Driver capabilities XML format

Element and attribute overview

As new virtualization engine support gets added to libvirt, and to handle cases like QEMU supporting a variety of emulations, a query interface has been added in 0.2.1 allowing to list the set of supported virtualization capabilities on the host:

    char * virConnectGetCapabilities (virConnectPtr conn);

The value returned is an XML document listing the virtualization capabilities of the host and virtualization engine to which @conn is connected. One can test it using virsh command line tool command 'capabilities', it dumps the XML associated to the current connection.

As can be seen seen in the example, the capabilities XML consists of the capabilities element which have exactly one host child element to report information on host capabilities, and zero or more guest element to express the set of architectures the host can run at the moment.

Host capabilities

The <host/> element consists of the following child elements:

uuid

The host UUID.

cpu

The host CPU architecture and features.

power_management

whether host is capable of memory suspend, disk hibernation, or hybrid suspend.

migration

This element exposes information on the hypervisor's migration capabilities, like live migration, supported URI transports, and so on.

topology

This element embodies the host internal topology. Management applications may want to learn this information when orchestrating new guests - e.g. due to reduce inter-NUMA node transfers.

secmodel

To find out default security labels for different security models you need to parse this element. In contrast with the former elements, this is repeated for each security model the libvirt daemon currently supports.

Guest capabilities

While the previous section aims at host capabilities, this one focuses on capabilities available to a guest using a given hypervisor. The <guest/> element will typically wrap up the following elements:

os_type

This expresses what kind of operating system the hypervisor is able to run. Possible values are:

xen

for XEN

linux

legacy alias for xen

hvm

Unmodified operating system

exe

Container based virtualization

uml

User Mode Linux

arch

This element brings some information on supported guest architecture.

features

This optional element encases possible features that can be used with a guest of described type.

Examples

For example, in the case of a 64-bit machine with hardware virtualization capabilities enabled in the chip and BIOS you will see:

<capabilities>
  <host>
    <cpu>
      <arch>x86_64</arch>
      <features>
        <vmx/>
      </features>
      <model>core2duo</model>
      <vendor>Intel</vendor>
      <topology sockets="1" cores="2" threads="1"/>
      <feature name="lahf_lm"/>
      <feature name='xtpr'/>
      ...
    </cpu>
    <power_management>
      <suspend_mem/>
      <suspend_disk/>
      <suspend_hybrid/>
    <power_management/>
  </host>

  <!-- xen-3.0-x86_64 -->
  <guest>
    <os_type>xen</os_type>
    <arch name="x86_64">
      <wordsize>64</wordsize>
      <domain type="xen"></domain>
      <emulator>/usr/lib64/xen/bin/qemu-dm</emulator>
    </arch>
    <features>
    </features>
  </guest>

  <!-- hvm-3.0-x86_32 -->
  <guest>
    <os_type>hvm</os_type>
    <arch name="i686">
      <wordsize>32</wordsize>
      <domain type="xen"></domain>
      <emulator>/usr/lib/xen/bin/qemu-dm</emulator>
      <machine>pc</machine>
      <machine>isapc</machine>
      <loader>/usr/lib/xen/boot/hvmloader</loader>
    </arch>
    <features>
      <cpuselection/>
      <deviceboot/>
    </features>
  </guest>
  ...
</capabilities>