docs/qmp/qmp-spec.txt - qemu-android - Git at Google

                       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.

 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:

 http://www.ietf.org/rfc/rfc4627.txt

 For convenience, json-object members and json-array elements 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.

 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

 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
 - The "id" member is a transaction identification associated with the
   command execution, it is optional and will be part of the response if
   provided

 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-object, "id": json-value }

  Where,

 - The "return" member contains the command returned data, which is defined
   in a per-command basis or 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. 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

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

 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 Simple 'stop' execution
 ---------------------------

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

 3.3 KVM information
 -------------------

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

 3.4 Parsing error
 ------------------

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

 3.5 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

 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.
	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.

	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:

	http://www.ietf.org/rfc/rfc4627.txt

	For convenience, json-object members and json-array elements 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.

	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

	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
	- The "id" member is a transaction identification associated with the
	command execution, it is optional and will be part of the response if
	provided

	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-object, "id": json-value }

	Where,

	- The "return" member contains the command returned data, which is defined
	in a per-command basis or 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. 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

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

	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 Simple 'stop' execution
	---------------------------

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

	3.3 KVM information
	-------------------

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

	3.4 Parsing error
	------------------

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

	3.5 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

	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.