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.

Some XML elements may be entirely omitted from the domaincapabilities XML, depending on what the libvirt driver has filled in. Applications should only act on what is explicitly reported in the domaincapabilities XML. For example, if <disk supported='yes'/> is present, you can safely assume the driver supports <disk> devices. If <disk supported='no'/> is present, you can safely assume the driver does NOT support <disk> devices. If the <disk> block is omitted entirely, the driver is not indicating one way or the other whether it supports <disk> devices, and applications should not interpret the missing block to mean any thing in particular.

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. Since not every hypervisor has a sense of machine types this element might be omitted in such drivers.
arch: The domain's architecture.

CPU Allocation

Before any devices capability occurs, there might be 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'>
    <enum name='firmware'>
      <value>bios</value>
      <value>efi</value>
    </enum>
    <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>
      <enum name='secure'>
        <value>yes</value>
        <value>no</value>
      </enum>
    </loader>
  </os>
  ...
<domainCapabilities>

The firmware enum corresponds to the firmware attribute of the os element in the domain XML. The presence of this enum means libvirt is capable of the so-called firmware auto-selection feature. And the listed firmware values represent the accepted input in the domain XML. Note that the firmware enum reports only those values for which a firmware "descriptor file" exists on the host. Firmware descriptor file is a small JSON document that describes details about a given BIOS or UEFI binary on the host, e.g. the firmware binary path, its architecture, supported machine types, NVRAM template, etc. This ensures that the reported values won't cause a failure on guest boot.

For the loader element, the following can occur:

value: List of known firmware binary paths. Currently this is used only to advertise the known location of OVMF binaries for QEMU. OVMF binaries will only be listed if they actually exist on host.
type: Whether the boot loader is a typical BIOS (rom) or a UEFI firmware (pflash). Each value sub-element under the type enum represents a possible value for the type attribute for the <loader/> element in the domain XML. E.g. the presence of pfalsh under the type enum means that a domain XML can use UEFI firmware via: <loader/> type="pflash" ...>/path/to/the/firmware/binary/</loader>.
readonly: Options for the readonly attribute of the <loader/> element in the domain XML.
secure: Options for the secure attribute of the <loader/> element in the domain XML. Note that the value yes is listed only if libvirt detects a firmware descriptor file that has path to an OVMF binary that supports Secure boot, and lists its architecture and supported machine type.

CPU configuration

The cpu element exposes options usable for configuring guest CPUs.

<domainCapabilities>
  ...
  <cpu>
    <mode name='host-passthrough' supported='yes'>
      <enum name='hostPassthroughMigratable'>
        <value>on</value>
        <value>off</value>
      </enum>
    </mode>
    <mode name='maximum' supported='yes'>
      <enum name='maximumMigratable'>
        <value>on</value>
        <value>off</value>
      </enum>
    </mode>
    <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' deprecated='no'>Broadwell</model>
      <model usable='yes' deprecated='no'>Broadwell-noTSX</model>
      <model usable='no' deprecated='yes'>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: The hostPassthroughMigratable enum shows possible values of the migratable attribute for the <cpu> element with mode='host-passthrough' in the domain XML.
host-model: If host-model is supported by the hypervisor, the mode describes the guest CPU which will be used when starting a domain with host-model CPU. 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 the fallback attribute of the model sub element: allow means not all specifics were accounted for and thus the CPU a guest will see may be different; forbid indicates that the CPU a guest will see should match this CPU definition.
custom: The mode element contains a list of supported CPU models, each described by a dedicated model element. The usable attribute specifies whether the model can be used directly on the host. When usable='no' the corresponding model cannot be used without disabling some features that the CPU of such model is expected to have. A special value unknown indicates libvirt does not have enough information to provide the usability data. The deprecated attribute reflects the hypervisor's policy on usage of this model (since 7.1.0).

I/O Threads

The iothread elements indicates whether or not I/O threads are supported.

<domainCapabilities>
  ...
  <iothread supported='yes'/>
  ...
<domainCapabilities>

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>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/>.

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 type attribute 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 type attribute 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 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.

RNG device

RNG device capabilities are exposed under the rng element. For instance:

<domainCapabilities>
  ...
  <devices>
    <rng supported='yes'>
      <enum name='model'>
        <value>virtio</value>
        <value>virtio-transitional</value>
        <value>virtio-non-transitional</value>
      </enum>
      <enum name='backendModel'>
        <value>random</value>
        <value>egd</value>
        <value>builtin</value>
      </enum>
    </rng>
    ...
  </devices>
</domainCapabilities>

model: Options for the model attribute of the <rng> element.
backendModel: Options for the model attribute of the <rng><backend> 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>
    <vmcoreinfo supported='yes'/>
    <genid supported='yes'/>
    <backingStoreInput supported='yes'/>
    <backup supported='yes'/>
    <sev>
      <cbitpos>47</cbitpos>
      <reduced-phys-bits>1</reduced-phys-bits>
    </sev>
  </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 version attribute of the gic element.

vmcoreinfo

Reports whether the vmcoreinfo feature can be enabled.

genid

Reports whether the genid feature can be used by the domain.

backingStoreInput

Reports whether the hypervisor will obey the <backingStore> elements configured for a <disk> when booting the guest, hotplugging the disk to a running guest, or similar. (Since 5.10)

backup

Reports whether the hypervisor supports the backup, checkpoint, and related features. (virDomainBackupBegin, virDomainCheckpointCreateXML etc). The presence of the backup element even if supported='no' implies that the VIR_DOMAIN_UNDEFINE_CHECKPOINTS_METADATA flag for virDomainUndefine is supported.

SEV capabilities

AMD Secure Encrypted Virtualization (SEV) capabilities are exposed under the sev element. SEV is an extension to the AMD-V architecture which supports running virtual machines (VMs) under the control of a hypervisor. When supported, guest owner can create a VM whose memory contents will be transparently encrypted with a key unique to that VM.

For more details on the SEV feature, please follow resources in the AMD developer's document store. In order to use SEV with libvirt have a look at SEV in domain XML

cbitpos: When memory encryption is enabled, one of the physical address bits (aka the C-bit) is utilized to mark if a memory page is protected. The C-bit position is Hypervisor dependent.
reducedPhysBits: When memory encryption is enabled, we lose certain bits in physical address space. The number of bits we lose is hypervisor dependent.