QMP: QMP/ -> docs/qmp/

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

3 files changed, 872 insertions, 0 deletions

diff --git a/docs/qmp/README b/docs/qmp/README
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+                          QEMU Monitor Protocol
+                          =====================
+
+Introduction
+-------------
+
+The QEMU Monitor Protocol (QMP) allows applications to communicate with
+QEMU's Monitor.
+
+QMP is JSON[1] based and currently has the following features:
+
+- Lightweight, text-based, easy to parse data format
+- Asynchronous messages support (ie. events)
+- Capabilities Negotiation
+
+For detailed information on QMP's usage, please, refer to the following files:
+
+o qmp-spec.txt      QEMU Monitor Protocol current specification
+o qmp-commands.txt  QMP supported commands (auto-generated at build-time)
+o qmp-events.txt    List of available asynchronous events
+
+There is also a simple Python script called 'qmp-shell' available.
+
+IMPORTANT: It's strongly recommended to read the 'Stability Considerations'
+section in the qmp-commands.txt file before making any serious use of QMP.
+
+
+[1] http://www.json.org
+
+Usage
+-----
+
+To enable QMP, you need a QEMU monitor instance in "control mode". There are
+two ways of doing this.
+
+The simplest one is using the '-qmp' command-line option. The following
+example makes QMP available on localhost port 4444:
+
+  $ qemu [...] -qmp tcp:localhost:4444,server
+
+However, in order to have more complex combinations, like multiple monitors,
+the '-mon' command-line option should be used along with the '-chardev' one.
+For instance, the following example creates one user monitor on stdio and one
+QMP monitor on localhost port 4444.
+
+   $ qemu [...] -chardev stdio,id=mon0 -mon chardev=mon0,mode=readline \
+                -chardev socket,id=mon1,host=localhost,port=4444,server \
+                -mon chardev=mon1,mode=control
+
+Please, refer to QEMU's manpage for more information.
+
+Simple Testing
+--------------
+
+To manually test QMP one can connect with telnet and issue commands by hand:
+
+$ telnet localhost 4444
+Trying 127.0.0.1...
+Connected to localhost.
+Escape character is '^]'.
+{"QMP": {"version": {"qemu": {"micro": 50, "minor": 13, "major": 0}, "package": ""}, "capabilities": []}}
+{ "execute": "qmp_capabilities" }
+{"return": {}}
+{ "execute": "query-version" }
+{"return": {"qemu": {"micro": 50, "minor": 13, "major": 0}, "package": ""}}
+
+Development Process
+-------------------
+
+When changing QMP's interface (by adding new commands, events or modifying
+existing ones) it's mandatory to update the relevant documentation, which is
+one (or more) of the files listed in the 'Introduction' section*.
+
+Also, it's strongly recommended to send the documentation patch first, before
+doing any code change. This is so because:
+
+  1. Avoids the code dictating the interface
+
+  2. Review can improve your interface.  Letting that happen before
+     you implement it can save you work.
+
+* The qmp-commands.txt file is generated from the qmp-commands.hx one, which
+  is the file that should be edited.
+
+Homepage
+--------
+
+http://wiki.qemu.org/QMP
diff --git a/docs/qmp/qmp-events.txt b/docs/qmp/qmp-events.txt
new file mode 100644
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--- /dev/null
+++ b/docs/qmp/qmp-events.txt
@@ -0,0 +1,502 @@
+                   QEMU Monitor Protocol Events
+                   ============================
+
+BALLOON_CHANGE
+--------------
+
+Emitted when the guest changes the actual BALLOON level. This
+value is equivalent to the 'actual' field return by the
+'query-balloon' command
+
+Data:
+
+- "actual": actual level of the guest memory balloon in bytes (json-number)
+
+Example:
+
+{ "event": "BALLOON_CHANGE",
+    "data": { "actual": 944766976 },
+    "timestamp": { "seconds": 1267020223, "microseconds": 435656 } }
+
+BLOCK_IMAGE_CORRUPTED
+---------------------
+
+Emitted when a disk image is being marked corrupt.
+
+Data:
+
+- "device": Device name (json-string)
+- "msg":    Informative message (e.g., reason for the corruption) (json-string)
+- "offset": If the corruption resulted from an image access, this is the access
+            offset into the image (json-int)
+- "size":   If the corruption resulted from an image access, this is the access
+            size (json-int)
+
+Example:
+
+{ "event": "BLOCK_IMAGE_CORRUPTED",
+    "data": { "device": "ide0-hd0",
+        "msg": "Prevented active L1 table overwrite", "offset": 196608,
+        "size": 65536 },
+    "timestamp": { "seconds": 1378126126, "microseconds": 966463 } }
+
+BLOCK_IO_ERROR
+--------------
+
+Emitted when a disk I/O error occurs.
+
+Data:
+
+- "device": device name (json-string)
+- "operation": I/O operation (json-string, "read" or "write")
+- "action": action that has been taken, it's one of the following (json-string):
+    "ignore": error has been ignored
+    "report": error has been reported to the device
+    "stop": error caused VM to be stopped
+
+Example:
+
+{ "event": "BLOCK_IO_ERROR",
+    "data": { "device": "ide0-hd1",
+              "operation": "write",
+              "action": "stop" },
+    "timestamp": { "seconds": 1265044230, "microseconds": 450486 } }
+
+Note: If action is "stop", a STOP event will eventually follow the
+BLOCK_IO_ERROR event.
+
+BLOCK_JOB_CANCELLED
+-------------------
+
+Emitted when a block job has been cancelled.
+
+Data:
+
+- "type":     Job type (json-string; "stream" for image streaming
+                                     "commit" for block commit)
+- "device":   Device name (json-string)
+- "len":      Maximum progress value (json-int)
+- "offset":   Current progress value (json-int)
+              On success this is equal to len.
+              On failure this is less than len.
+- "speed":    Rate limit, bytes per second (json-int)
+
+Example:
+
+{ "event": "BLOCK_JOB_CANCELLED",
+     "data": { "type": "stream", "device": "virtio-disk0",
+               "len": 10737418240, "offset": 134217728,
+               "speed": 0 },
+     "timestamp": { "seconds": 1267061043, "microseconds": 959568 } }
+
+BLOCK_JOB_COMPLETED
+-------------------
+
+Emitted when a block job has completed.
+
+Data:
+
+- "type":     Job type (json-string; "stream" for image streaming
+                                     "commit" for block commit)
+- "device":   Device name (json-string)
+- "len":      Maximum progress value (json-int)
+- "offset":   Current progress value (json-int)
+              On success this is equal to len.
+              On failure this is less than len.
+- "speed":    Rate limit, bytes per second (json-int)
+- "error":    Error message (json-string, optional)
+              Only present on failure.  This field contains a human-readable
+              error message.  There are no semantics other than that streaming
+              has failed and clients should not try to interpret the error
+              string.
+
+Example:
+
+{ "event": "BLOCK_JOB_COMPLETED",
+     "data": { "type": "stream", "device": "virtio-disk0",
+               "len": 10737418240, "offset": 10737418240,
+               "speed": 0 },
+     "timestamp": { "seconds": 1267061043, "microseconds": 959568 } }
+
+BLOCK_JOB_ERROR
+---------------
+
+Emitted when a block job encounters an error.
+
+Data:
+
+- "device": device name (json-string)
+- "operation": I/O operation (json-string, "read" or "write")
+- "action": action that has been taken, it's one of the following (json-string):
+    "ignore": error has been ignored, the job may fail later
+    "report": error will be reported and the job canceled
+    "stop": error caused job to be paused
+
+Example:
+
+{ "event": "BLOCK_JOB_ERROR",
+    "data": { "device": "ide0-hd1",
+              "operation": "write",
+              "action": "stop" },
+    "timestamp": { "seconds": 1265044230, "microseconds": 450486 } }
+
+BLOCK_JOB_READY
+---------------
+
+Emitted when a block job is ready to complete.
+
+Data:
+
+- "device": device name (json-string)
+
+Example:
+
+{ "event": "BLOCK_JOB_READY",
+    "data": { "device": "ide0-hd1" },
+    "timestamp": { "seconds": 1265044230, "microseconds": 450486 } }
+
+Note: The "ready to complete" status is always reset by a BLOCK_JOB_ERROR
+event.
+
+DEVICE_DELETED
+-----------------
+
+Emitted whenever the device removal completion is acknowledged
+by the guest.
+At this point, it's safe to reuse the specified device ID.
+Device removal can be initiated by the guest or by HMP/QMP commands.
+
+Data:
+
+- "device": device name (json-string, optional)
+- "path": device path (json-string)
+
+{ "event": "DEVICE_DELETED",
+  "data": { "device": "virtio-net-pci-0",
+            "path": "/machine/peripheral/virtio-net-pci-0" },
+  "timestamp": { "seconds": 1265044230, "microseconds": 450486 } }
+
+DEVICE_TRAY_MOVED
+-----------------
+
+It's emitted whenever the tray of a removable device is moved by the guest
+or by HMP/QMP commands.
+
+Data:
+
+- "device": device name (json-string)
+- "tray-open": true if the tray has been opened or false if it has been closed
+               (json-bool)
+
+{ "event": "DEVICE_TRAY_MOVED",
+  "data": { "device": "ide1-cd0",
+            "tray-open": true
+  },
+  "timestamp": { "seconds": 1265044230, "microseconds": 450486 } }
+
+NIC_RX_FILTER_CHANGED
+-----------------
+
+The event is emitted once until the query command is executed,
+the first event will always be emitted.
+
+Data:
+
+- "name": net client name (json-string)
+- "path": device path (json-string)
+
+{ "event": "NIC_RX_FILTER_CHANGED",
+  "data": { "name": "vnet0",
+            "path": "/machine/peripheral/vnet0/virtio-backend" },
+  "timestamp": { "seconds": 1368697518, "microseconds": 326866 } }
+}
+
+RESET
+-----
+
+Emitted when the Virtual Machine is reseted.
+
+Data: None.
+
+Example:
+
+{ "event": "RESET",
+    "timestamp": { "seconds": 1267041653, "microseconds": 9518 } }
+
+RESUME
+------
+
+Emitted when the Virtual Machine resumes execution.
+
+Data: None.
+
+Example:
+
+{ "event": "RESUME",
+    "timestamp": { "seconds": 1271770767, "microseconds": 582542 } }
+
+RTC_CHANGE
+----------
+
+Emitted when the guest changes the RTC time.
+
+Data:
+
+- "offset": Offset between base RTC clock (as specified by -rtc base), and
+new RTC clock value (json-number)
+
+Example:
+
+{ "event": "RTC_CHANGE",
+    "data": { "offset": 78 },
+    "timestamp": { "seconds": 1267020223, "microseconds": 435656 } }
+
+SHUTDOWN
+--------
+
+Emitted when the Virtual Machine is powered down.
+
+Data: None.
+
+Example:
+
+{ "event": "SHUTDOWN",
+    "timestamp": { "seconds": 1267040730, "microseconds": 682951 } }
+
+Note: If the command-line option "-no-shutdown" has been specified, a STOP
+event will eventually follow the SHUTDOWN event.
+
+SPICE_CONNECTED, SPICE_DISCONNECTED
+-----------------------------------
+
+Emitted when a SPICE client connects or disconnects.
+
+Data:
+
+- "server": Server information (json-object)
+  - "host": IP address (json-string)
+  - "port": port number (json-string)
+  - "family": address family (json-string, "ipv4" or "ipv6")
+- "client": Client information (json-object)
+  - "host": IP address (json-string)
+  - "port": port number (json-string)
+  - "family": address family (json-string, "ipv4" or "ipv6")
+
+Example:
+
+{ "timestamp": {"seconds": 1290688046, "microseconds": 388707},
+  "event": "SPICE_CONNECTED",
+  "data": {
+    "server": { "port": "5920", "family": "ipv4", "host": "127.0.0.1"},
+    "client": {"port": "52873", "family": "ipv4", "host": "127.0.0.1"}
+}}
+
+SPICE_INITIALIZED
+-----------------
+
+Emitted after initial handshake and authentication takes place (if any)
+and the SPICE channel is up'n'running
+
+Data:
+
+- "server": Server information (json-object)
+  - "host": IP address (json-string)
+  - "port": port number (json-string)
+  - "family": address family (json-string, "ipv4" or "ipv6")
+  - "auth": authentication method (json-string, optional)
+- "client": Client information (json-object)
+  - "host": IP address (json-string)
+  - "port": port number (json-string)
+  - "family": address family (json-string, "ipv4" or "ipv6")
+  - "connection-id": spice connection id.  All channels with the same id
+                     belong to the same spice session (json-int)
+  - "channel-type": channel type.  "1" is the main control channel, filter for
+                    this one if you want track spice sessions only (json-int)
+  - "channel-id": channel id.  Usually "0", might be different needed when
+                  multiple channels of the same type exist, such as multiple
+                  display channels in a multihead setup (json-int)
+  - "tls": whevener the channel is encrypted (json-bool)
+
+Example:
+
+{ "timestamp": {"seconds": 1290688046, "microseconds": 417172},
+  "event": "SPICE_INITIALIZED",
+  "data": {"server": {"auth": "spice", "port": "5921",
+                      "family": "ipv4", "host": "127.0.0.1"},
+           "client": {"port": "49004", "family": "ipv4", "channel-type": 3,
+                      "connection-id": 1804289383, "host": "127.0.0.1",
+                      "channel-id": 0, "tls": true}
+}}
+
+STOP
+----
+
+Emitted when the Virtual Machine is stopped.
+
+Data: None.
+
+Example:
+
+{ "event": "STOP",
+    "timestamp": { "seconds": 1267041730, "microseconds": 281295 } }
+
+SUSPEND
+-------
+
+Emitted when guest enters S3 state.
+
+Data: None.
+
+Example:
+
+{ "event": "SUSPEND",
+     "timestamp": { "seconds": 1344456160, "microseconds": 309119 } }
+
+SUSPEND_DISK
+------------
+
+Emitted when the guest makes a request to enter S4 state.
+
+Data: None.
+
+Example:
+
+{ "event": "SUSPEND_DISK",
+     "timestamp": { "seconds": 1344456160, "microseconds": 309119 } }
+
+Note: QEMU shuts down when entering S4 state.
+
+VNC_CONNECTED
+-------------
+
+Emitted when a VNC client establishes a connection.
+
+Data:
+
+- "server": Server information (json-object)
+  - "host": IP address (json-string)
+  - "service": port number (json-string)
+  - "family": address family (json-string, "ipv4" or "ipv6")
+  - "auth": authentication method (json-string, optional)
+- "client": Client information (json-object)
+  - "host": IP address (json-string)
+  - "service": port number (json-string)
+  - "family": address family (json-string, "ipv4" or "ipv6")
+
+Example:
+
+{ "event": "VNC_CONNECTED",
+    "data": {
+        "server": { "auth": "sasl", "family": "ipv4",
+                    "service": "5901", "host": "0.0.0.0" },
+        "client": { "family": "ipv4", "service": "58425",
+                    "host": "127.0.0.1" } },
+    "timestamp": { "seconds": 1262976601, "microseconds": 975795 } }
+
+
+Note: This event is emitted before any authentication takes place, thus
+the authentication ID is not provided.
+
+VNC_DISCONNECTED
+----------------
+
+Emitted when the connection is closed.
+
+Data:
+
+- "server": Server information (json-object)
+  - "host": IP address (json-string)
+  - "service": port number (json-string)
+  - "family": address family (json-string, "ipv4" or "ipv6")
+  - "auth": authentication method (json-string, optional)
+- "client": Client information (json-object)
+  - "host": IP address (json-string)
+  - "service": port number (json-string)
+  - "family": address family (json-string, "ipv4" or "ipv6")
+  - "x509_dname": TLS dname (json-string, optional)
+  - "sasl_username": SASL username (json-string, optional)
+
+Example:
+
+{ "event": "VNC_DISCONNECTED",
+    "data": {
+        "server": { "auth": "sasl", "family": "ipv4",
+                    "service": "5901", "host": "0.0.0.0" },
+        "client": { "family": "ipv4", "service": "58425",
+                    "host": "127.0.0.1", "sasl_username": "luiz" } },
+    "timestamp": { "seconds": 1262976601, "microseconds": 975795 } }
+
+VNC_INITIALIZED
+---------------
+
+Emitted after authentication takes place (if any) and the VNC session is
+made active.
+
+Data:
+
+- "server": Server information (json-object)
+  - "host": IP address (json-string)
+  - "service": port number (json-string)
+  - "family": address family (json-string, "ipv4" or "ipv6")
+  - "auth": authentication method (json-string, optional)
+- "client": Client information (json-object)
+  - "host": IP address (json-string)
+  - "service": port number (json-string)
+  - "family": address family (json-string, "ipv4" or "ipv6")
+  - "x509_dname": TLS dname (json-string, optional)
+  - "sasl_username": SASL username (json-string, optional)
+
+Example:
+
+{ "event": "VNC_INITIALIZED",
+    "data": {
+        "server": { "auth": "sasl", "family": "ipv4",
+                    "service": "5901", "host": "0.0.0.0"},
+        "client": { "family": "ipv4", "service": "46089",
+                    "host": "127.0.0.1", "sasl_username": "luiz" } },
+        "timestamp": { "seconds": 1263475302, "microseconds": 150772 } }
+
+WAKEUP
+------
+
+Emitted when the guest has woken up from S3 and is running.
+
+Data: None.
+
+Example:
+
+{ "event": "WATCHDOG",
+     "timestamp": { "seconds": 1344522075, "microseconds": 745528 } }
+
+WATCHDOG
+--------
+
+Emitted when the watchdog device's timer is expired.
+
+Data:
+
+- "action": Action that has been taken, it's one of the following (json-string):
+            "reset", "shutdown", "poweroff", "pause", "debug", or "none"
+
+Example:
+
+{ "event": "WATCHDOG",
+     "data": { "action": "reset" },
+     "timestamp": { "seconds": 1267061043, "microseconds": 959568 } }
+
+Note: If action is "reset", "shutdown", or "pause" the WATCHDOG event is
+followed respectively by the RESET, SHUTDOWN, or STOP events.
+
+GUEST_PANICKED
+--------------
+
+Emitted when guest OS panic is detected.
+
+Data:
+
+- "action": Action that has been taken (json-string, currently always "pause").
+
+Example:
+
+{ "event": "GUEST_PANICKED",
+     "data": { "action": "pause" } }
diff --git a/docs/qmp/qmp-spec.txt b/docs/qmp/qmp-spec.txt
new file mode 100644
index 0000000000..a27789692b
--- /dev/null
+++ b/docs/qmp/qmp-spec.txt
@@ -0,0 +1,282 @@
+           QEMU Monitor Protocol Specification - Version 0.1
+
+1. Introduction
+===============
+
+This document specifies the QEMU Monitor Protocol (QMP), a JSON-based protocol
+which is available for applications to control QEMU at the machine-level.
+
+To enable QMP support, QEMU has to be run in "control mode". This is done by
+starting QEMU with the appropriate command-line options. Please, refer to the
+QEMU manual page for more information.
+
+2. Protocol Specification
+=========================
+
+This section details the protocol format. For the purpose of this document
+"Client" is any application which is communicating with QEMU in control mode,
+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 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 success response is issued when the command execution has finished
+without errors.
+
+The format 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 error response is issued when the command execution could not be
+completed because of an error condition.
+
+The format 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 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": "0.12.50", "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 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.
+
+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.