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diff --git a/src/docs/qmp-spec.txt b/src/docs/qmp-spec.txt new file mode 100644 index 0000000..4fb10a5 --- /dev/null +++ b/src/docs/qmp-spec.txt @@ -0,0 +1,341 @@ + QEMU Machine Protocol Specification + +0. About This Document +====================== + +Copyright (C) 2009-2015 Red Hat, Inc. + +This work is licensed under the terms of the GNU GPL, version 2 or +later. See the COPYING file in the top-level directory. + +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: + +http://www.ietf.org/rfc/rfc7159.txt + +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. + +2.5 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 field 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. |
