QEMU Machine Protocol Specification

1. Introduction

This document specifies the QEMU Machine Protocol (QMP), a JSON-based protocol which is available for applications to operate QEMU at the machine-level. It is also in use by the QEMU Guest Agent (QGA), which is available for host applications to interact with the guest operating system.

2. Protocol Specification

This section details the protocol format. For the purpose of this document “Client” is any application which is using QMP to communicate with QEMU and “Server” is QEMU itself.

JSON data structures, when mentioned in this document, are always in the following format:

json-DATA-STRUCTURE-NAME

Where DATA-STRUCTURE-NAME is any valid JSON data structure, as defined by the JSON standard:

The protocol is always encoded in UTF-8 except for synchronization bytes (documented below); although thanks to json-string escape sequences, the server will reply using only the strict ASCII subset.

For convenience, json-object members mentioned in this document will be in a certain order. However, in real protocol usage they can be in ANY order, thus no particular order should be assumed. On the other hand, use of json-array elements presumes that preserving order is important unless specifically documented otherwise. Repeating a key within a json-object gives unpredictable results.

Also for convenience, the server will accept an extension of ‘single-quoted’ strings in place of the usual “double-quoted” json-string, and both input forms of strings understand an additional escape sequence of “’” for a single quote. The server will only use double quoting on output.

2.1 General Definitions

2.1.1 All interactions transmitted by the Server are json-objects, always terminating with CRLF

2.1.2 All json-objects members are mandatory when not specified otherwise

2.2 Server Greeting

Right when connected the Server will issue a greeting message, which signals that the connection has been successfully established and that the Server is ready for capabilities negotiation (for more information refer to section ‘4. Capabilities Negotiation’).

The greeting message format is:

{ "QMP": { "version": json-object, "capabilities": json-array } }

Where,

  • The “version” member contains the Server’s version information (the format is the same of the query-version command)
  • The “capabilities” member specify the availability of features beyond the baseline specification; the order of elements in this array has no particular significance, so a client must search the entire array when looking for a particular capability

2.2.1 Capabilities

As of the date this document was last revised, no server or client capability strings have been defined.

2.3 Issuing Commands

The format for command execution is:

{ "execute": json-string, "arguments": json-object, "id": json-value }

Where,

  • The “execute” member identifies the command to be executed by the Server
  • The “arguments” member is used to pass any arguments required for the execution of the command, it is optional when no arguments are required. Each command documents what contents will be considered valid when handling the json-argument
  • The “id” member is a transaction identification associated with the command execution, it is optional and will be part of the response if provided. The “id” member can be any json-value, although most clients merely use a json-number incremented for each successive command

2.4 Commands Responses

There are two possible responses which the Server will issue as the result of a command execution: success or error.

2.4.1 success

The format of a success response is:

{ "return": json-value, "id": json-value }

Where,

  • The “return” member contains the data returned by the command, which is defined on a per-command basis (usually a json-object or json-array of json-objects, but sometimes a json-number, json-string, or json-array of json-strings); it is an empty json-object if the command does not return data
  • The “id” member contains the transaction identification associated with the command execution if issued by the Client

2.4.2 error

The format of an error response is:

{ "error": { "class": json-string, "desc": json-string }, "id":
json-value }

Where,

  • The “class” member contains the error class name (eg. “GenericError”)
  • The “desc” member is a human-readable error message. Clients should not attempt to parse this message.
  • The “id” member contains the transaction identification associated with the command execution if issued by the Client

NOTE: Some errors can occur before the Server is able to read the “id” member, in these cases the “id” member will not be part of the error response, even if provided by the client.

2.5 Asynchronous events

As a result of state changes, the Server may send messages unilaterally to the Client at any time, when not in the middle of any other response. They are called “asynchronous events”.

The format of asynchronous events is:

{ "event": json-string, "data": json-object, "timestamp": { "seconds":
json-number, "microseconds": json-number } }

Where,

  • The “event” member contains the event’s name
  • The “data” member contains event specific data, which is defined in a per-event basis, it is optional
  • The “timestamp” member contains the exact time of when the event occurred in the Server. It is a fixed json-object with time in seconds and microseconds relative to the Unix Epoch (1 Jan 1970); if there is a failure to retrieve host time, both members of the timestamp will be set to -1.

For a listing of supported asynchronous events, please, refer to the qmp-events.txt file.

Some events are rate-limited to at most one per second. If additional “similar” events arrive within one second, all but the last one are dropped, and the last one is delayed. “Similar” normally means same event type. See qmp-events.txt for details.

2.6 QGA Synchronization

When using QGA, an additional synchronization feature is built into the protocol. If the Client sends a raw 0xFF sentinel byte (not valid JSON), then the Server will reset its state and discard all pending data prior to the sentinel. Conversely, if the Client makes use of the ‘guest-sync-delimited’ command, the Server will send a raw 0xFF sentinel byte prior to its response, to aid the Client in discarding any data prior to the sentinel.

3. QMP Examples

This section provides some examples of real QMP usage, in all of them “C” stands for “Client” and “S” stands for “Server”.

3.1 Server greeting

S: { "QMP": { "version": { "qemu": { "micro": 50, "minor": 6, "major": 1
}, "package": ""}, "capabilities": []}}

3.2 Client QMP negotiation

C: { "execute": "qmp\_capabilities" } S: { "return": {}}

3.3 Simple ‘stop’ execution

C: { "execute": "stop" } S: { "return": {} }

3.4 KVM information

C: { "execute": "query-kvm", "id": "example" } S: { "return": {
"enabled": true, "present": true }, "id": "example"}

3.5 Parsing error

C: { "execute": } S: { "error": { "class": "GenericError", "desc":
"Invalid JSON syntax" } }

3.6 Powerdown event

S: { "timestamp": { "seconds": 1258551470, "microseconds": 802384 },
"event": "POWERDOWN" }

4. Capabilities Negotiation

When a Client successfully establishes a connection, the Server is in Capabilities Negotiation mode.

In this mode only the qmp_capabilities command is allowed to run, all other commands will return the CommandNotFound error. Asynchronous messages are not delivered either.

Clients should use the qmp_capabilities command to enable capabilities advertised in the Server’s greeting (section ‘2.2 Server Greeting’) they support.

When the qmp_capabilities command is issued, and if it does not return an error, the Server enters in Command mode where capabilities changes take effect, all commands (except qmp_capabilities) are allowed and asynchronous messages are delivered.

5 Compatibility Considerations

All protocol changes or new features which modify the protocol format in an incompatible way are disabled by default and will be advertised by the capabilities array (section ‘2.2 Server Greeting’). Thus, Clients can check that array and enable the capabilities they support.

The QMP Server performs a type check on the arguments to a command. It generates an error if a value does not have the expected type for its key, or if it does not understand a key that the Client included. The strictness of the Server catches wrong assumptions of Clients about the Server’s schema. Clients can assume that, when such validation errors occur, they will be reported before the command generated any side effect.

However, Clients must not assume any particular:

  • Length of json-arrays
  • Size of json-objects; in particular, future versions of QEMU may add new keys and Clients should be able to ignore them.
  • Order of json-object members or json-array elements
  • Amount of errors generated by a command, that is, new errors can be added to any existing command in newer versions of the Server

Any command or member name beginning with “x-” is deemed experimental, and may be withdrawn or changed in an incompatible manner in a future release.

Of course, the Server does guarantee to send valid JSON. But apart from this, a Client should be “conservative in what they send, and liberal in what they accept”.

6. Downstream extension of QMP

We recommend that downstream consumers of QEMU do not modify QMP. Management tools should be able to support both upstream and downstream versions of QMP without special logic, and downstream extensions are inherently at odds with that.

However, we recognize that it is sometimes impossible for downstreams to avoid modifying QMP. Both upstream and downstream need to take care to preserve long-term compatibility and interoperability.

To help with that, QMP reserves JSON object member names beginning with ‘__’ (double underscore) for downstream use (“downstream names”). This means upstream will never use any downstream names for its commands, arguments, errors, asynchronous events, and so forth.

Any new names downstream wishes to add must begin with ‘__’. To ensure compatibility with other downstreams, it is strongly recommended that you prefix your downstream names with ‘__RFQDN_’ where RFQDN is a valid, reverse fully qualified domain name which you control. For example, a qemu-kvm specific monitor command would be:

(qemu) __org.linux-kvm_enable_irqchip

Downstream must not change the server greeting (section 2.2) other than to offer additional capabilities. But see below for why even that is discouraged.

Section ‘5 Compatibility Considerations’ applies to downstream as well as to upstream, obviously. It follows that downstream must behave exactly like upstream for any input not containing members with downstream names (“downstream members”), except it may add members with downstream names to its output.

Thus, a client should not be able to distinguish downstream from upstream as long as it doesn’t send input with downstream members, and properly ignores any downstream members in the output it receives.

Advice on downstream modifications:

  1. Introducing new commands is okay. If you want to extend an existing command, consider introducing a new one with the new behaviour instead.
  2. Introducing new asynchronous messages is okay. If you want to extend an existing message, consider adding a new one instead.
  3. Introducing new errors for use in new commands is okay. Adding new errors to existing commands counts as extension, so 1. applies.
  4. New capabilities are strongly discouraged. Capabilities are for evolving the basic protocol, and multiple diverging basic protocol dialects are most undesirable.