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
fimware 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
). Eachvalue
sub-element under thetype
enum represents a possible value for thetype
attribute for the <loader/> element in the domain XML. E.g. the presence ofpfalsh
under thetype
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 valueyes
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'/> <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-model
is supported by the hypervisor, themode
describes the guest CPU which will be used when starting a domain withhost-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 thefallback
attribute of themodel
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 dedicatedmodel
element. Theusable
attribute specifies whether the model can be used on the host. A special valueunknown
indicates libvirt does not have enough information to provide the usability data.
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 ofmode="subsystem"
. -
capsType
- Options for the
type
attribute of the <hostdev/> element in case ofmode="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> </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 thegic
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.
backup ¶
Reports whether the hypervisor supports the backup, checkpoint, and
related features. (virDomainBackupBegin
,
virDomainCheckpointCreateXML
etc).
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.