libvirt: Driver capabilities XML format
Description: libvirt, virtualization, virtualization API

# Driver capabilities XML format

- Element and attribute overview
  - Host capabilities
  - Guest capabilities
  - Examples

## 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. Possible subelements are:
  pae
    If present, 32-bit guests can use PAE address space extensions, since 0.4.1
  nonpae
    If present, 32-bit guests can be run without requiring PAE, since 0.4.1
  ia64_be
    If present, IA64 guests can be run in big-endian mode, since 0.4.1
  acpi
    If this element is present, the `default` attribute describes whether the hypervisor exposes ACPI to the guest by default, and the `toggle` attribute describes whether the user can override this default. Since 0.4.1
  apic
    If this element is present, the `default` attribute describes whether the hypervisor exposes APIC to the guest by default, and the `toggle` attribute describes whether the user can override this default. Since 0.4.1
  cpuselection
    If this element is present, the hypervisor supports the `<cpu>` element within a domain definition for fine-grained control over the CPU presented to the guest. Since 0.7.5
  deviceboot
    If this element is present, the `<boot order='...'/>` element can be used inside devices, rather than the older boot specification by category. Since 0.8.8
  disksnapshot
    If this element is present, the `default` attribute describes whether external disk snapshots are supported. If absent, external snapshots may still be supported, but it requires attempting the API and checking for an error to find out for sure. Since 1.2.3

### 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>