Right when connected the Server will issue a greeting message, which signals
that the connection has been successfully established and that the Server is
-waiting for commands.
+ready for capabilities negotiation (for more information refer to section
+'4. Capabilities Negotiation').
The format is:
The format is:
-{ "error": { "class": json-string, "data": json-object, "desc": json-string },
- "id": json-value }
+{ "error": { "class": json-string, "desc": json-string }, "id": json-value }
Where,
-- The "class" member contains the error class name (eg. "ServiceUnavailable")
-- The "data" member contains specific error data and is defined in a
- per-command basis, it will be an empty json-object if the error has no data
+- 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
------------------
C: { "execute": }
-S: {"error": {"class": "JSONParsing", "desc": "Invalid JSON syntax", "data":
-{}}}
+S: {"error": {"class": "GenericError", "desc": "Invalid JSON syntax" } }
3.5 Powerdown event
-------------------
S: {"timestamp": {"seconds": 1258551470, "microseconds": 802384}, "event":
"POWERDOWN"}
-4. Compatibility Considerations
---------------------------------
+4. Capabilities Negotiation
+----------------------------
-In order to achieve maximum compatibility between versions, Clients must not
-assume any particular:
+When a Client successfully establishes a connection, the Server is in
+Capabilities Negotiation mode.
-- Size of json-objects or length of json-arrays
+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
-Additionally, Clients should always:
+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 downstram 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.
-- Check the capabilities json-array at connection time
-- Check the availability of commands with 'query-commands' before issuing them
+2. Introducing new asynchronous messages is okay. If you want to extend
+ an existing message, consider adding a new one instead.
-5. Recommendations to Client implementors
------------------------------------------
+3. Introducing new errors for use in new commands is okay. Adding new
+ errors to existing commands counts as extension, so 1. applies.
-5.1 The Server should be always started in pause mode, thus the Client is
- able to perform any setup procedure without the risk of race conditions
- and related problems
+4. New capabilities are strongly discouraged. Capabilities are for
+ evolving the basic protocol, and multiple diverging basic protocol
+ dialects are most undesirable.
projects / qemu.git / blobdiff