Documentation/QMP
QEMU Machine Protocol
The QEMU Machine Protocol (QMP) is a JSON-based protocol which allows applications to control a QEMU instance.
Features:
- Lightweight, text-based, easy to parse data format
- Asynchronous messages support (events)
- Capabilities negotiation
- API/ABI stability guarantees
Please, also check the README file.
Examples
The first example explains some important details about QMP. The others are simpler and run on top of the first one.
In all examples 'C' stands for 'Client' and 'S' stands for 'Server'.
Capabilities Negotiation
When a new QMP connection is established, QMP sends its greeting message and enters capabilities negotiation mode. In this mode, only the qmp_capabilities command works. To exit capabilities negotiation mode and enter command mode, the qmp_capabilities command must be issued:
S: {
"QMP": {
"version": {
"qemu": {
"micro": 0,
"minor": 6,
"major": 1
},
"package": ""
},
"capabilities": [
]
}
}
C: { "execute": "qmp_capabilities" }
S: { "return": {}}
The { "return": {} } response is QMP's success response. An error response will contain the "error" keyword instead of "return".
Eject a medium
C: { "execute": "eject", "arguments": { "device": "ide1-cd0" } }
S: { "return": {}}
Query VM status
C: { "execute": "query-status" }
S: {
"return": {
"status": "running",
"singlestep": false,
"running": true
}
}
Asynchronous message
S: { "event": "BLOCK_IO_ERROR",
"data": { "device": "ide0-hd1",
"operation": "write",
"action": "stop" },
"timestamp": { "seconds": 1265044230, "microseconds": 450486 } }
Development
Main developer in charge is Luiz Capitulino. All QMP-related discussions happen on the qemu-devel mailing list.
Luiz's QMP queue can be found at:
http://repo.or.cz/w/qemu/qmp-unstable.git/shortlog/refs/heads/queue/qmp
IMPORTANT: This branch is constantly rebased!
TODO
In order of importance.
Drop the qmp-commands.hx file
The qmp-commands.hx file is basically a command table. Today, with the advent of the QAPI, the qapi-schema.json file serves the same purpose. We should drop qmp-commands.hx in favor of qapi-schema.json.
This is more or less what it takes to do that:
- Convert do_device_add() and do_qmp_capabilities() to propagate errors (ie, fill an Error ** object instead of doing qerror_report() calls)
- Do something about qmp_netdev_add(), qmp_qom_get() and qmp_qom_set() (drop them or add QAPI wrappers?)
- Change QMP code in monitor.c to use the QAPI to do command parameter validation and command dispatching (just like qemu-ga does)
- Move QMP examples from qmp-commands.hx to qapi-schema.json
- Drop qmp-commands.hx
Add full introspection support
This is about adding a command which returns all QMP types, commands (input and output parameters) and events.
Amos Kong is working on this:
http://marc.info/?l=qemu-devel&m=137059998004419&w=2
Add events support to the QAPI
Today events are exclusively part of the text protocol. This means that they are open coded, QEMU subsystems can't make use of them and they are not listed in qapi-schema.json.
We would solve all those problems by adding event support to the QAPI.
QMP testing
There should be a QMP test-suite. Either in QEMU itself or externally, like in kvm-autotest (which seems to be the best option).
HMP command conversion to the QAPI
Our long term plan is to make HMP, the human monitor, independent of QEMU. This means that HMP code shouldn't call QEMU code other than the monitor.
To achieve this we have to split existing HMP-only commands (eg. do_wav_capture() or do_ioport_read()) into a QMP part and an HMP part. Just like new QMP commands.
Trying it
By hand
1. Start QMP on a TCP socket, so that telnet can be used
# qemu [...] -qmp tcp:localhost:4444,server,nowait
2. Run telnet
$ telnet localhost 4444
3. You should see QMP's greeting banner
{"QMP": {"version": {"qemu": {"micro": 0, "minor": 6, "major": 1}, "package": ""}, "capabilities": []}}
4. Issue the qmp_capabilities command, so that QMP enters command mode
{ "execute": "qmp_capabilities" }
5. You can now issue commands. For example, to get a list of QMP supported commands, issue query-commands
{ "execute": "query-commands" }
There's an optimization to this procedure in case you plan to use it often:
1. Install programs socat and rlwrap. If you're running Fedora, you can do
# yum install socat rlwrap
2. Add the following sections to your QEMU config file (or create a qemu-qmp.conf one):
[chardev "qmp"] backend = "socket" path = "path to the QMP unix socket" server = "on" wait = "off" [mon "qmp"] mode = "control" chardev = "qmp" pretty = "on"
3. Run QEMU
# qemu [...] -readconfig qemu-qmp.conf
4. Run rlwrap
# rlwrap -C qmp socat STDIO UNIX:path-to-the-QMP-unix-socket
You can now issue commands, rlwrap will give you readline support (including persistent history).
qmp-shell script
This script is available under the QMP directory in QEMU's source-tree. It automates a bit the testing work, as it can construct commands objects for you.
1. Start QMP on a unix socket
# qemu [...] -qmp unix:./qmp-sock,server
2. Run the script
# qmp-shell ./qmp-sock
3. You should get the following prompt
(QEMU)
4. You can now issue commands. For example, let's add a new device
(QEMU) device_add driver=e1000 id=net1