QMP/qmp-spec.txt

   1            QEMU Monitor Protocol Specification - Version 0.1
   2
   3 1. Introduction
   4 ===============
   5
   6 This document specifies the QEMU Monitor Protocol (QMP), a JSON-based protocol
   7 which is available for applications to control QEMU at the machine-level.
   8
   9 To enable QMP support, QEMU has to be run in "control mode". This is done by
  10 starting QEMU with the appropriate command-line options. Please, refer to the
  11 QEMU manual page for more information.
  12
  13 2. Protocol Specification
  14 =========================
  15
  16 This section details the protocol format. For the purpose of this document
  17 "Client" is any application which is communicating with QEMU in control mode,
  18 and "Server" is QEMU itself.
  19
  20 JSON data structures, when mentioned in this document, are always in the
  21 following format:
  22
  23     json-DATA-STRUCTURE-NAME
  24
  25 Where DATA-STRUCTURE-NAME is any valid JSON data structure, as defined by
  26 the JSON standard:
  27
  28 http://www.ietf.org/rfc/rfc4627.txt
  29
  30 For convenience, json-object members and json-array elements mentioned in
  31 this document will be in a certain order. However, in real protocol usage
  32 they can be in ANY order, thus no particular order should be assumed.
  33
  34 2.1 General Definitions
  35 -----------------------
  36
  37 2.1.1 All interactions transmitted by the Server are json-objects, always
  38       terminating with CRLF
  39
  40 2.1.2 All json-objects members are mandatory when not specified otherwise
  41
  42 2.2 Server Greeting
  43 -------------------
  44
  45 Right when connected the Server will issue a greeting message, which signals
  46 that the connection has been successfully established and that the Server is
  47 ready for capabilities negotiation (for more information refer to section
  48 '4. Capabilities Negotiation').
  49
  50 The format is:
  51
  52 { "QMP": { "version": json-object, "capabilities": json-array } }
  53
  54  Where,
  55
  56 - The "version" member contains the Server's version information (the format
  57   is the same of the 'query-version' command)
  58 - The "capabilities" member specify the availability of features beyond the
  59   baseline specification
  60
  61 2.3 Issuing Commands
  62 --------------------
  63
  64 The format for command execution is:
  65
  66 { "execute": json-string, "arguments": json-object, "id": json-value }
  67
  68  Where,
  69
  70 - The "execute" member identifies the command to be executed by the Server
  71 - The "arguments" member is used to pass any arguments required for the
  72   execution of the command, it is optional when no arguments are required
  73 - The "id" member is a transaction identification associated with the
  74   command execution, it is optional and will be part of the response if
  75   provided
  76
  77 2.4 Commands Responses
  78 ----------------------
  79
  80 There are two possible responses which the Server will issue as the result
  81 of a command execution: success or error.
  82
  83 2.4.1 success
  84 -------------
  85
  86 The success response is issued when the command execution has finished
  87 without errors.
  88
  89 The format is:
  90
  91 { "return": json-object, "id": json-value }
  92
  93  Where,
  94
  95 - The "return" member contains the command returned data, which is defined
  96   in a per-command basis or an empty json-object if the command does not
  97   return data
  98 - The "id" member contains the transaction identification associated
  99   with the command execution (if issued by the Client)
 100
 101 2.4.2 error
 102 -----------
 103
 104 The error response is issued when the command execution could not be
 105 completed because of an error condition.
 106
 107 The format is:
 108
 109 { "error": { "class": json-string, "desc": json-string }, "id": json-value }
 110
 111  Where,
 112
 113 - The "class" member contains the error class name (eg. "GenericError")
 114 - The "desc" member is a human-readable error message. Clients should
 115   not attempt to parse this message.
 116 - The "id" member contains the transaction identification associated with
 117   the command execution (if issued by the Client)
 118
 119 NOTE: Some errors can occur before the Server is able to read the "id" member,
 120 in these cases the "id" member will not be part of the error response, even
 121 if provided by the client.
 122
 123 2.5 Asynchronous events
 124 -----------------------
 125
 126 As a result of state changes, the Server may send messages unilaterally
 127 to the Client at any time. They are called 'asynchronous events'.
 128
 129 The format is:
 130
 131 { "event": json-string, "data": json-object,
 132   "timestamp": { "seconds": json-number, "microseconds": json-number } }
 133
 134  Where,
 135
 136 - The "event" member contains the event's name
 137 - The "data" member contains event specific data, which is defined in a
 138   per-event basis, it is optional
 139 - The "timestamp" member contains the exact time of when the event occurred
 140   in the Server. It is a fixed json-object with time in seconds and
 141   microseconds
 142
 143 For a listing of supported asynchronous events, please, refer to the
 144 qmp-events.txt file.
 145
 146 3. QMP Examples
 147 ===============
 148
 149 This section provides some examples of real QMP usage, in all of them
 150 'C' stands for 'Client' and 'S' stands for 'Server'.
 151
 152 3.1 Server greeting
 153 -------------------
 154
 155 S: {"QMP": {"version": {"qemu": "0.12.50", "package": ""}, "capabilities": []}}
 156
 157 3.2 Simple 'stop' execution
 158 ---------------------------
 159
 160 C: { "execute": "stop" }
 161 S: {"return": {}}
 162
 163 3.3 KVM information
 164 -------------------
 165
 166 C: { "execute": "query-kvm", "id": "example" }
 167 S: {"return": {"enabled": true, "present": true}, "id": "example"}
 168
 169 3.4 Parsing error
 170 ------------------
 171
 172 C: { "execute": }
 173 S: {"error": {"class": "GenericError", "desc": "Invalid JSON syntax" } }
 174
 175 3.5 Powerdown event
 176 -------------------
 177
 178 S: {"timestamp": {"seconds": 1258551470, "microseconds": 802384}, "event":
 179 "POWERDOWN"}
 180
 181 4. Capabilities Negotiation
 182 ----------------------------
 183
 184 When a Client successfully establishes a connection, the Server is in
 185 Capabilities Negotiation mode.
 186
 187 In this mode only the 'qmp_capabilities' command is allowed to run, all
 188 other commands will return the CommandNotFound error. Asynchronous messages
 189 are not delivered either.
 190
 191 Clients should use the 'qmp_capabilities' command to enable capabilities
 192 advertised in the Server's greeting (section '2.2 Server Greeting') they
 193 support.
 194
 195 When the 'qmp_capabilities' command is issued, and if it does not return an
 196 error, the Server enters in Command mode where capabilities changes take
 197 effect, all commands (except 'qmp_capabilities') are allowed and asynchronous
 198 messages are delivered.
 199
 200 5 Compatibility Considerations
 201 ------------------------------
 202
 203 All protocol changes or new features which modify the protocol format in an
 204 incompatible way are disabled by default and will be advertised by the
 205 capabilities array (section '2.2 Server Greeting'). Thus, Clients can check
 206 that array and enable the capabilities they support.
 207
 208 The QMP Server performs a type check on the arguments to a command.  It
 209 generates an error if a value does not have the expected type for its
 210 key, or if it does not understand a key that the Client included.  The
 211 strictness of the Server catches wrong assumptions of Clients about
 212 the Server's schema.  Clients can assume that, when such validation
 213 errors occur, they will be reported before the command generated any
 214 side effect.
 215
 216 However, Clients must not assume any particular:
 217
 218 - Length of json-arrays
 219 - Size of json-objects; in particular, future versions of QEMU may add
 220   new keys and Clients should be able to ignore them.
 221 - Order of json-object members or json-array elements
 222 - Amount of errors generated by a command, that is, new errors can be added
 223   to any existing command in newer versions of the Server
 224
 225 Of course, the Server does guarantee to send valid JSON.  But apart from
 226 this, a Client should be "conservative in what they send, and liberal in
 227 what they accept".
 228
 229 6. Downstream extension of QMP
 230 ------------------------------
 231
 232 We recommend that downstream consumers of QEMU do *not* modify QMP.
 233 Management tools should be able to support both upstream and downstream
 234 versions of QMP without special logic, and downstream extensions are
 235 inherently at odds with that.
 236
 237 However, we recognize that it is sometimes impossible for downstreams to
 238 avoid modifying QMP.  Both upstream and downstream need to take care to
 239 preserve long-term compatibility and interoperability.
 240
 241 To help with that, QMP reserves JSON object member names beginning with
 242 '__' (double underscore) for downstream use ("downstream names").  This
 243 means upstream will never use any downstream names for its commands,
 244 arguments, errors, asynchronous events, and so forth.
 245
 246 Any new names downstream wishes to add must begin with '__'.  To
 247 ensure compatibility with other downstreams, it is strongly
 248 recommended that you prefix your downstram names with '__RFQDN_' where
 249 RFQDN is a valid, reverse fully qualified domain name which you
 250 control.  For example, a qemu-kvm specific monitor command would be:
 251
 252     (qemu) __org.linux-kvm_enable_irqchip
 253
 254 Downstream must not change the server greeting (section 2.2) other than
 255 to offer additional capabilities.  But see below for why even that is
 256 discouraged.
 257
 258 Section '5 Compatibility Considerations' applies to downstream as well
 259 as to upstream, obviously.  It follows that downstream must behave
 260 exactly like upstream for any input not containing members with
 261 downstream names ("downstream members"), except it may add members
 262 with downstream names to its output.
 263
 264 Thus, a client should not be able to distinguish downstream from
 265 upstream as long as it doesn't send input with downstream members, and
 266 properly ignores any downstream members in the output it receives.
 267
 268 Advice on downstream modifications:
 269
 270 1. Introducing new commands is okay.  If you want to extend an existing
 271    command, consider introducing a new one with the new behaviour
 272    instead.
 273
 274 2. Introducing new asynchronous messages is okay.  If you want to extend
 275    an existing message, consider adding a new one instead.
 276
 277 3. Introducing new errors for use in new commands is okay.  Adding new
 278    errors to existing commands counts as extension, so 1. applies.
 279
 280 4. New capabilities are strongly discouraged.  Capabilities are for
 281    evolving the basic protocol, and multiple diverging basic protocol
 282    dialects are most undesirable.