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

# Domain capabilities XML format

- Overview
- Element and attribute overview
  - CPU Allocation
  - BIOS bootloader
  - Devices
    - Hard drives, floppy disks, CDROMs
    - Host device assignment

## 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.