.. role:: since ============================== Domain capabilities XML format ============================== .. contents:: 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 is present, you can safely assume the driver supports devices. If is present, you can safely assume the driver does NOT support devices. If the block is omitted entirely, the driver is not indicating one way or the other whether it supports 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:`Since 1.2.7` ): ``virConnectGetDomainCapabilities`` (`API docs `__) The root element that emulator capability XML document starts with has name ``domainCapabilities``. It contains at least four direct child elements: :: /usr/bin/qemu-system-x86_64 kvm pc-i440fx-2.1 x86_64 ... ``path`` The full path to the emulator binary. Since not every hypervisor has a notion of emulator binary this element might be omitted in such drivers. ``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: :: ... ... ``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. :: ... bios efi /usr/share/OVMF/OVMF_CODE.fd rom pflash yes no yes no ... 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 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: type="pflash" ...>/path/to/the/firmware/binary/. ``readonly`` Options for the ``readonly`` attribute of the element in the domain XML. ``secure`` Options for the ``secure`` attribute of the 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 `__. :: ... on off on off Broadwell Intel Broadwell Broadwell-noTSX EPYC-Milan Haswell ... ... 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 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. The optional ``maxphysaddr`` element reports physical address size of the host CPU if this value is available and applicable for the requested domain type. This is useful for computing baseline CPU definition which should be compatible with several hosts. ``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. A special value ``unknown`` indicates libvirt does not have enough information to provide the usability data. When ``usable='no'`` the corresponding model cannot be used without disabling some features that the CPU of such model is expected to have. :since:`Since 10.9.0` each CPU model with ``usable='no'`` is followed by a corresponding ``blockers`` element containing a list of features blocking usability of the CPU model. Models marked as usable (``usable='yes'``) can be safely used in domain XMLs with ``check='none'`` as the hypervisor guarantees the model can be used on the current host and additional checks done by libvirt are redundant. :since:`Since 10.2.0` libvirt automatically detects this situation and avoids the redundant checks even when ``check='partial'`` is used, with older releases disabling libvirt checks via ``check='none'`` for such models is recommended to avoid needless issues with starting domains when libvirt's definition of a particular model differs from hypervisor's definition. The ``deprecated`` attribute reflects the hypervisor's policy on usage of this model :since:`(since 7.1.0)`. The ``vendor`` attribute :since:`(since 8.9.0)` contains the vendor of the CPU model for users who want to use CPU models with specific vendors only. CPU models with undefined vendor will be listed with ``vendor='unkwnown'``. The ``canonical`` attribute :since:`(since 10.10.0)` contains a canonical name of the CPU model if the model is actually an alias to another one. I/O Threads ~~~~~~~~~~~ The ``iothread`` elements indicates whether or not `I/O threads `__ are supported. :: ... ... Memory Backing ~~~~~~~~~~~~~~ The ``memory backing`` element indicates whether or not `memory backing `__ is supported. :: ... anonymous file memfd ... ``sourceType`` Options for the ``type`` attribute of the element. Devices ~~~~~~~ Another set of XML elements describe the supported devices and their capabilities. All devices occur as children of the main ``devices`` element. :: ... disk cdrom floppy lun ... Reported capabilities are expressed as an enumerated list of available options for each of the element or attribute. For example, the 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: :: ... disk cdrom floppy lun ide fdc scsi virtio xen usb sata sd ... ``diskDevice`` Options for the ``device`` attribute of the element. ``bus`` Options for the ``bus`` attribute of the element for a . Graphical framebuffers ^^^^^^^^^^^^^^^^^^^^^^ Graphics device capabilities are exposed under the ``graphics`` element. For instance: :: ... sdl vnc spice ... ``type`` Options for the ``type`` attribute of the element. Video device ^^^^^^^^^^^^ Video device capabilities are exposed under the ``video`` element. For instance: :: ... ... ``modelType`` Options for the ``type`` attribute of the