The Qemu Guest Agent permits access to a VM via a virtual serial socket. This has the advantage that the VM can be accessed via the VM node (hypervisor) without network connection or remote desktop protocol.

This is a requirement to be able to take disk-only-snapshots without downtime by issueing a disk-freeze (shadow-copy) within the guest prior to taking the snapshot.

Furthermore it is useful for orchestrating CPU and memory hotplugging:

• guest OS must be told to enable CPU/memory after hot-plug
• guest OS must release a CPU/memory prior to hot-unplug

The XML must be amended by the following XML snippet in the devices section:

<channel type="unix">
  <source mode="bind"/>
  <target type="virtio" name="org.qemu.guest_agent.0"/>
</channel>

This will create a new virtual serial device within the VM and a new socket under /var/lib/libvirt/qemu/channel/target/

Make sure that this directory exists and is protected:

mkdir -p /var/lib/libvirt/qemu/channel/target/
chown root:qemu /var/lib/libvirt/qemu/channel/target/
chmod 770 /var/lib/libvirt/qemu/channel/target/

The sockets are named /var/lib/libvirt/qemu/channel/target/${VMNAME}.org.qemu.guest_agent.0.

Please note: since libvirt will automatically listen on that socket one can not use a tool like qemu-ga-client to do anything on it, everything has to go via libvirt.

In the VM one must install and start the qemu-guest-agent.

On Gentoo this means:

emerge qemu-guest-agent
/etc/init.d/qemu-guest-agentqemu-guest-agent
rc-update add qemu-guest-agent default

See the links for an example on how to read a file (it is non-trivial and the returned content is base64-encoded, so it must be scripted and there is no point in duplicating it here).

Shutdown a VM via qemu-ga

The advantage of using qemu-ga instead of the ACPI-based mechanism to shutdown a VM is that you get a confirmation if the qemu-ga was able to issue shutdown -P within the VM whereas with ACPI you won't even know whether the guest OS has received the event.

virsh shutdown --mode agent $VMNAME

Example:

~ # virsh shutdown --mode agent bc58f697-1f21-4613-9f4d-469cdaff0621Domain bc58f697-1f21-4613-9f4d-469cdaff0621 is being shutdown

Manually ping qemu-ga via libvirt

Libvirt issues a guest-sync before any other command as a way to ensure availability of qemu-ga. Nevertheless it may be useful to run commands directly, like the guest-ping.

virsh qemu-agent-command $VMNAME '{"execute":"guest-ping"}'

Example:

~ # virsh qemu-agent-command bc58f697-1f21-4613-9f4d-469cdaff0621 '{"execute":"guest-ping"}'

{"return":{}}

Getting list of available qemu-ga commands

VMNAME=
virsh qemu-agent-command $VMNAME '{"execute":"guest-info"}'

Example:

~ # virsh qemu-agent-command bc58f697-1f21-4613-9f4d-469cdaff0621 '{"execute":"guest-info"}'

{"return":
  {"version":"2.1.2","supported_commands":[
    {"enabled":true,"name":"guest-set-vcpus","success-response":true},
    {"enabled":true,"name":"guest-get-vcpus","success-response":true},
    {"enabled":true,"name":"guest-network-get-interfaces","success-response":true},
    {"enabled":true,"name":"guest-suspend-hybrid","success-response":false},
    {"enabled":true,"name":"guest-suspend-ram","success-response":false},
    {"enabled":true,"name":"guest-suspend-disk","success-response":false},
    {"enabled":true,"name":"guest-fstrim","success-response":true},
    {"enabled":true,"name":"guest-fsfreeze-thaw","success-response":true},
    {"enabled":true,"name":"guest-fsfreeze-freeze","success-response":true},
    {"enabled":true,"name":"guest-fsfreeze-status","success-response":true},
    {"enabled":true,"name":"guest-file-flush","success-response":true},
    {"enabled":true,"name":"guest-file-seek","success-response":true},
    {"enabled":true,"name":"guest-file-write","success-response":true},
    {"enabled":true,"name":"guest-file-read","success-response":true},
    {"enabled":true,"name":"guest-file-close","success-response":true},
    {"enabled":true,"name":"guest-file-open","success-response":true},
    {"enabled":true,"name":"guest-shutdown","success-response":false},
    {"enabled":true,"name":"guest-info","success-response":true},
    {"enabled":true,"name":"guest-set-time","success-response":true},
    {"enabled":true,"name":"guest-get-time","success-response":true},
    {"enabled":true,"name":"guest-ping","success-response":true},
    {"enabled":true,"name":"guest-sync","success-response":true},
    {"enabled":true,"name":"guest-sync-delimited","success-response":true}
  ]}
}

• http://wiki.libvirt.org/page/Qemu_guest_agent
• http://wiki.qemu.org/Features/QAPI/GuestAgent