# Driver capabilities XML format

## Element and attribute overview

As new virtualization engine support gets added to libvirt, and to handle cases like QEMU supporting a variety of emulations, a query interface has been added in 0.2.1 allowing to list the set of supported virtualization capabilities on the host:

    char * virConnectGetCapabilities (virConnectPtr conn);

The value returned is an XML document listing the virtualization capabilities of the host and virtualization engine to which `conn is connected. One can test it using virsh command line tool command 'capabilities', it dumps the XML associated to the current connection.

As can be seen in the Examples, the capabilities XML consists of the capabilities element which have exactly one host child element to report information on host capabilities, and zero or more guest element to express the set of architectures the host can run at the moment.

### Host capabilities

The <host/> element consists of the following child elements:

uuid

  The host UUID.

cpu

  The host CPU architecture and features.

  Note that, while this element contains a topology sub-element, the information contained therein is fairly high-level and likely not very useful when it comes to optimizing guest vCPU placement. Look into the topology element, described below, for more detailed information.

power_management

  whether host is capable of memory suspend, disk hibernation, or hybrid suspend.

migration_features

  This element exposes information on the hypervisor's migration capabilities, like live migration, supported URI transports, and so on.

topology

  This element describes the host CPU topology in detail.

  Management applications may want to use this information when defining new guests: for example, in order to ensure that all vCPUs are scheduled on CPUs that are in the same NUMA node or even CPU core.

  The cells sub-element contains a list of NUMA nodes, each one represented by a single cell element. Within each cell, a cpus sub-element contains a list of logical CPUs, each one represented by a single cpu element. In both cases, the num attribute of the top-level element contains the number of children.

  Each cpu element contains the following attributes:

  id

    CPU identifier. Can be used to refer to it in the context of CPU tuning.

  socket_id

    Identifier for the physical package the CPU is in.

  die_id

    Identifier for the die the CPU is in.

    Note that, even if this attribute is present, you might not be able to define guests with multiple CPU dies.

  cluster_id

    Identifier for the cluster the CPU is in.

    Note that, even if this attribute is present, you might not be able to define guests with multiple CPU clusters.

  core_id

    Identifier for the core the CPU is in.

  siblings

    List of CPUs that are in the same core.

    The list will include the current CPU, plus all other CPUs that have the same values for socket_id, die_id, cluster_id and core_id.

secmodel

  To find out default security labels for different security models you need to parse this element. In contrast with the former elements, this is repeated for each security model the libvirt daemon currently supports.

#### Host CPU model and features

As described previously in this section, libvirt exposes to users the list of Host CPU features. Libvirt has a special way to expose this list: Instead of providing the full (and thereby often very long) set of features, libvirt specifies a CPU model as baseline and additional features on top of it (as can be seen in Examples where the baseline is Skylake-Client-noTSX-IBRS).

The ideal case would be that the baseline CPU model definition matches exactly the CPU present in the system, and no additional feature is needed to express the capabilities of the CPU. For example, if you are running on a Server CPU you bought as Icelake type, the returned CPU model name could be Icelake-Server. However, this ideal situation rarely happens, for two main reasons:

- Manufacturers do not ship a single type of CPU under a given name, there are various different SKUs with different features under the same name. Yet it is not practical to have a database listing all of those variants.

- Some features might be in the hardware but unavailable for various reasons (BIOS and kernel configuration, disabled for security, ...). One typical example where this situation happens is related to the TSX mitigation. As a mitigation to the TAA side channel attack, the Linux kernel disables by default TSX and its 2 features, rtm and hle. Since many Linux distributions keep this safer default behavior, these 2 features appear as missing.

Because for backward compatibility reasons, host capabilities cannot list features that would need to be removed from the baseline model to describe the host CPU, libvirt has to often use a rather old CPU model, for example, Broadwell rather than Icelake. Therefore, users should not expect the reported CPU model name to have any implications other than that of a named baseline to build the complete available feature set of the Host CPU.

Domain capabilities do not have such limitation, and the host-model CPU definition would show the correct CPU model in almost all cases.

### Guest capabilities

While the previous section (Host capabilities) aims at host capabilities, this one focuses on capabilities available to a guest using a given hypervisor. The <guest/> element will typically wrap up the following elements:

os_type

  This expresses what kind of operating system the hypervisor is able to run. Possible values are:

  xen

    for XEN PV

  linux

    legacy alias for xen

  xenpvh

    for XEN PVH

  hvm

    Unmodified operating system

  exe

    Container based virtualization

arch

  This element brings some information on supported guest architecture. Possible subelements are:

  wordsize

    Size of CPU word in bits, for example 64.

  emulator

    Emulator (device model) path, for use in emulator element of domain XML.

  loader

    Loader path, for use in loader element of domain XML.

  machine

    Machine type, for use in machine attribute of os/type element in domain XML. For example Xen supports xenfv for HVM, xenpv for PV, or xenpvh for PVH.

  domain

    The type attribute of this element specifies the type of hypervisor required to run the domain. Use in type attribute of the domain root element.

features

  This optional element encases possible features that can be used with a guest of described type. Possible subelements are:

  pae

    If present, 32-bit guests can use PAE address space extensions, since 0.4.1

  nonpae

    If present, 32-bit guests can be run without requiring PAE, since 0.4.1

  ia64_be

    If present, IA64 guests can be run in big-endian mode, since 0.4.1

  acpi

    If this element is present, the default attribute describes whether the hypervisor exposes ACPI to the guest by default, and the toggle attribute describes whether the user can override this default. Since 0.4.1

  apic

    If this element is present, the default attribute describes whether the hypervisor exposes APIC to the guest by default, and the toggle attribute describes whether the user can override this default. Since 0.4.1

  cpuselection

    If this element is present, the hypervisor supports the <cpu> element within a domain definition for fine-grained control over the CPU presented to the guest. Since 0.7.5

  deviceboot

    If this element is present, the <boot order='...'/> element can be used inside devices, rather than the older boot specification by category. Since 0.8.8

  disksnapshot

    If this element is present, the default attribute describes whether creating external disk snapshots is supported. If absent, creating external snapshots may still be supported, but it requires attempting the API and checking for an error to find out for sure. Since 1.2.3

  externalSnapshot

    If this element is present, the hypervisor supports deleting and reverting external snapshots including memory state. Support for creation of external snapshots is reported via the disksnapshot feature flag. Management applications can now switch from internal snapshots to external snapshots. Since 9.9.0

### Examples

For example, in the case of a 64-bit machine with hardware virtualization capabilities enabled in the chip and BIOS you will see:

    <capabilities>

      <host>
        <uuid>7b55704c-29f4-11b2-a85c-9dc6ff50623f</uuid>
        <cpu>
          <arch>x86_64</arch>
          <model>Skylake-Client-noTSX-IBRS</model>
          <vendor>Intel</vendor>
          <microcode version='236'/>
          <signature family='6' model='142' stepping='12'/>
          <counter name='tsc' frequency='2303997000' scaling='no'/>
          <topology sockets='1' dies='1' clusters='1' cores='4' threads='2'/>
          <maxphysaddr mode='emulate' bits='39'/>
          <feature name='ds'/>
          <feature name='acpi'/>
          <feature name='ss'/>
          <feature name='ht'/>
          <feature name='tm'/>
          <feature name='pbe'/>
          <feature name='dtes64'/>
          <feature name='monitor'/>
          <feature name='ds_cpl'/>
          <feature name='vmx'/>
          <feature name='smx'/>
          <feature name='est'/>
          <feature name='tm2'/>
          <feature name='xtpr'/>
          <feature name='pdcm'/>
          <feature name='osxsave'/>
          <feature name='tsc_adjust'/>
          <feature name='sgx'/>
          <feature name='clflushopt'/>
          <feature name='intel-pt'/>
          <feature name='md-clear'/>
          <feature name='stibp'/>
          <feature name='arch-capabilities'/>
          <feature name='ssbd'/>
          <feature name='xsaves'/>
          <feature name='sgx1'/>
          <feature name='sgx-debug'/>
          <feature name='sgx-mode64'/>
          <feature name='sgx-provisionkey'/>
          <feature name='sgx-tokenkey'/>
          <feature name='pdpe1gb'/>
          <feature name='invtsc'/>
          <feature name='rdctl-no'/>
          <feature name='ibrs-all'/>
          <feature name='skip-l1dfl-vmentry'/>
          <feature name='mds-no'/>
          <feature name='tsx-ctrl'/>
          <pages unit='KiB' size='4'/>
          <pages unit='KiB' size='2048'/>
          <pages unit='KiB' size='1048576'/>
        </cpu>
        <power_management>
          <suspend_mem/>
        </power_management>
        <iommu support='yes'/>
        <migration_features>
          <live/>
          <uri_transports>
            <uri_transport>tcp</uri_transport>
            <uri_transport>rdma</uri_transport>
          </uri_transports>
        </migration_features>
        <topology>
          <cells num='1'>
            <cell id='0'>
              <memory unit='KiB'>32498112</memory>
              <pages unit='KiB' size='4'>6813808</pages>
              <pages unit='KiB' size='2048'>2048</pages>
              <pages unit='KiB' size='1048576'>1</pages>
              <distances>
                <sibling id='0' value='10'/>
              </distances>
              <cpus num='8'>
                <cpu id='0' socket_id='0' die_id='0' cluster_id='0' core_id='0' siblings='0,4'/>
                <cpu id='1' socket_id='0' die_id='0' cluster_id='0' core_id='1' siblings='1,5'/>
                <cpu id='2' socket_id='0' die_id='0' cluster_id='0' core_id='2' siblings='2,6'/>
                <cpu id='3' socket_id='0' die_id='0' cluster_id='0' core_id='3' siblings='3,7'/>
                <cpu id='4' socket_id='0' die_id='0' cluster_id='0' core_id='0' siblings='0,4'/>
                <cpu id='5' socket_id='0' die_id='0' cluster_id='0' core_id='1' siblings='1,5'/>
                <cpu id='6' socket_id='0' die_id='0' cluster_id='0' core_id='2' siblings='2,6'/>
                <cpu id='7' socket_id='0' die_id='0' cluster_id='0' core_id='3' siblings='3,7'/>
              </cpus>
            </cell>
          </cells>
        </topology>
        <cache>
          <bank id='0' level='3' type='both' size='8' unit='MiB' cpus='0-7'/>
        </cache>
        <secmodel>
          <model>none</model>
          <doi>0</doi>
        </secmodel>
        <secmodel>
          <model>dac</model>
          <doi>0</doi>
          <baselabel type='kvm'>+77:+77</baselabel>
          <baselabel type='qemu'>+77:+77</baselabel>
        </secmodel>
      </host>

      <guest>
        <os_type>hvm</os_type>
        <arch name='x86_64'>
          <wordsize>64</wordsize>
          <emulator>/usr/bin/qemu-system-x86_64</emulator>
          <machine maxCpus='255'>pc-i440fx-7.1</machine>
          <machine canonical='pc-i440fx-7.1' maxCpus='255'>pc</machine>
          <machine maxCpus='288'>pc-q35-5.2</machine>
          <machine maxCpus='255'>pc-i440fx-2.12</machine>
          <machine maxCpus='255'>pc-i440fx-2.0</machine>
          <machine maxCpus='255'>pc-i440fx-6.2</machine>
          <machine maxCpus='288'>pc-q35-4.2</machine>
          <machine maxCpus='255'>pc-i440fx-2.5</machine>
          <machine maxCpus='255'>pc-i440fx-4.2</machine>
          <machine maxCpus='255'>pc-i440fx-5.2</machine>
          <machine maxCpus='255' deprecated='yes'>pc-i440fx-1.5</machine>
          <machine maxCpus='255'>pc-q35-2.7</machine>
          <machine maxCpus='288'>pc-q35-7.1</machine>
          <machine canonical='pc-q35-7.1' maxCpus='288'>q35</machine>
          <machine maxCpus='255'>pc-i440fx-2.2</machine>
          <machine maxCpus='255'>pc-i440fx-2.7</machine>
          <machine maxCpus='288'>pc-q35-6.1</machine>
          <machine maxCpus='255'>pc-q35-2.4</machine>
          <machine maxCpus='288'>pc-q35-2.10</machine>
          <machine maxCpus='1'>x-remote</machine>
          <machine maxCpus='288'>pc-q35-5.1</machine>
          <machine maxCpus='255' deprecated='yes'>pc-i440fx-1.7</machine>
          <machine maxCpus='288'>pc-q35-2.9</machine>
          <machine maxCpus='255'>pc-i440fx-2.11</machine>
          <machine maxCpus='288'>pc-q35-3.1</machine>
          <machine maxCpus='255'>pc-i440fx-6.1</machine>
          <machine maxCpus='288'>pc-q35-4.1</machine>
          <machine maxCpus='255'>pc-i440fx-2.4</machine>
          <machine maxCpus='255'>pc-i440fx-4.1</machine>
          <machine maxCpus='255'>pc-i440fx-5.1</machine>
          <machine maxCpus='255'>pc-i440fx-2.9</machine>
          <machine maxCpus='1'>isapc</machine>
          <machine maxCpus='255' deprecated='yes'>pc-i440fx-1.4</machine>
          <machine maxCpus='255'>pc-q35-2.6</machine>
          <machine maxCpus='255'>pc-i440fx-3.1</machine>
          <machine maxCpus='288'>pc-q35-2.12</machine>
          <machine maxCpus='288'>pc-q35-7.0</machine>
          <machine maxCpus='255'>pc-i440fx-2.1</machine>
          <machine maxCpus='288'>pc-q35-6.0</machine>
          <machine maxCpus='255'>pc-i440fx-2.6</machine>
          <machine maxCpus='288'>pc-q35-4.0.1</machine>
          <machine maxCpus='255'>pc-i440fx-7.0</machine>
          <machine maxCpus='255' deprecated='yes'>pc-i440fx-1.6</machine>
          <machine maxCpus='288'>pc-q35-5.0</machine>
          <machine maxCpus='288'>pc-q35-2.8</machine>
          <machine maxCpus='255'>pc-i440fx-2.10</machine>
          <machine maxCpus='288'>pc-q35-3.0</machine>
          <machine maxCpus='255'>pc-i440fx-6.0</machine>
          <machine maxCpus='288'>pc-q35-4.0</machine>
          <machine maxCpus='288'>microvm</machine>
          <machine maxCpus='255'>pc-i440fx-2.3</machine>
          <machine maxCpus='255'>pc-i440fx-4.0</machine>
          <machine maxCpus='255'>pc-i440fx-5.0</machine>
          <machine maxCpus='255'>pc-i440fx-2.8</machine>
          <machine maxCpus='288'>pc-q35-6.2</machine>
          <machine maxCpus='255'>pc-q35-2.5</machine>
          <machine maxCpus='255'>pc-i440fx-3.0</machine>
          <machine maxCpus='288'>pc-q35-2.11</machine>
          <domain type='qemu'/>
          <domain type='kvm'/>
        </arch>
        <features>
          <acpi default='on' toggle='yes'/>
          <apic default='on' toggle='no'/>
          <cpuselection/>
          <deviceboot/>
          <disksnapshot default='on' toggle='no'/>
          <externalSnapshot/>
        </features>
      </guest>

    </capabilities>