QEMU guest agent | Virtualization Guide

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20 QEMU guest agent #Edit source

20.1 Running QEMU GA commands
20.2 virsh commands that require QEMU GA
20.3 Enhancing libvirt commands
20.4 More information

The QEMU guest agent (GA) runs inside the VM Guest and allows the VM Host Server to run commands in the guest operating system via libvirt. It supports many functions—for example, getting details about guest file systems, freezing and thawing file systems, or suspending or rebooting a guest.

QEMU GA is included in the qemu-guest-agent package and is installed, configured, and activated by default on KVM virtual machines.

QEMU GA is installed in Xen virtual machines, but it is not activated by default. Although it is possible to use QEMU GA with Xen virtual machines, there is no integration with libvirt as described below for KVM virtual machines. To use QEMU GA with Xen a channel device must be added to the VM Guest configuration. The channel device includes a Unix domain socket path on the VM Host Server for communicating with QEMU GA.

<channel type='unix'>
  <source mode='bind' path='/example/path'/>
  <target type='xen' name='org.qemu.guest_agent.0'/>
</channel>

20.1 Running QEMU GA commands #Edit source

QEMU GA includes many native commands that do not have direct libvirt counterparts. Refer to Section 20.4, “More information” to find the complete list. You can run all of the QEMU GA commands by using libvirt's general purpose command qemu-agent-command:

virsh qemu-agent-command DOMAIN_NAME '{"execute":"QEMU_GA_COMMAND"}'

For example:

> sudo virsh qemu-agent-command sle15sp2 '{"execute":"guest-info"}' --pretty
{
"return": {
  "version": "4.2.0",
  "supported_commands": [
    {
      "enabled": true,
      "name": "guest-get-osinfo",
      "success-response": true
    },
[...]

20.2 `virsh` commands that require QEMU GA #Edit source

Several virsh commands require QEMU GA for their functionality. Here are some of them:

virsh guestinfo: Prints information about the guest from the guest's point of view.
virsh guestvcpus: Queries or changes the state of virtual CPUs from the guest's point of view.
virsh set-user-password: Sets the password for a user account in the guest.
virsh domfsinfo: Shows a list of mounted file systems within the running domain.
virsh dompmsuspend: Suspends a running guest.

20.3 Enhancing `libvirt` commands #Edit source

If QEMU GA is enabled inside the guest, a number of virsh subcommands have enhanced functionality when run in the agent mode. The following list includes only some examples of them. For a complete list, see the virsh manual page and search for the agent string.

virsh shutdown --mode agent and virsh reboot --mode agent: This method of shutting down or rebooting leaves the guest clean for its next run, similar to the ACPI method.
virsh domfsfreeze and virsh domfsthaw: Instructs the guest to make its file system quiescent—to flush all I/O operations in the cache and leave volumes in a consistent state, so that no checks will be needed when they are remounted.
virsh setvcpus --guest: Changes the number of CPUs assigned to a guest.
virsh domifaddr --source agent: Queries the QEMU GA for the guest's IP address.
virsh vcpucount --guest: Prints information about the virtual CPU counts from the perspective of the guest.

20.4 More information #Edit source

A complete list of commands supported by the QEMU GA is at https://www.qemu.org/docs/master/interop/qemu-ga-ref.html.
The virsh manual page (man 1 virsh) includes descriptions of the commands that support the QEMU GA interface.

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