qemu/docs/specs/vhost-user.txt

Vhost-user Protocol
===================

Copyright (c) 2014 Virtual Open Systems Sarl.

This work is licensed under the terms of the GNU GPL, version 2 or later.
See the COPYING file in the top-level directory.
===================

This protocol is aiming to complement the ioctl interface used to control the
vhost implementation in the Linux kernel. It implements the control plane needed
to establish virtqueue sharing with a user space process on the same host. It
uses communication over a Unix domain socket to share file descriptors in the
ancillary data of the message.

The protocol defines 2 sides of the communication, master and slave. Master is
the application that shares its virtqueues, in our case QEMU. Slave is the
consumer of the virtqueues.

In the current implementation QEMU is the Master, and the Slave is intended to
be a software Ethernet switch running in user space, such as Snabbswitch.

Master and slave can be either a client (i.e. connecting) or server (listening)
in the socket communication.

Message Specification
---------------------

Note that all numbers are in the machine native byte order. A vhost-user message
consists of 3 header fields and a payload:

------------------------------------
| request | flags | size | payload |
------------------------------------

 * Request: 32-bit type of the request
 * Flags: 32-bit bit field:
   - Lower 2 bits are the version (currently 0x01)
   - Bit 2 is the reply flag - needs to be sent on each reply from the slave
   - Bit 3 is the need_reply flag - see VHOST_USER_PROTOCOL_F_REPLY_ACK for
     details.
 * Size - 32-bit size of the payload


Depending on the request type, payload can be:

 * A single 64-bit integer
   -------
   | u64 |
   -------

   u64: a 64-bit unsigned integer

 * A vring state description
   ---------------
  | index | num |
  ---------------

   Index: a 32-bit index
   Num: a 32-bit number

 * A vring address description
   --------------------------------------------------------------
   | index | flags | size | descriptor | used | available | log |
   --------------------------------------------------------------

   Index: a 32-bit vring index
   Flags: a 32-bit vring flags
   Descriptor: a 64-bit user address of the vring descriptor table
   Used: a 64-bit user address of the vring used ring
   Available: a 64-bit user address of the vring available ring
   Log: a 64-bit guest address for logging

 * Memory regions description
   ---------------------------------------------------
   | num regions | padding | region0 | ... | region7 |
   ---------------------------------------------------

   Num regions: a 32-bit number of regions
   Padding: 32-bit

   A region is:
   -----------------------------------------------------
   | guest address | size | user address | mmap offset |
   -----------------------------------------------------

   Guest address: a 64-bit guest address of the region
   Size: a 64-bit size
   User address: a 64-bit user address
   mmap offset: 64-bit offset where region starts in the mapped memory

* Log description
   ---------------------------
   | log size | log offset |
   ---------------------------
   log size: size of area used for logging
   log offset: offset from start of supplied file descriptor
       where logging starts (i.e. where guest address 0 would be logged)

In QEMU the vhost-user message is implemented with the following struct:

typedef struct VhostUserMsg {
    VhostUserRequest request;
    uint32_t flags;
    uint32_t size;
    union {
        uint64_t u64;
        struct vhost_vring_state state;
        struct vhost_vring_addr addr;
        VhostUserMemory memory;
        VhostUserLog log;
    };
} QEMU_PACKED VhostUserMsg;

Communication
-------------

The protocol for vhost-user is based on the existing implementation of vhost
for the Linux Kernel. Most messages that can be sent via the Unix domain socket
implementing vhost-user have an equivalent ioctl to the kernel implementation.

The communication consists of master sending message requests and slave sending
message replies. Most of the requests don't require replies. Here is a list of
the ones that do:

 * VHOST_GET_FEATURES
 * VHOST_GET_PROTOCOL_FEATURES
 * VHOST_GET_VRING_BASE
 * VHOST_SET_LOG_BASE (if VHOST_USER_PROTOCOL_F_LOG_SHMFD)

[ Also see the section on REPLY_ACK protocol extension. ]

There are several messages that the master sends with file descriptors passed
in the ancillary data:

 * VHOST_SET_MEM_TABLE
 * VHOST_SET_LOG_BASE (if VHOST_USER_PROTOCOL_F_LOG_SHMFD)
 * VHOST_SET_LOG_FD
 * VHOST_SET_VRING_KICK
 * VHOST_SET_VRING_CALL
 * VHOST_SET_VRING_ERR

If Master is unable to send the full message or receives a wrong reply it will
close the connection. An optional reconnection mechanism can be implemented.

Any protocol extensions are gated by protocol feature bits,
which allows full backwards compatibility on both master
and slave.
As older slaves don't support negotiating protocol features,
a feature bit was dedicated for this purpose:
#define VHOST_USER_F_PROTOCOL_FEATURES 30

Starting and stopping rings
----------------------
Client must only process each ring when it is started.

Client must only pass data between the ring and the
backend, when the ring is enabled.

If ring is started but disabled, client must process the
ring without talking to the backend.

For example, for a networking device, in the disabled state
client must not supply any new RX packets, but must process
and discard any TX packets.

If VHOST_USER_F_PROTOCOL_FEATURES has not been negotiated, the ring is initialized
in an enabled state.

If VHOST_USER_F_PROTOCOL_FEATURES has been negotiated, the ring is initialized
in a disabled state. Client must not pass data to/from the backend until ring is enabled by
VHOST_USER_SET_VRING_ENABLE with parameter 1, or after it has been disabled by
VHOST_USER_SET_VRING_ENABLE with parameter 0.

Each ring is initialized in a stopped state, client must not process it until
ring is started, or after it has been stopped.

Client must start ring upon receiving a kick (that is, detecting that file
descriptor is readable) on the descriptor specified by
VHOST_USER_SET_VRING_KICK, and stop ring upon receiving
VHOST_USER_GET_VRING_BASE.

While processing the rings (whether they are enabled or not), client must
support changing some configuration aspects on the fly.

Multiple queue support
----------------------

Multiple queue is treated as a protocol extension, hence the slave has to
implement protocol features first. The multiple queues feature is supported
only when the protocol feature VHOST_USER_PROTOCOL_F_MQ (bit 0) is set.

The max number of queues the slave supports can be queried with message
VHOST_USER_GET_PROTOCOL_FEATURES. Master should stop when the number of
requested queues is bigger than that.

As all queues share one connection, the master uses a unique index for each
queue in the sent message to identify a specified queue. One queue pair
is enabled initially. More queues are enabled dynamically, by sending
message VHOST_USER_SET_VRING_ENABLE.

Migration
---------

During live migration, the master may need to track the modifications
the slave makes to the memory mapped regions. The client should mark
the dirty pages in a log. Once it complies to this logging, it may
declare the VHOST_F_LOG_ALL vhost feature.

To start/stop logging of data/used ring writes, server may send messages
VHOST_USER_SET_FEATURES with VHOST_F_LOG_ALL and VHOST_USER_SET_VRING_ADDR with
VHOST_VRING_F_LOG in ring's flags set to 1/0, respectively.

All the modifications to memory pointed by vring "descriptor" should
be marked. Modifications to "used" vring should be marked if
VHOST_VRING_F_LOG is part of ring's flags.

Dirty pages are of size:
#define VHOST_LOG_PAGE 0x1000

The log memory fd is provided in the ancillary data of
VHOST_USER_SET_LOG_BASE message when the slave has
VHOST_USER_PROTOCOL_F_LOG_SHMFD protocol feature.

The size of the log is supplied as part of VhostUserMsg
which should be large enough to cover all known guest
addresses. Log starts at the supplied offset in the
supplied file descriptor.
The log covers from address 0 to the maximum of guest
regions. In pseudo-code, to mark page at "addr" as dirty:

page = addr / VHOST_LOG_PAGE
log[page / 8] |= 1 << page % 8

Where addr is the guest physical address.

Use atomic operations, as the log may be concurrently manipulated.

Note that when logging modifications to the used ring (when VHOST_VRING_F_LOG
is set for this ring), log_guest_addr should be used to calculate the log
offset: the write to first byte of the used ring is logged at this offset from
log start. Also note that this value might be outside the legal guest physical
address range (i.e. does not have to be covered by the VhostUserMemory table),
but the bit offset of the last byte of the ring must fall within
the size supplied by VhostUserLog.

VHOST_USER_SET_LOG_FD is an optional message with an eventfd in
ancillary data, it may be used to inform the master that the log has
been modified.

Once the source has finished migration, rings will be stopped by
the source. No further update must be done before rings are
restarted.

Protocol features
-----------------

#define VHOST_USER_PROTOCOL_F_MQ             0
#define VHOST_USER_PROTOCOL_F_LOG_SHMFD      1
#define VHOST_USER_PROTOCOL_F_RARP           2
#define VHOST_USER_PROTOCOL_F_REPLY_ACK      3

Message types
-------------

 * VHOST_USER_GET_FEATURES

      Id: 1
      Equivalent ioctl: VHOST_GET_FEATURES
      Master payload: N/A
      Slave payload: u64

      Get from the underlying vhost implementation the features bitmask.
      Feature bit VHOST_USER_F_PROTOCOL_FEATURES signals slave support for
      VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES.

 * VHOST_USER_SET_FEATURES

      Id: 2
      Ioctl: VHOST_SET_FEATURES
      Master payload: u64

      Enable features in the underlying vhost implementation using a bitmask.
      Feature bit VHOST_USER_F_PROTOCOL_FEATURES signals slave support for
      VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES.

 * VHOST_USER_GET_PROTOCOL_FEATURES

      Id: 15
      Equivalent ioctl: VHOST_GET_FEATURES
      Master payload: N/A
      Slave payload: u64

      Get the protocol feature bitmask from the underlying vhost implementation.
      Only legal if feature bit VHOST_USER_F_PROTOCOL_FEATURES is present in
      VHOST_USER_GET_FEATURES.
      Note: slave that reported VHOST_USER_F_PROTOCOL_FEATURES must support
      this message even before VHOST_USER_SET_FEATURES was called.

 * VHOST_USER_SET_PROTOCOL_FEATURES

      Id: 16
      Ioctl: VHOST_SET_FEATURES
      Master payload: u64

      Enable protocol features in the underlying vhost implementation.
      Only legal if feature bit VHOST_USER_F_PROTOCOL_FEATURES is present in
      VHOST_USER_GET_FEATURES.
      Note: slave that reported VHOST_USER_F_PROTOCOL_FEATURES must support
      this message even before VHOST_USER_SET_FEATURES was called.

 * VHOST_USER_SET_OWNER

      Id: 3
      Equivalent ioctl: VHOST_SET_OWNER
      Master payload: N/A

      Issued when a new connection is established. It sets the current Master
      as an owner of the session. This can be used on the Slave as a
      "session start" flag.

 * VHOST_USER_RESET_OWNER

      Id: 4
      Master payload: N/A

      This is no longer used. Used to be sent to request disabling
      all rings, but some clients interpreted it to also discard
      connection state (this interpretation would lead to bugs).
      It is recommended that clients either ignore this message,
      or use it to disable all rings.

 * VHOST_USER_SET_MEM_TABLE

      Id: 5
      Equivalent ioctl: VHOST_SET_MEM_TABLE
      Master payload: memory regions description

      Sets the memory map regions on the slave so it can translate the vring
      addresses. In the ancillary data there is an array of file descriptors
      for each memory mapped region. The size and ordering of the fds matches
      the number and ordering of memory regions.

 * VHOST_USER_SET_LOG_BASE

      Id: 6
      Equivalent ioctl: VHOST_SET_LOG_BASE
      Master payload: u64
      Slave payload: N/A

      Sets logging shared memory space.
      When slave has VHOST_USER_PROTOCOL_F_LOG_SHMFD protocol
      feature, the log memory fd is provided in the ancillary data of
      VHOST_USER_SET_LOG_BASE message, the size and offset of shared
      memory area provided in the message.


 * VHOST_USER_SET_LOG_FD

      Id: 7
      Equivalent ioctl: VHOST_SET_LOG_FD
      Master payload: N/A

      Sets the logging file descriptor, which is passed as ancillary data.

 * VHOST_USER_SET_VRING_NUM

      Id: 8
      Equivalent ioctl: VHOST_SET_VRING_NUM
      Master payload: vring state description

      Set the size of the queue.

 * VHOST_USER_SET_VRING_ADDR

      Id: 9
      Equivalent ioctl: VHOST_SET_VRING_ADDR
      Master payload: vring address description
      Slave payload: N/A

      Sets the addresses of the different aspects of the vring.

 * VHOST_USER_SET_VRING_BASE

      Id: 10
      Equivalent ioctl: VHOST_SET_VRING_BASE
      Master payload: vring state description

      Sets the base offset in the available vring.

 * VHOST_USER_GET_VRING_BASE

      Id: 11
      Equivalent ioctl: VHOST_USER_GET_VRING_BASE
      Master payload: vring state description
      Slave payload: vring state description

      Get the available vring base offset.

 * VHOST_USER_SET_VRING_KICK

      Id: 12
      Equivalent ioctl: VHOST_SET_VRING_KICK
      Master payload: u64

      Set the event file descriptor for adding buffers to the vring. It
      is passed in the ancillary data.
      Bits (0-7) of the payload contain the vring index. Bit 8 is the
      invalid FD flag. This flag is set when there is no file descriptor
      in the ancillary data. This signals that polling should be used
      instead of waiting for a kick.

 * VHOST_USER_SET_VRING_CALL

      Id: 13
      Equivalent ioctl: VHOST_SET_VRING_CALL
      Master payload: u64

      Set the event file descriptor to signal when buffers are used. It
      is passed in the ancillary data.
      Bits (0-7) of the payload contain the vring index. Bit 8 is the
      invalid FD flag. This flag is set when there is no file descriptor
      in the ancillary data. This signals that polling will be used
      instead of waiting for the call.

 * VHOST_USER_SET_VRING_ERR

      Id: 14
      Equivalent ioctl: VHOST_SET_VRING_ERR
      Master payload: u64

      Set the event file descriptor to signal when error occurs. It
      is passed in the ancillary data.
      Bits (0-7) of the payload contain the vring index. Bit 8 is the
      invalid FD flag. This flag is set when there is no file descriptor
      in the ancillary data.

 * VHOST_USER_GET_QUEUE_NUM

      Id: 17
      Equivalent ioctl: N/A
      Master payload: N/A
      Slave payload: u64

      Query how many queues the backend supports. This request should be
      sent only when VHOST_USER_PROTOCOL_F_MQ is set in queried protocol
      features by VHOST_USER_GET_PROTOCOL_FEATURES.

 * VHOST_USER_SET_VRING_ENABLE

      Id: 18
      Equivalent ioctl: N/A
      Master payload: vring state description

      Signal slave to enable or disable corresponding vring.
      This request should be sent only when VHOST_USER_F_PROTOCOL_FEATURES
      has been negotiated.

 * VHOST_USER_SEND_RARP

      Id: 19
      Equivalent ioctl: N/A
      Master payload: u64

      Ask vhost user backend to broadcast a fake RARP to notify the migration
      is terminated for guest that does not support GUEST_ANNOUNCE.
      Only legal if feature bit VHOST_USER_F_PROTOCOL_FEATURES is present in
      VHOST_USER_GET_FEATURES and protocol feature bit VHOST_USER_PROTOCOL_F_RARP
      is present in VHOST_USER_GET_PROTOCOL_FEATURES.
      The first 6 bytes of the payload contain the mac address of the guest to
      allow the vhost user backend to construct and broadcast the fake RARP.

VHOST_USER_PROTOCOL_F_REPLY_ACK:
-------------------------------
The original vhost-user specification only demands replies for certain
commands. This differs from the vhost protocol implementation where commands
are sent over an ioctl() call and block until the client has completed.

With this protocol extension negotiated, the sender (QEMU) can set the
"need_reply" [Bit 3] flag to any command. This indicates that
the client MUST respond with a Payload VhostUserMsg indicating success or
failure. The payload should be set to zero on success or non-zero on failure,
unless the message already has an explicit reply body.

The response payload gives QEMU a deterministic indication of the result
of the command. Today, QEMU is expected to terminate the main vhost-user
loop upon receiving such errors. In future, qemu could be taught to be more
resilient for selective requests.

For the message types that already solicit a reply from the client, the
presence of VHOST_USER_PROTOCOL_F_REPLY_ACK or need_reply bit being set brings
no behavioural change. (See the 'Communication' section for details.)