docs/specs/vhost-user.txt

   1 Vhost-user Protocol
   2 ===================
   3
   4 Copyright (c) 2014 Virtual Open Systems Sarl.
   5
   6 This work is licensed under the terms of the GNU GPL, version 2 or later.
   7 See the COPYING file in the top-level directory.
   8 ===================
   9
  10 This protocol is aiming to complement the ioctl interface used to control the
  11 vhost implementation in the Linux kernel. It implements the control plane needed
  12 to establish virtqueue sharing with a user space process on the same host. It
  13 uses communication over a Unix domain socket to share file descriptors in the
  14 ancillary data of the message.
  15
  16 The protocol defines 2 sides of the communication, master and slave. Master is
  17 the application that shares its virtqueues, in our case QEMU. Slave is the
  18 consumer of the virtqueues.
  19
  20 In the current implementation QEMU is the Master, and the Slave is intended to
  21 be a software Ethernet switch running in user space, such as Snabbswitch.
  22
  23 Master and slave can be either a client (i.e. connecting) or server (listening)
  24 in the socket communication.
  25
  26 Message Specification
  27 ---------------------
  28
  29 Note that all numbers are in the machine native byte order. A vhost-user message
  30 consists of 3 header fields and a payload:
  31
  32 ------------------------------------
  33 | request | flags | size | payload |
  34 ------------------------------------
  35
  36  * Request: 32-bit type of the request
  37  * Flags: 32-bit bit field:
  38    - Lower 2 bits are the version (currently 0x01)
  39    - Bit 2 is the reply flag - needs to be sent on each reply from the slave
  40  * Size - 32-bit size of the payload
  41
  42
  43 Depending on the request type, payload can be:
  44
  45  * A single 64-bit integer
  46    -------
  47    | u64 |
  48    -------
  49
  50    u64: a 64-bit unsigned integer
  51
  52  * A vring state description
  53    ---------------
  54   | index | num |
  55   ---------------
  56
  57    Index: a 32-bit index
  58    Num: a 32-bit number
  59
  60  * A vring address description
  61    --------------------------------------------------------------
  62    | index | flags | size | descriptor | used | available | log |
  63    --------------------------------------------------------------
  64
  65    Index: a 32-bit vring index
  66    Flags: a 32-bit vring flags
  67    Descriptor: a 64-bit user address of the vring descriptor table
  68    Used: a 64-bit user address of the vring used ring
  69    Available: a 64-bit user address of the vring available ring
  70    Log: a 64-bit guest address for logging
  71
  72  * Memory regions description
  73    ---------------------------------------------------
  74    | num regions | padding | region0 | ... | region7 |
  75    ---------------------------------------------------
  76
  77    Num regions: a 32-bit number of regions
  78    Padding: 32-bit
  79
  80    A region is:
  81    -----------------------------------------------------
  82    | guest address | size | user address | mmap offset |
  83    -----------------------------------------------------
  84
  85    Guest address: a 64-bit guest address of the region
  86    Size: a 64-bit size
  87    User address: a 64-bit user address
  88    mmap offset: 64-bit offset where region starts in the mapped memory
  89
  90 In QEMU the vhost-user message is implemented with the following struct:
  91
  92 typedef struct VhostUserMsg {
  93     VhostUserRequest request;
  94     uint32_t flags;
  95     uint32_t size;
  96     union {
  97         uint64_t u64;
  98         struct vhost_vring_state state;
  99         struct vhost_vring_addr addr;
 100         VhostUserMemory memory;
 101     };
 102 } QEMU_PACKED VhostUserMsg;
 103
 104 Communication
 105 -------------
 106
 107 The protocol for vhost-user is based on the existing implementation of vhost
 108 for the Linux Kernel. Most messages that can be sent via the Unix domain socket
 109 implementing vhost-user have an equivalent ioctl to the kernel implementation.
 110
 111 The communication consists of master sending message requests and slave sending
 112 message replies. Most of the requests don't require replies. Here is a list of
 113 the ones that do:
 114
 115  * VHOST_GET_FEATURES
 116  * VHOST_GET_VRING_BASE
 117
 118 There are several messages that the master sends with file descriptors passed
 119 in the ancillary data:
 120
 121  * VHOST_SET_MEM_TABLE
 122  * VHOST_SET_LOG_FD
 123  * VHOST_SET_VRING_KICK
 124  * VHOST_SET_VRING_CALL
 125  * VHOST_SET_VRING_ERR
 126
 127 If Master is unable to send the full message or receives a wrong reply it will
 128 close the connection. An optional reconnection mechanism can be implemented.
 129
 130 Multi queue support
 131 -------------------
 132 The protocol supports multiple queues by setting all index fields in the sent
 133 messages to a properly calculated value.
 134
 135 Message types
 136 -------------
 137
 138  * VHOST_USER_GET_FEATURES
 139
 140       Id: 1
 141       Equivalent ioctl: VHOST_GET_FEATURES
 142       Master payload: N/A
 143       Slave payload: u64
 144
 145       Get from the underlying vhost implementation the features bitmask.
 146
 147  * VHOST_USER_SET_FEATURES
 148
 149       Id: 2
 150       Ioctl: VHOST_SET_FEATURES
 151       Master payload: u64
 152
 153       Enable features in the underlying vhost implementation using a bitmask.
 154
 155  * VHOST_USER_SET_OWNER
 156
 157       Id: 3
 158       Equivalent ioctl: VHOST_SET_OWNER
 159       Master payload: N/A
 160
 161       Issued when a new connection is established. It sets the current Master
 162       as an owner of the session. This can be used on the Slave as a
 163       "session start" flag.
 164
 165  * VHOST_USER_RESET_OWNER
 166
 167       Id: 4
 168       Equivalent ioctl: VHOST_RESET_OWNER
 169       Master payload: N/A
 170
 171       Issued when a new connection is about to be closed. The Master will no
 172       longer own this connection (and will usually close it).
 173
 174  * VHOST_USER_SET_MEM_TABLE
 175
 176       Id: 5
 177       Equivalent ioctl: VHOST_SET_MEM_TABLE
 178       Master payload: memory regions description
 179
 180       Sets the memory map regions on the slave so it can translate the vring
 181       addresses. In the ancillary data there is an array of file descriptors
 182       for each memory mapped region. The size and ordering of the fds matches
 183       the number and ordering of memory regions.
 184
 185  * VHOST_USER_SET_LOG_BASE
 186
 187       Id: 6
 188       Equivalent ioctl: VHOST_SET_LOG_BASE
 189       Master payload: u64
 190
 191       Sets the logging base address.
 192
 193  * VHOST_USER_SET_LOG_FD
 194
 195       Id: 7
 196       Equivalent ioctl: VHOST_SET_LOG_FD
 197       Master payload: N/A
 198
 199       Sets the logging file descriptor, which is passed as ancillary data.
 200
 201  * VHOST_USER_SET_VRING_NUM
 202
 203       Id: 8
 204       Equivalent ioctl: VHOST_SET_VRING_NUM
 205       Master payload: vring state description
 206
 207       Sets the number of vrings for this owner.
 208
 209  * VHOST_USER_SET_VRING_ADDR
 210
 211       Id: 9
 212       Equivalent ioctl: VHOST_SET_VRING_ADDR
 213       Master payload: vring address description
 214       Slave payload: N/A
 215
 216       Sets the addresses of the different aspects of the vring.
 217
 218  * VHOST_USER_SET_VRING_BASE
 219
 220       Id: 10
 221       Equivalent ioctl: VHOST_SET_VRING_BASE
 222       Master payload: vring state description
 223
 224       Sets the base offset in the available vring.
 225
 226  * VHOST_USER_GET_VRING_BASE
 227
 228       Id: 11
 229       Equivalent ioctl: VHOST_USER_GET_VRING_BASE
 230       Master payload: vring state description
 231       Slave payload: vring state description
 232
 233       Get the available vring base offset.
 234
 235  * VHOST_USER_SET_VRING_KICK
 236
 237       Id: 12
 238       Equivalent ioctl: VHOST_SET_VRING_KICK
 239       Master payload: u64
 240
 241       Set the event file descriptor for adding buffers to the vring. It
 242       is passed in the ancillary data.
 243       Bits (0-7) of the payload contain the vring index. Bit 8 is the
 244       invalid FD flag. This flag is set when there is no file descriptor
 245       in the ancillary data. This signals that polling should be used
 246       instead of waiting for a kick.
 247
 248  * VHOST_USER_SET_VRING_CALL
 249
 250       Id: 13
 251       Equivalent ioctl: VHOST_SET_VRING_CALL
 252       Master payload: u64
 253
 254       Set the event file descriptor to signal when buffers are used. It
 255       is passed in the ancillary data.
 256       Bits (0-7) of the payload contain the vring index. Bit 8 is the
 257       invalid FD flag. This flag is set when there is no file descriptor
 258       in the ancillary data. This signals that polling will be used
 259       instead of waiting for the call.
 260
 261  * VHOST_USER_SET_VRING_ERR
 262
 263       Id: 14
 264       Equivalent ioctl: VHOST_SET_VRING_ERR
 265       Master payload: u64
 266
 267       Set the event file descriptor to signal when error occurs. It
 268       is passed in the ancillary data.
 269       Bits (0-7) of the payload contain the vring index. Bit 8 is the
 270       invalid FD flag. This flag is set when there is no file descriptor
 271       in the ancillary data.