Domain capabilities XML format

Overview

Sometimes, when a new domain is to be created it may come handy to know the capabilities of the hypervisor so the correct combination of devices and drivers is used. For example, when management application is considering the mode for a host device's passthrough there are several options depending not only on host, but on hypervisor in question too. If the hypervisor is qemu then it needs to be more recent to support VFIO, while legacy KVM is achievable just fine with older qemus.

The main difference between virConnectGetCapabilities and the emulator capabilities API is, the former one aims more on the host capabilities (e.g. NUMA topology, security models in effect, etc.) while the latter one specializes on the hypervisor capabilities.

While the Driver Capabilities provides the host capabilities (e.g NUMA topology, security models in effect, etc.), the Domain Capabilities provides the hypervisor specific capabilities for Management Applications to query and make decisions regarding what to utilize.

The Domain Capabilities can provide information such as the correct combination of devices and drivers that are supported. Knowing which host and hypervisor specific options are available or supported would allow the management application to choose an appropriate mode for a pass-through host device as well as which adapter to utilize.

Element and attribute overview

A new query interface was added to the virConnect API's to retrieve the XML listing of the set of domain capabilities (Since 1.2.7):

virConnectGetDomainCapabilities

The root element that emulator capability XML document starts with has name domainCapabilities. It contains at least four direct child elements:

<domainCapabilities>
  <path>/usr/bin/qemu-system-x86_64</path>
  <domain>kvm</domain>
  <machine>pc-i440fx-2.1</machine>
  <arch>x86_64</arch>
  ...
</domainCapabilities>

path: The full path to the emulator binary.
domain: Describes the virtualization type (or so called domain type).
machine: The domain's machine type.
arch: The domain's architecture.

CPU Allocation

Before any devices capability occurs, there might be a info on domain wide capabilities, e.g. virtual CPUs:

<domainCapabilities>
  ...
  <vcpu max='255'/>
  ...
</domainCapabilities>

vcpu: The maximum number of supported virtual CPUs

BIOS bootloader

Sometimes users might want to tweak some BIOS knobs or use UEFI. For cases like that, os element exposes what values can be passed to its children.

<domainCapabilities>
  ...
  <os supported='yes'>
    <loader supported='yes'>
      <value>/usr/share/OVMF/OVMF_CODE.fd</value>
      <enum name='type'>
        <value>rom</value>
        <value>pflash</value>
      </enum>
      <enum name='readonly'>
        <value>yes</value>
        <value>no</value>
      </enum>
    </loader>
  </os>
  ...
<domainCapabilities>

For the loader element, the following can occur:

value: List of known loader paths. Currently this is only used to advertise known locations of OVMF binaries for qemu. Binaries will only be listed if they actually exist on disk.
type: Whether loader is a typical BIOS (rom) or an UEFI binary (pflash). This refers to type attribute of the <loader/> element.
readonly: Options for the readonly attribute of the <loader/> element.

Devices

The final set of XML elements describe the supported devices and their capabilities. All devices occur as children of the main devices element.

<domainCapabilities>
  ...
  <devices>
    <disk supported='yes'>
      <enum name='diskDevice'>
        <value>disk</value>
        <value>cdrom</value>
        <value>floppy</value>
        <value>lun</value>
      </enum>
      ...
    </disk>
    <hostdev supported='no'/>
  </devices>
</domainCapabilities>

Reported capabilities are expressed as an enumerated list of available options for each of the element or attribute. For example, the <disk/> element has an attribute device which can support the values disk, cdrom, floppy, or lun.

Hard drives, floppy disks, CDROMs

Disk capabilities are exposed under disk element. For instance:

<domainCapabilities>
  ...
  <devices>
    <disk supported='yes'>
      <enum name='diskDevice'>
        <value>disk</value>
        <value>cdrom</value>
        <value>floppy</value>
        <value>lun</value>
      </enum>
      <enum name='bus'>
        <value>ide</value>
        <value>fdc</value>
        <value>scsi</value>
        <value>virtio</value>
        <value>xen</value>
        <value>usb</value>
        <value>uml</value>
        <value>sata</value>
        <value>sd</value>
      </enum>
    </disk>
    ...
  </devices>
</domainCapabilities>

diskDevice: Options for the device attribute of the <disk/> element.
bus: Options for the bus attribute of the <target/> element for a <disk/>.

Host device assignment

Some host devices can be passed through to a guest (e.g. USB, PCI and SCSI). Well, only if the following is enabled:

<domainCapabilities>
  ...
  <devices>
    <hostdev supported='yes'>
      <enum name='mode'>
        <value>subsystem</value>
        <value>capabilities</value>
      </enum>
      <enum name='startupPolicy'>
        <value>default</value>
        <value>mandatory</value>
        <value>requisite</value>
        <value>optional</value>
      </enum>
      <enum name='subsysType'>
        <value>usb</value>
        <value>pci</value>
        <value>scsi</value>
      </enum>
      <enum name='capsType'>
        <value>storage</value>
        <value>misc</value>
        <value>net</value>
      </enum>
      <enum name='pciBackend'>
        <value>default</value>
        <value>kvm</value>
        <value>vfio</value>
        <value>xen</value>
      </enum>
    </hostdev>
  </devices>
</domainCapabilities>

mode: Options for the mode attribute of the <hostdev/> element.
startupPolicy: Options for the startupPolicy attribute of the <hostdev/> element.
subsysType: Options for the type attribute of the <hostdev/> element in case of mode="subsystem".
capsType: Options for the type attribute of the <hostdev/> element in case of mode="capabilities".
pciBackend: Options for the name attribute of the <driver/> element.