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QEMU Guest Agent

It is a daemon program running inside the domain which is supposed to help management applications with executing functions which need assistance of the guest OS. For example, freezing and thawing filesystems, entering suspend. However, guest agent (GA) is not bullet proof, and hostile guest OS can send spurious replies.

Setting QEMU GA up

Currently, QEMU exposes GA via virtio serial port. There are some attempts to allow applications to use qemu monitor for communication with GA, but virtio serial port should remain supported. These are internals, though.

To be able to use GA users needs to create virtio serial port with special name org.qemu.guest_agent.0. In other words, one needs to add this to his/her domain XML under <devices>:

<channel type='unix'>
   <source mode='bind' path='/var/lib/libvirt/qemu/f16x86_64.agent'/>
   <target type='virtio' name='org.qemu.guest_agent.0'/>
</channel>

If you use libvirt 1.0.6 or newer, you can omit the path='...' attribute of the <source> element, and libvirt will manage things automatically on your behalf.

Usage

Libvirt does not guarantee any support of direct use of the guest agent. If you don't mind using libvirt-qemu.so, you can use the virDomainQemuAgentCommand API (exposed by virsh qemu-agent-command); but be aware that this is unsupported, and any changes you make to the agent that change state behind libvirt's back may cause libvirt to misbehave. Meanwhile, the guest agent may be used internally by several supported libvirt APIs, often by passing flags to request its use. For example, virDomainShutdownFlags supports:

VIR_DOMAIN_SHUTDOWN_DEFAULT        = 0,
VIR_DOMAIN_SHUTDOWN_ACPI_POWER_BTN = (1 << 0),
VIR_DOMAIN_SHUTDOWN_GUEST_AGENT    = (1 << 1),

Simmilar applies for the reboot API:

VIR_DOMAIN_REBOOT_DEFAULT        = 0,
VIR_DOMAIN_REBOOT_ACPI_POWER_BTN = (1 << 0),
VIR_DOMAIN_REBOOT_GUEST_AGENT    = (1 << 1),

In virsh, users can select shutdown/reboot method via --mode acpi|agent:

virsh reboot --mode agent $DOMAIN
virsh shutdown --mode agent $DOMAIN

If users is creating snapshots he/she might want to freeze before and thaw filesystems after. This can be done by specifying VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE flag to virDomainSnapshotCreate API or:

virsh snapshot-create --quiesce $DOMAIN
virsh snapshot-create-as --quiesce $DOMAIN

in virsh.

Warning: Like any monitor command, GA commands can block indefinitely esp. when GA is not running. Currently, there is no reliable method/API to tell if GA is running. Libvirt implements some basic check to determine if there is a qemu-ga instance running or not. Basically a guest-sync command is issued prior every useful command. If it returns, then libvirt considers guest agent up and running and issues the real command. However, guest agent can die or be terminated meanwhile. The guest-sync command is issued with a timeout which if hit guest agent is considered as not present.

Configure guest agent without libvirt interference

In some cases, users might want to configure a guest agent in their domain XML but don't want libvirt to connect to guest agent socket at all. Because libvirt connects to a guest agent channel if and only if it is a virtio channel with org.qemu.guest_agent.0 name, all one need to do is void at least one of these conditions. However, the most feasible way is to change the target's name:

<channel type='unix'>
   <source mode='bind' path='/var/lib/libvirt/qemu/f16x86_64.agent'/>
   <target type='virtio' name='org.qemu.guest_agent.1'/>
</channel>

If you do change name, you'll need to tell it to qemu GA as well:

 # qemu-ga -p /dev/virtio-ports/$name

Guest requirements

One thing has been already mentioned - the domain configuration for virtio serial port. However, guest must have the GA installed. Fortunately, it is already distributed as rpm, so all one needs to do is:

yum install qemu-guest-agent

Related links

If you want to read further: