Notes about QEMU monitor

Notes about QEMU monitor.

1. Introduction

The QEMU monitor is used to give complex commands to the QEMU emulator. You can use it to:

Remove or insert removable media images (such as CD-ROM or floppies).
Freeze/unfreeze the Virtual Machine (VM) and save or restore its state from a disk file.
Inspect the VM state without an external debugger
…

1.1 commands

1	(qemu) help

2. Usage

QEMU启动时，需要使用-monitor选项指定做为console设备，官方文档说明如下:

-monitor dev
    Redirect the monitor to host device dev (same devices as the serialport).
    The default device is vc in graphical mode and stdio in non graphical mode.
    Use -monitor none to disable the default monitor.

1 2	-serial mon:stdio creates a multiplexed stdio backend connected to the serial port and the QEMU monitor

1 2	(qemu) info kvm kvm support: enabled

上述这种方式更偏向用户直接输入命令进行交互，称为HMP(Human Machine Protocol)，程序使用这种方式不是太方便。QEMU还提供了另外一种基于JSON的QMP(QEMU Machine Protocol)来满足自动化处理的需求。Libvirt就是使用QMP来控制QEMU实例。

3. QMP

QMP规范可以参考:

QMP协议的工作流程如下:

连接建立后服务器发送欢迎信息，进入能力协商(capabilities negotiation)模式
客户端发送{“execute”:”qmp_capablities”}
成功则服务器返回{“return”:{}}，否则return中会含有error。
客户端发送命令
服务器以异步消息返回结果

3.1 Basic usage

1	$ qemu [...] -qmp tcp:localhost:4444,server=on,wait=off

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$ qemu [...] -chardev stdio,id=mon0 -mon chardev=mon0,mode=readline \
             -chardev socket,id=mon1,host=localhost,port=4444,server=on,wait=off \
             -mon chardev=mon1,mode=control,pretty=on

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": 0,
                "minor": 0,
                "major": 3
            },
            "package": "v3.0.0"
        },
        "capabilities": [
            "oob"
        ]
    }
}

3.2 qmp-shell

1	$ qemu [...] -qmp unix:/tmp/qmp-sock,server=on,wait=off

Run ‘qmp-shell’ (located in the source tree, under: “scripts/qmp/“) to connect to the just-launched QEMU:

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$ ./qmp-shell -p -v /tmp/qmp-sock
[...]
(QEMU)

Virtual CPU hotplug

参考资料:

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