MonitorProtocol

Monitor Protocol

The QEMU Monitor Protocol (QMP) project goal is to implement a JSON based protocol for QEMU.

Problem

QEMU does not provide a way for applications to control it at the "machine level", that is, the current existing interface (called Monitor) was designed for human interaction only.

This is very problematic for applications, for the following reasons:

• It is needed to parse output meant to be read by humans
• It is not reliable, because it is not always possible to know whether a command has been successfully executed or not
• It lacks support for asynchronous events

Solution

Improve QEMU's Monitor to support a machine protocol with the following features:

• Use a lightweight, text-based, easy to parse data format (JSON has been chosen)
• Protocol stability should be guaranteed
• Asynchronous events support
• Parallel execution of commands
• A standard way to query protocol capabilities

A draft specification already exists and can be found here.

Libqmp

Although this is still under discussion, we are also going to provide a C library with a stable ABI.

The short-term plan is to have a simple API to send commands and receive responses, this way C clients do not need to have their own JSON implementation.

The long-term plan is to provide a full management API for QEMU.

Development

The development takes place in the QMP unstable tree:

http://repo.or.cz/w/qemu/qmp-unstable.git

NOTE: all branches in this repository are constantly rebased.

Important branches:

• master: contains a functional QMP implementation for emission only (includes asynchronous messages). Protocol documentation can be found in the QMP directory
• conversions-qobject: output in QObject conversions
• conversions-qerror: error handling conversions

Project status (2009-11-10)

Most of the infrastructure has been merged, the fundamental missing parts are a JSON encoder/decoder and an error framework.

Patches exist for a JSON decoder (QJSON) and an error framework (QError), but they have not been merged yet.

Additionally, all command handlers need to be converted to use QObjects. This work is in progress, but most handlers depend on the JSON decoder to be merged.

Main building blocks

• QObject Layer (merged)
• QObject Monitor infrastructure (merged)
• JSON decoder (in progress)
• JSON encoder (not started)
• Error framework (under review)

Conversion work

• Input through QDict (merged)
• Output in QObject style (in progress)
• Error handling (not started)

Conversion work detailed status

	Command	Info
Handlers	61	35
Fully Converted	7	16
Percentage	11%	45.7%

The following tables have a per-function status. There is one table for command handlers and another one for info handlers.

Status can be:

• merged: already merged upstream
• converted: handler is converted but not submitted yet
• never fails: does not need error handling

NOTE: Handlers used by Libvirt are marked with yellow.

Command handlers

Handler name	Output in QObject	Error handling	Comments
do_acl_add()
do_acl_policy()
do_acl_remove()
do_acl_reset()
do_acl_show()
do_balloon()	merged	never fails
do_boot_set()
do_change()
do_closefd()	merged
do_commit()
do_cont()	merged	never fails	TODO: encrypted images
do_cpu_set()
do_delvm()
do_device_add()
do_device_del()
do_eject()	merged
do_gdbserver()
do_getfd()	merged
do_help_cmd()
do_info()	merged
do_inject_mce()
do_inject_nmi()
do_ioport_read()
do_ioport_write()
do_loadvm()
do_log()
do_logfile()
do_memory_dump()
do_memory_save()	merged
do_migrate()	merged
do_migrate_cancel()	merged	never fails
do_migrate_set_downtime()
do_migrate_set_speed()	merged	never fails
do_mouse_button()
do_mouse_move()
do_mouse_set()
do_pci_device_hot_remove()	merged
do_physical_memory_dump()
do_physical_memory_save()	merged
do_print()
do_quit()	merged
do_savevm()
do_screen_dump()
do_sendkey()
do_set_link()
do_singlestep()
do_stop()	merged	never fails
do_stop_capture()
do_sum()
do_system_powerdown()	merged	nerver fails
do_system_reset()	merged	never fails
do_usb_add()
do_usb_del()
do_watchdog_action()
do_wav_capture()
drive_hot_add()
net_host_device_add()
net_host_device_remove()
net_slirp_hostfwd_add()
net_slirp_hostfwd_remove()
pci_device_hot_add()	converted

Info handlers

Handler name	Status	Comments
bdrv_info()	converted
bdrv_info_stats()	converted
do_info_balloon()	merged
do_info_capture()
do_info_cpus()	merged
do_info_cpu_stats()
do_info_history()
do_info_hpet()	converted
do_info_jit()
do_info_kvm()	converted
do_info_mice()	converted
do_info_migrate()	converted
do_info_name()	converted
do_info_network()	converted
do_info_numa()
do_info_profile()
do_info_qdm()
do_info_qtree()
do_info_registers()
do_info_roms()
do_info_snapshots()
do_info_status()	converted
do_info_usernet()	converted
do_info_uuid()	converted
do_info_version()	merged
do_info_vnc()	converted
irq_info()
mem_info()
pci_info()
pcmcia_info()
pic_info()
qemu_chr_info()	converted
tlb_info()
usb_host_info()
usb_info()

History

This is the fourth proposal for a machine protocol, past discussions can be found in the following links:

• http://www.mail-archive.com/qemu-devel@nongnu.org/msg14593.html
• http://lists.gnu.org/archive/html/qemu-devel/2009-01/msg00655.html
• http://lists.gnu.org/archive/html/qemu-devel/2009-06/msg01584.html

Contact

Primary contact is Luiz Capitulino <lcapitulino at redhat dot com> but most project discussions happen on the QEMU-devel mailing list:

http://lists.nongnu.org/mailman/listinfo/qemu-devel