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 totypeattribute of the <loader/> element.
- readonly
- Options for the readonlyattribute of the <loader/> element.
CPU configuration ¶
      The cpu element exposes options usable for configuring
      guest CPUs.
    
<domainCapabilities>
  ...
  <cpu>
    <mode name='host-passthrough' supported='yes'/>
    <mode name='host-model' supported='yes'>
      <model fallback='allow'>Broadwell</model>
      <vendor>Intel</vendor>
      <feature policy='disable' name='aes'/>
      <feature policy='require' name='vmx'/>
    </mode>
    <mode name='custom' supported='yes'>
      <model usable='no'>Broadwell</model>
      <model usable='yes'>Broadwell-noTSX</model>
      <model usable='no'>Haswell</model>
      ...
    </mode>
  </cpu>
  ...
<domainCapabilities>
        
      Each CPU mode understood by libvirt is described with a
      mode element which tells whether the particular mode
      is supported and provides (when applicable) more details about it:
    
- host-passthrough
- No mode specific details are provided.
- host-model
- 
        If host-modelis supported by the hypervisor, themodedescribes the guest CPU which will be used when starting a domain withhost-modelCPU. The hypervisor specifics (such as unsupported CPU models or features, machine type, etc.) may be accounted for in this guest CPU specification and thus the CPU can be different from the one shown in host capabilities XML. This is indicated by thefallbackattribute of themodelsub element:allowmeans not all specifics were accounted for and thus the CPU a guest will see may be different;forbidindicates that the CPU a guest will see should match this CPU definition.
- custom
- 
        The modeelement contains a list of supported CPU models, each described by a dedicatedmodelelement. Theusableattribute specifies whether the model can be used on the host. A special valueunknownindicates libvirt does not have enough information to provide the usability data.
Devices ¶
      Another 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 the 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 deviceattribute of the <disk/> element.
- bus
- Options for the busattribute of the <target/> element for a <disk/>.
Graphical framebuffers ¶
Graphics device capabilities are exposed under the
    graphics element. For instance:
<domainCapabilities>
  ...
  <devices>
    <graphics supported='yes'>
      <enum name='type'>
        <value>sdl</value>
        <value>vnc</value>
        <value>spice</value>
      </enum>
    </graphics>
    ...
  </devices>
</domainCapabilities>
        - type
- Options for the typeattribute of the <graphics/> element.
Video device ¶
Video device capabilities are exposed under the
    video element. For instance:
<domainCapabilities>
  ...
  <devices>
    <video supported='yes'>
      <enum name='modelType'>
        <value>vga</value>
        <value>cirrus</value>
        <value>vmvga</value>
        <value>qxl</value>
        <value>virtio</value>
      </enum>
    </video>
    ...
  </devices>
</domainCapabilities>
        - modelType
- Options for the typeattribute of the <video><model> element.
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 modeattribute of the <hostdev/> element.
- startupPolicy
- Options for the startupPolicyattribute of the <hostdev/> element.
- subsysType
- Options for the typeattribute of the <hostdev/> element in case ofmode="subsystem".
- capsType
- Options for the typeattribute of the <hostdev/> element in case ofmode="capabilities".
- pciBackend
- Options for the nameattribute of the <driver/> element.
Features ¶
One more set of XML elements describe the supported features and
    their capabilities. All features occur as children of the main
    features element.
<domainCapabilities>
  ...
  <features>
    <gic supported='yes'>
      <enum name='version'>
        <value>2</value>
        <value>3</value>
      </enum>
    </gic>
  </features>
</domainCapabilities>
        Reported capabilities are expressed as an enumerated list of
    possible values for each of the elements or attributes. For example, the
    gic element has an attribute version which can
    support the values 2 or 3.
For information about the purpose of each feature, see the relevant section in the domain XML documentation.
GIC capabilities ¶
GIC capabilities are exposed under the gic element.
- version
- Options for the versionattribute of thegicelement.