registers in BAR0.
+
+Device-Specific Regions
+"""""""""""""""""""""""
+
+A device can define regions additional to the standard ones (e.g. PCI indexes
+0-8). This is achieved by including a VFIO_REGION_INFO_CAP_TYPE capability
+in the region info reply of a device-specific region. Such regions are reflected
+in ``struct vfio_device_info.num_regions``. Thus, for PCI devices this value can
+be equal to, or higher than, VFIO_PCI_NUM_REGIONS.
+
+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-user 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.
+
+Socket
+------
+
+A server can serve:
+
+1) one or more clients, and/or
+2) one or more virtual devices, belonging to one or more clients.
+
+The current protocol specification requires a dedicated socket per
+client/server connection. It is a server-side implementation detail whether a
+single server handles multiple virtual devices from the same or multiple
+clients. The location of the socket is implementation-specific. Multiplexing
+clients, devices, and servers over the same socket is not supported in this
+version of the protocol.
+
+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.
+
+Command Concurrency
+-------------------
+A client may pipeline multiple commands without waiting for previous command
+replies.  The server will process commands in the order they are received.  A
+consequence of this is if a client issues a command with the *No_reply* bit,
+then subseqently issues a command without *No_reply*, the older command will
+have been processed before the reply to the younger command is sent by the
+server.  The client must be aware of the device's capability to process
+concurrent commands if pipelining is used.  For example, pipelining allows
+multiple client threads to concurently access device memory; the client must
+ensure these acceses obey device semantics.
+
+An example is a frame buffer device, where the device may allow concurrent
+access to different areas of video memory, but may have indeterminate behavior
+if concurrent acceses are performed to command or status registers.
+
+Note that unrelated messages sent from the sevrer to the client can appear in
+between a client to server request/reply and vice versa.
+
+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).
+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
+reset the VFIO device when it detects the socket has been disconnected.
+Error recovery will be driven by the guest's device error handling
+behavior.
+
+Client Disconnection
+^^^^^^^^^^^^^^^^^^^^
+The client disconnecting from the server primarily means that the client
+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 disconnection should not result in a server exit:
+
+1) A single server serving multiple clients.
+2) A multi-process QEMU upgrading itself step by step, which is not 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
+the control stack will know about it and can clean up resources
+accordingly.
+
+Request Retry and Response Timeout
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+A failed command is a command that has been successfully sent and has been
+responded to with an error code. Failure to send the command in the first place
+(e.g. because the socket is disconnected) is a different type of error examined
+earlier in the disconnect section.
+
+.. Note::
+   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.
+
+Defining a retry and timeout scheme is deferred to a future version of the
+protocol.
+
+.. _Commands:
+
+Commands
+--------
+The following table lists the VFIO message command IDs, and whether the
+message command 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_WRITE              | 11      | server -> client  |
++----------------------------------+---------+-------------------+
+| VFIO_USER_VM_INTERRUPT           | 12      | server -> client  |
++----------------------------------+---------+-------------------+
+| VFIO_USER_DEVICE_RESET           | 13      | client -> server  |
++----------------------------------+---------+-------------------+
+| VFIO_USER_DIRTY_PAGES            | 14      | client -> server  |
++----------------------------------+---------+-------------------+
+
+
+.. Note:: Some VFIO defines cannot be reused since their values are
+   architecture-specific (e.g. VFIO_IOMMU_MAP_DMA).
+
+Header
+------
+All messages, both command messages and reply messages, are preceded by a
+header that contains basic information about the message. The header is
+followed by message-specific data described in the sections below.
+
++----------------+--------+-------------+
+| Name           | Offset | Size        |
++================+========+=============+
+| Message ID     | 0      | 2           |
++----------------+--------+-------------+
+| Command        | 2      | 2           |
++----------------+--------+-------------+
+| Message size   | 4      | 4           |
++----------------+--------+-------------+
+| Flags          | 8      | 4           |
++----------------+--------+-------------+
+|                | +-----+------------+ |
+|                | | Bit | Definition | |
+|                | +=====+============+ |
+|                | | 0-3 | Type       | |
+|                | +-----+------------+ |
+|                | | 4   | No_reply   | |
+|                | +-----+------------+ |
+|                | | 5   | Error      | |
+|                | +-----+------------+ |
++----------------+--------+-------------+
+| Error          | 12     | 4           |
++----------------+--------+-------------+
+| <message data> | 16     | variable    |
++----------------+--------+-------------+
+
+* *Message ID* identifies the message, and is echoed in the command's reply
+  message. Message IDs belong entirely to the sender, can be re-used (even
+  concurrently) and the receiver must not make any assumptions about their
+  uniqueness.
+* *Command* specifies the command to be executed, listed in Commands_.
+* *Message size* contains the size of the entire message, including the header.
+* *Flags* contains attributes of the message:
+
+  * The *Type* bits indicate the message type.
+
+    *  *Command* (value 0x0) indicates a command message.
+    *  *Reply* (value 0x1) indicates a reply message acknowledging a previous
+       command with the same message ID.
+  * *No_reply* in a command message indicates that no reply is needed for this command.
+    This is commonly used when multiple commands are sent, and only the last needs
+    acknowledgement.
+  * *Error* in a reply message indicates the command being acknowledged had
+    an error. In this case, the *Error* field will be valid.
+
+* *Error* in a reply message is an optional UNIX errno value. It may be zero
+  even if the Error bit is set in Flags. It is reserved in a command message.
+
+Each command message in Commands_ must be replied to with a reply message, unless the
+message sets the *No_Reply* bit.  The reply consists of the header with the *Reply*
+bit set, plus any additional data.
+
+VFIO_USER_VERSION
+-----------------
+
+This is the initial message sent by the client after the socket connection is
+established:
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+-------------------------------------------+
+| Name         | Value                                     |
++==============+===========================================+
+| Message ID   | <ID>                                      |
++--------------+-------------------------------------------+
+| Command      | 1                                         |
++--------------+-------------------------------------------+
+| Message size | 16 + version header + version data length |
++--------------+-------------------------------------------+
+| Flags        | Reply bit set in reply                    |
++--------------+-------------------------------------------+
+| Error        | 0/errno                                   |
++--------------+-------------------------------------------+
+| Version      | version header                            |
++--------------+-------------------------------------------+
+
+Version Header Format
+^^^^^^^^^^^^^^^^^^^^^
+
++---------------+--------+------------------------------------------------+
+| Name          | Offset | Size                                           |
++===============+========+================================================+
+| version major | 16     | 2                                              |
++---------------+--------+------------------------------------------------+
+| version minor | 18     | 2                                              |
++---------------+--------+------------------------------------------------+
+| version data  | 22     | variable (including terminating NUL            |
+|               |        | character). Optional.                          |
++---------------+--------+------------------------------------------------+
+
+Version Data Format
+^^^^^^^^^^^^^^^^^^^
+
+The version data is an optional JSON byte array with the following format:
+
++--------------+------------------+-------------------------------------------+
+| Name         | Type             | Description                               |
++==============+==================+===========================================+
+| capabilities | collection of    | Contains common capabilities that the     |
+|              | name/value pairs | sender supports. Optional.                |
++--------------+------------------+-------------------------------------------+
+
+Capabilities:
+
++---------------+------------------+------------------------------------------+
+| Name          | Type             | Description                              |
++===============+==================+==========================================+
+| ``max_fds``   | number           | Maximum number of file descriptors that  |
+|               |                  | can be received by the sender. Optional. |
+|               |                  | If not specified then the receiver must  |
+|               |                  | assume ``max_fds=1``.                    |
++---------------+------------------+------------------------------------------+
+| ``migration`` | collection of    | Migration capability parameters. If      |
+|               | name/value pairs | missing then migration is not supported  |
+|               |                  | by the sender.                           |
++---------------+------------------+------------------------------------------+
+
+The migration capability contains the following name/value pairs:
+
++------------+--------+-------------------------------------------------------+
+| Name       | Type   | Description                                           |
++============+========+=======================================================+
+| ``pgsize`` | number | Page size of dirty pages bitmap. The smallest between |
+|            |        | the client and the server is used.                    |
++------------+--------+-------------------------------------------------------+
+
+
+.. _Version:
+
+Versioning and Feature Support
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Upon accepting a connection, the client must send a VFIO_USER_VERSION message
+proposing a protocol version and a set of capabilities. The server 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 server
+  requires a capability the client did not include, it 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.
+
+When making a change to this specification, the protocol version number must
+be included in the form "added in version X.Y"
+
+
+VFIO_USER_DMA_MAP
+-----------------
+
+Message Format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 2                      |
++--------------+------------------------+
+| Message size | 16 + table size        |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| Table        | array of table entries |
++--------------+------------------------+
+
+This command message is sent by the client to the server to inform it of the
+memory regions the server can access. It must be sent before the server can
+perform any DMA to the client. It is normally sent directly after the version
+handshake is completed, but may also occur when memory is added to the client,
+or if the client uses a vIOMMU. If the client does not expect the server to
+perform DMA then it does not need to send to the server VFIO_USER_DMA_MAP
+commands. If the server does not need to perform DMA then it can ignore such
+commands but it must still reply to them. The table is an array of the
+following structure:
+
+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* contains the following region attributes:
+
+  * *Mappable* indicates that the region can be mapped via the mmap() system
+    call using the file descriptor provided in the message meta-data.
+
+This structure is 32 bytes in size, so the message size is:
+16 + (# of table entries * 32).
+
+If a DMA region being added can be directly mapped by the server, an array of
+file descriptors must be sent as part of the message meta-data. Each mappable
+region entry must have a corresponding file descriptor. On AF_UNIX sockets, the
+file descriptors must be passed as SCM_RIGHTS type ancillary data. Otherwise,
+if a DMA region cannot be directly mapped by the server, it can be accessed by
+the server using VFIO_USER_DMA_READ and VFIO_USER_DMA_WRITE messages, explained
+in `Read and Write Operations`_. A command to map over an existing region must
+be failed by the server with ``EEXIST`` set in error field in the reply.
+
+
+VFIO_USER_DMA_UNMAP
+-------------------
+
+Message Format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 3                      |
++--------------+------------------------+
+| Message size | 16 + table size        |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| Table        | array of table entries |
++--------------+------------------------+
+
+This command message is sent by the client to the server to inform it that a
+DMA region, previously made available via a VFIO_USER_DMA_MAP command message,
+is no longer available for DMA. It typically occurs when memory is subtracted
+from the client or if the client uses a vIOMMU. If the client does not expect
+the server to perform DMA then it does not need to send to the server
+VFIO_USER_DMA_UNMAP commands. If the server does not need to perform DMA then
+it can ignore such commands but it must still reply to them. The table is an
+array of the following structure:
+
+Table entry format
+^^^^^^^^^^^^^^^^^^
+
++--------------+--------+---------------------------------------+
+| Name         | Offset | Size                                  |
++==============+========+=======================================+
+| Address      | 0      | 8                                     |
++--------------+--------+---------------------------------------+
+| Size         | 8      | 8                                     |
++--------------+--------+---------------------------------------+
+| Offset       | 16     | 8                                     |
++--------------+--------+---------------------------------------+
+| Protections  | 24     | 4                                     |
++--------------+--------+---------------------------------------+
+| Flags        | 28     | 4                                     |
++--------------+--------+---------------------------------------+
+|              | +-----+--------------------------------------+ |
+|              | | Bit | Definition                           | |
+|              | +=====+======================================+ |
+|              | | 0   | VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP | |
+|              | +-----+--------------------------------------+ |
++--------------+--------+---------------------------------------+
+| VFIO Bitmaps | 32     | variable                              |
++--------------+--------+---------------------------------------+
+
+* *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* contains the following region attributes:
+
+  * *VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP* indicates that a dirty page bitmap
+    must be populated before unmapping the DMA region. The client must provide
+    a ``struct vfio_bitmap`` in the VFIO bitmaps field for each region, with
+    the ``vfio_bitmap.pgsize`` and ``vfio_bitmap.size`` fields initialized.
+
+* *VFIO Bitmaps* contains one ``struct vfio_bitmap`` per region if
+  ``VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP`` is set in Flags.
+
+VFIO bitmap format
+^^^^^^^^^^^^^^^^^^
+
++--------+--------+------+
+| Name   | Offset | Size |
++========+========+======+
+| pgsize | 0      | 8    |
++--------+--------+------+
+| size   | 8      | 8    |
++--------+--------+------+
+| data   | 16     | 8    |
++--------+--------+------+
+
+* *pgsize* is the page size for the bitmap, in bytes.
+* *size* is the size for the bitmap, in bytes, excluding the VFIO bitmap header.
+* *data* This field is unused in vfio-user.
+
+The VFIO bitmap structure is defined in ``<linux/vfio.h>``
+(``struct vfio_bitmap``).
+
+If ``VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP`` is not set in Flags then the size
+of the message is: 16 + (# of table entries * 32).
+If ``VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP`` is set in Flags then the size of
+the message is: 16 + (# of table entries * 56).
+
+Upon receiving a VFIO_USER_DMA_UNMAP command, if the file descriptor is mapped
+then the server must release all references to that DMA region before replying,
+which includes potentially in flight DMA transactions. Removing a portion of a
+DMA region is possible. If the VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP bit is set
+in the request, the server must append to the header the ``struct vfio_bitmap``
+received in the command followed by the bitmap, for each region. Thus, the
+message size the client should expect is the size of the header plus the size
+of ``struct vfio_bitmap`` plus ``vfio_bitmap.size`` bytes for each region. Each
+bit in the bitmap represents one page of size ``vfio_bitmap.pgsize``.
+
+
+VFIO_USER_DEVICE_GET_INFO
+-------------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+----------------------------+
+| Name         | Value                      |
++==============+============================+
+| Message ID   | <ID>                       |
++--------------+----------------------------+
+| Command      | 4                          |
++--------------+----------------------------+
+| Message size | 16 in command, 32 in reply |
++--------------+----------------------------+
+| Flags        | Reply bit set in reply     |
++--------------+----------------------------+
+| Error        | 0/errno                    |
++--------------+----------------------------+
+| Device info  | VFIO device info           |
++--------------+----------------------------+
+
+This command 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 command
+message.  The VFIO device info structure is defined in ``<linux/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                        |
++-------------+--------+--------------------------+
+
+* *argsz* is the size of the VFIO device info structure.
+* *flags* contains the following device attributes.
+
+  * VFIO_DEVICE_FLAGS_RESET indicates that the device supports the
+    VFIO_USER_DEVICE_RESET message.
+  * VFIO_DEVICE_FLAGS_PCI indicates that the device is a PCI device.
+
+* *num_regions* is the number of memory regions that the device exposes.
+* *num_irqs* is the number of distinct interrupt types that 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                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 5                      |
++--------------+------------------------+
+| Message size | 48 + any caps          |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| Region info  | VFIO region info       |
++--------------+------------------------+
+
+This command 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 ``<linux/vfio.h>`` (``struct vfio_region_info``). Since the client
+does not know the size of the capabilities, the size of the reply it should
+expect is 48 plus any capabilities whose size is indicated in the size field of
+the reply header.
+
+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                            |
++------------+--------+------------------------------+
+
+* *argsz* is the size of the VFIO region info structure plus the
+  size of any region capabilities returned.
+* *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 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 command 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
+  ``<linux/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 ``<linux/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     |
++------------------+----------------------------------+
+
+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
+``<linux/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 ``<linux/vfio.h>`` (``struct
+vfio_region_info_cap_sparse_mmap``).
+
+VFIO Region Type
+^^^^^^^^^^^^^^^^
+
++------------------+---------------------------+
+| Name             | Value                     |
++==================+===========================+
+| id               | VFIO_REGION_INFO_CAP_TYPE |
++------------------+---------------------------+
+| version          | 0x1                       |
++------------------+---------------------------+
+| next             | <next>                    |
++------------------+---------------------------+
+| region info type | VFIO region info type     |
++------------------+---------------------------+
+
+This capability is defined when a region is specific to the device.
+
+VFIO region info type
+^^^^^^^^^^^^^^^^^^^^^
+
+The VFIO region info type is defined in ``<linux/vfio.h>``
+(``struct vfio_region_info_cap_type``).
+
++---------+--------+------+
+| Name    | Offset | Size |
++=========+========+======+
+| type    | 0      | 4    |
++---------+--------+------+
+| subtype | 4      | 4    |
++---------+--------+------+
+
+The only device-specific region type and subtype supported by vfio-user is
+VFIO_REGION_TYPE_MIGRATION (3) and VFIO_REGION_SUBTYPE_MIGRATION (1).
+
+VFIO Device Migration Info
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The beginning of the subregion must contain
+``struct vfio_device_migration_info``, defined in ``<linux/vfio.h>``. This
+subregion is accessed like any other part of a standard vfio-user PCI region
+using VFIO_USER_REGION_READ/VFIO_USER_REGION_WRITE.
+
++---------------+--------+-----------------------------+
+| Name          | Offset | Size                        |
++===============+========+=============================+
+| device_state  | 0      | 4                           |
++---------------+--------+-----------------------------+
+|               | +-----+----------------------------+ |
+|               | | Bit | Definition                 | |
+|               | +=====+============================+ |
+|               | | 0   | VFIO_DEVICE_STATE_RUNNING  | |
+|               | +-----+----------------------------+ |
+|               | | 1   | VFIO_DEVICE_STATE_SAVING   | |
+|               | +-----+----------------------------+ |
+|               | | 2   | VFIO_DEVICE_STATE_RESUMING | |
+|               | +-----+----------------------------+ |
++---------------+--------+-----------------------------+
+| reserved      | 4      | 4                           |
++---------------+--------+-----------------------------+
+| pending_bytes | 8      | 8                           |
++---------------+--------+-----------------------------+
+| data_offset   | 16     | 8                           |
++---------------+--------+-----------------------------+
+| data_size     | 24     | 8                           |
++---------------+--------+-----------------------------+
+
+* *device_state* defines the state of the device:
+
+  The client initiates device state transition by writing the intended state.
+  The server must respond only after it has succesfully transitioned to the new
+  state. If an error occurs then the server must respond to the
+  VFIO_USER_REGION_WRITE operation with the Error field set accordingly and
+  must remain at the previous state, or in case of internal error it must
+  transtition to the error state, defined as
+  VFIO_DEVICE_STATE_RESUMING | VFIO_DEVICE_STATE_SAVING. The client must
+  re-read the device state in order to determine it afresh.
+
+  The following device states are defined:
+
+  +-----------+---------+----------+-----------------------------------+
+  | _RESUMING | _SAVING | _RUNNING | Description                       |
+  +===========+=========+==========+===================================+
+  | 0         | 0       | 0        | Device is stopped.                |
+  +-----------+---------+----------+-----------------------------------+
+  | 0         | 0       | 1        | Device is running, default state. |
+  +-----------+---------+----------+-----------------------------------+
+  | 0         | 1       | 0        | Stop-and-copy state               |
+  +-----------+---------+----------+-----------------------------------+
+  | 0         | 1       | 1        | Pre-copy state                    |
+  +-----------+---------+----------+-----------------------------------+
+  | 1         | 0       | 0        | Resuming                          |
+  +-----------+---------+----------+-----------------------------------+
+  | 1         | 0       | 1        | Invalid state                     |
+  +-----------+---------+----------+-----------------------------------+
+  | 1         | 1       | 0        | Error state                       |
+  +-----------+---------+----------+-----------------------------------+
+  | 1         | 1       | 1        | Invalid state                     |
+  +-----------+---------+----------+-----------------------------------+
+
+  Valid state transitions are shown in the following table:
+
+  +-------------------------+---------+---------+---------------+----------+----------+
+  | |darr| From / To |rarr| | Stopped | Running | Stop-and-copy | Pre-copy | Resuming |
+  +=========================+=========+=========+===============+==========+==========+
+  | Stopped                 |    \-   |    0    |       0       |    0     |     0    |
+  +-------------------------+---------+---------+---------------+----------+----------+
+  | Running                 |    1    |    \-   |       1       |    1     |     1    |
+  +-------------------------+---------+---------+---------------+----------+----------+
+  | Stop-and-copy           |    1    |    0    |       \-      |    0     |     0    |
+  +-------------------------+---------+---------+---------------+----------+----------+
+  | Pre-copy                |    0    |    0    |       1       |    \-    |     0    |
+  +-------------------------+---------+---------+---------------+----------+----------+
+  | Resuming                |    0    |    1    |       0       |    0     |     \-   |
+  +-------------------------+---------+---------+---------------+----------+----------+
+
+  A device is migrated to the destination as follows:
+
+  * The source client transitions the device state from the running state to
+    the pre-copy state. This transition is optional for the client but must be
+    supported by the server. The souce server starts sending device state data
+    to the source client through the migration region while the device is
+    running.
+
+  * The source client transitions the device state from the running state or the
+    pre-copy state to the stop-and-copy state. The source server stops the
+    device, saves device state and sends it to the source client through the
+    migration region.
+
+  The source client is responsible for sending the migration data to the
+  destination client.
+
+  A device is resumed on the destination as follows:
+
+  * The destination client transitions the device state from the running state
+    to the resuming state. The destination server uses the device state data
+    received through the migration region to resume the device.
+
+  * The destination client provides saved device state to the destination
+    server and then transitions the device to back to the running state.
+
+* *reserved* This field is reserved and any access to it must be ignored by the
+  server.
+
+* *pending_bytes* Remaining bytes to be migrated by the server. This field is
+  read only.
+
+* *data_offset* Offset in the migration region where the client must:
+
+  * read from, during the pre-copy or stop-and-copy state, or
+
+  * write to, during the resuming state.
+
+  This field is read only.
+
+* *data_size* Contains the size, in bytes, of the amount of data copied to:
+
+  * the source migration region by the source server during the pre-copy or
+    stop-and copy state, or
+
+  * the destination migration region by the destination client during the
+    resuming state.
+
+Device-specific data must be stored at any position after
+`struct vfio_device_migration_info`. Note that the migration region can be
+memory mappable, even partially. In practise, only the migration data portion
+can be memory mapped.
+
+The client processes device state data during the pre-copy and the
+stop-and-copy state in the following iterative manner:
+
+  1. The client reads `pending_bytes` to mark a new iteration. Repeated reads
+     of this field is an idempotent operation. If there are no migration data
+     to be consumed then the next step depends on the current device state:
+
+     * pre-copy: the client must try again.
+
+     * stop-and-copy: this procedure can end and the device can now start
+       resuming on the destination.
+
+  2. The client reads `data_offset`; at thich point the server must make
+     available a portion of migration data at this offset to be read by the
+     client, which must happen *before* completing the read operation. The
+     amount of data to be read must be stored in the `data_size` field, which
+     the client reads next.
+
+  3. The client reads `data_size` to determine the amount of migration data
+     available.
+
+  4. The client reads and processes the migration data.
+
+  5. Go to step 1.
+
+Note that the client can transition the device from the pre-copy state to the
+stop-and-copy state at any time; `pending_bytes` does not need to become zero.
+
+The client initializes the device state on the destination by setting the
+device state in the resuming state and writing the migration data to the
+destination migration region at `data_offset` offset. The client can write the
+source migration data in an iterative manner and the server must consume this
+data before completing each write operation, updating the `data_offset` field.
+The server must apply the source migration data on the device resume state. The
+client must write data on the same order and transction size as read.
+
+If an error occurs then the server must fail the read or write operation. It is
+an implementation detail of the client how to handle errors.
+
+VFIO_USER_DEVICE_GET_IRQ_INFO
+-----------------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 6                      |
++--------------+------------------------+
+| Message size | 32                     |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| IRQ info     | VFIO IRQ info          |
++--------------+------------------------+
+
+This command 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 ``<linux/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                         |
++-------+--------+---------------------------+
+
+* *argsz* is the size of the VFIO IRQ info structure.
+* *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 sub-indexes 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 command message.
+* count describes the number of interrupts of the queried type.
+
+VFIO_USER_DEVICE_SET_IRQS
+-------------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 7                      |
++--------------+------------------------+
+| Message size | 36 + any data          |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| IRQ set      | VFIO IRQ set           |
++--------------+------------------------+
+
+This command message is sent by the client to the server to set actions for
+device interrupt types. The VFIO IRQ set structure is defined in
+``<linux/vfio.h>`` (``struct vfio_irq_set``).
+
+VFIO IRQ set format
+^^^^^^^^^^^^^^^^^^^
+
++-------+--------+------------------------------+
+| Name  | Offset | Size                         |
++=======+========+==============================+
+| argsz | 16     | 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                     |
++-------+--------+------------------------------+
+
+* *argsz* is the size of the VFIO IRQ set structure, including any *data* field.
+* *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 command.
+    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
+    server triggers the interrupt.
+
+* *index* is the index of IRQ type being setup.
+* *start* is the start of the sub-index 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:
+
+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 server memory with VFIO_USER_REGION_READ and
+VFIO_USER_REGION_WRITE commands. These share a common data structure that
+appears after the 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 client memory with VFIO_USER_DMA_READ and
+VFIO_USER_DMA_WRITE messages. These also share a common data structure that
+appears after the message header.
+
+DMA Read/Write Data
+^^^^^^^^^^^^^^^^^^^
+
++---------+--------+----------+
+| Name    | Offset | Size     |
++=========+========+==========+
+| Address | 16     | 8        |
++---------+--------+----------+
+| Count   | 24     | 4        |
++---------+--------+----------+
+| Data    | 28     | variable |
++---------+--------+----------+
+
+* *Address* is the area of client memory being accessed. This address must have
+  been previously 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.
+
+VFIO_USER_REGION_READ
+---------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 8                      |
++--------------+------------------------+
+| Message size | 32 + data size         |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| Read info    | REGION read/write data |
++--------------+------------------------+
+
+This command message is sent from the client to the server to read from server
+memory.  In the command messages, there is no data, and the count is the amount
+of data to be read. The reply message must include the data read, and its count
+field is the amount of data read.
+
+VFIO_USER_REGION_WRITE
+----------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 9                      |
++--------------+------------------------+
+| Message size | 32 + data size         |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| Write info   | REGION read/write data |
++--------------+------------------------+
+
+This command message is sent from the client to the server to write to server
+memory.  The command message must contain the data to be written, and its count
+field must contain the amount of write data. The count field in the reply
+message must be zero.
+
+VFIO_USER_DMA_READ
+------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 10                     |
++--------------+------------------------+
+| Message size | 28 + data size         |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| DMA info     | DMA read/write data    |
++--------------+------------------------+
+
+This command message is sent from the server to the client to read from client
+memory.  In the command message, there is no data, and the count must will be
+the amount of data to be read. The reply message must include the data read,
+and its count field must be the amount of data read.
+
+VFIO_USER_DMA_WRITE
+-------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 11                     |
++--------------+------------------------+
+| Message size | 28 + data size         |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| DMA info     | DMA read/write data    |
++--------------+------------------------+
+
+This command message is sent from the server to the client to write to client
+memory.  The command message must contain the data to be written, and its count
+field must contain the amount of write data. The count field in the reply
+message must be zero.
+
+VFIO_USER_VM_INTERRUPT
+----------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++----------------+------------------------+
+| Name           | Value                  |
++================+========================+
+| Message ID     | <ID>                   |
++----------------+------------------------+
+| Command        | 12                     |
++----------------+------------------------+
+| Message size   | 20                     |
++----------------+------------------------+
+| Flags          | Reply bit set in reply |
++----------------+------------------------+
+| Error          | 0/errno                |
++----------------+------------------------+
+| Interrupt info | <interrupt>            |
++----------------+------------------------+
+
+This command message is sent from the server to the client to signal the device
+has raised an interrupt.
+
+Interrupt info format
+^^^^^^^^^^^^^^^^^^^^^
+
++-----------+--------+------+
+| Name      | Offset | Size |
++===========+========+======+
+| Sub-index | 16     | 4    |
++-----------+--------+------+
+
+* *Sub-index* is relative to the IRQ index, e.g., the vector number used in PCI
+  MSI/X type interrupts.
+
+VFIO_USER_DEVICE_RESET
+----------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 13                     |
++--------------+------------------------+
+| Message size | 16                     |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+
+This command message is sent from the client to the server to reset the device.
+
+VFIO_USER_DIRTY_PAGES
+---------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------------+------------------------+
+| Name               | Value                  |
++====================+========================+
+| Message ID         | <ID>                   |
++--------------------+------------------------+
+| Command            | 14                     |
++--------------------+------------------------+
+| Message size       | 16                     |
++--------------------+------------------------+
+| Flags              | Reply bit set in reply |
++--------------------+------------------------+
+| Error              | 0/errno                |
++--------------------+------------------------+
+| VFIO Dirty bitmap  | <dirty bitmap>         |
++--------------------+------------------------+
+
+This command is analogous to VFIO_IOMMU_DIRTY_PAGES. It is sent by the client
+to the server in order to control logging of dirty pages, usually during a live
+migration. The VFIO dirty bitmap structure is defined in ``<linux/vfio.h>``
+(``struct vfio_iommu_type1_dirty_bitmap``).
+
+VFIO Dirty Bitmap Format
+^^^^^^^^^^^^^^^^^^^^^^^^
+
++-------+--------+-----------------------------------------+
+| Name  | Offset | Size                                    |
++=======+========+=========================================+
+| argsz | 0      | 4                                       |
++-------+--------+-----------------------------------------+
+| flags | 4      | 4                                       |
++-------+--------+-----------------------------------------+
+|       | +-----+----------------------------------------+ |
+|       | | Bit | Definition                             | |
+|       | +=====+========================================+ |
+|       | | 0   | VFIO_IOMMU_DIRTY_PAGES_FLAG_START      | |
+|       | +-----+----------------------------------------+ |
+|       | | 1   | VFIO_IOMMU_DIRTY_PAGES_FLAG_STOP       | |
+|       | +-----+----------------------------------------+ |
+|       | | 2   | VFIO_IOMMU_DIRTY_PAGES_FLAG_GET_BITMAP | |
+|       | +-----+----------------------------------------+ |
++-------+--------+-----------------------------------------+
+| data  | 8      | 4                                       |
++-------+--------+-----------------------------------------+
+
+* *argsz* is the size of the VFIO dirty bitmap info structure.
+
+* *flags* defines the action to be performed by the server:
+
+  * *VFIO_IOMMU_DIRTY_PAGES_FLAG_START* instructs the server to start logging
+    pages it dirties. Logging continues until explicitly disabled by
+    VFIO_IOMMU_DIRTY_PAGES_FLAG_STOP.
+
+  * *VFIO_IOMMU_DIRTY_PAGES_FLAG_STOP* instructs the server to stop logging
+    dirty pages.
+
+  * *VFIO_IOMMU_DIRTY_PAGES_FLAG_GET_BITMAP* requests from the server to return
+    the dirty bitmap for a specific IOVA range. The IOVA range is specified by
+    "VFIO dirty bitmap get" structure, which must immediatelly follow the
+    "VFIO dirty bitmap" structure, explained next. This operation is only valid
+    if logging of dirty pages has been previously started. The server must
+    respond the same way it does for VFIO_USER_DMA_UNMAP (the dirty pages
+    bitmap must follow the response header).
+
+  These flags are mutually exclusive with each other.
+
+* *data* This field is unused in vfio-user.
+
+VFIO Dirty Bitmap Get Format
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
++--------+--------+------+
+| Name   | Offset | Size |
++========+========+======+
+| iova   | 0      | 8    |
++--------+--------+------+
+| size   | 8      | 8    |
++--------+--------+------+
+| bitmap | 16     | 24   |
++--------+--------+------+
+
+* *iova* is the IOVA offset
+
+* *size* is the size of the IOVA region
+
+* *bitmap* is the VFIO bitmap (``struct vfio_bitmap``), with the same semantics
+  as VFIO_USER_DMA_UNMAP.
+
+
+Appendices
+==========
+
+Unused VFIO ioctl() commands
+----------------------------
+
+The following VFIO commands do not have an equivalent vfio-user command:
+
+* 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 may have to be handled by the client in their
+corresponding vfio-user form. This will be addressed in a future protocol
+version.
+
+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-user
+implementation, the IOMMU is implemented in SW by the client, and is not
+visible to the server. The simplest idea would be that the client put each
+device into its own group and container.
+
+Backend Program Conventions
+---------------------------
+
+vfio-user backend program conventions are based on the vhost-user ones.
+
+* The backend program must not daemonize itself.
+* No assumptions must be made as to what access the backend program has on the
+  system.
+* File descriptors 0, 1 and 2 must exist, must have regular
+  stdin/stdout/stderr semantics, and can be redirected.
+* The backend program must honor the SIGTERM signal.
+* The backend program must accept the following commands line options:
+
+  * ``--socket-path=PATH``: path to UNIX domain socket,
+  * ``--fd=FDNUM``: file descriptor for UNIX domain socket, incompatible with
+    ``--socket-path``
+* The backend program must be accompanied with a JSON file stored under
+  ``/usr/share/vfio-user``.