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<h1>Domain capabilities XML format</h1>
<ul>
<li>
<a href="#Overview">Overview</a>
</li>
<li>
<a href="#elements">Element and attribute overview</a>
<ul>
<li>
<a href="#elementsCPUAllocation">CPU Allocation</a>
</li>
<li>
<a href="#elementsOSBIOS">BIOS bootloader</a>
</li>
<li>
<a href="#elementsCPU">CPU configuration</a>
</li>
<li>
<a href="#elementsIothreads">I/O Threads</a>
</li>
<li>
<a href="#elementsDevices">Devices</a>
<ul>
<li>
<a href="#elementsDisks">Hard drives, floppy disks, CDROMs</a>
</li>
<li>
<a href="#elementsGraphics">Graphical framebuffers</a>
</li>
<li>
<a href="#elementsVideo">Video device</a>
</li>
<li>
<a href="#elementsHostDev">Host device assignment</a>
</li>
</ul>
</li>
<li>
<a href="#elementsFeatures">Features</a>
<ul>
<li>
<a href="#elementsGIC">GIC capabilities</a>
</li>
<li>
<a href="#elementsvmcoreinfo">vmcoreinfo</a>
</li>
<li>
<a href="#elementsgenid">genid</a>
</li>
<li>
<a href="#elementsSEV">SEV capabilities</a>
</li>
</ul>
</li>
</ul>
</li>
</ul>
<h2>
<a id="Overview">Overview</a>
<a class="headerlink" href="#Overview" title="Permalink to this headline">¶</a>
</h2>
<p>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.</p>
<p>The main difference between
<a href="/html/libvirt-libvirt-host.html#virConnectGetCapabilities"><code>virConnectGetCapabilities</code></a>
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.</p>
<p>While the <a href="formatcaps.html">Driver Capabilities</a> 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.</p>
<p>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.</p>
<p>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.</p>
<h2>
<a id="elements">Element and attribute overview</a>
<a class="headerlink" href="#elements" title="Permalink to this headline">¶</a>
</h2>
<p> A new query interface was added to the virConnect API's to retrieve the
XML listing of the set of domain capabilities (<span class="since">Since
1.2.7</span>):</p>
<pre>
<a href="/html/libvirt-libvirt-domain.html#virConnectGetDomainCapabilities">virConnectGetDomainCapabilities</a>
</pre>
<p>The root element that emulator capability XML document starts with has
name <code>domainCapabilities</code>. It contains at least four direct
child elements:</p>
<pre>
<domainCapabilities>
<path>/usr/bin/qemu-system-x86_64</path>
<domain>kvm</domain>
<machine>pc-i440fx-2.1</machine>
<arch>x86_64</arch>
...
</domainCapabilities>
</pre>
<dl>
<dt>
<code>path</code>
</dt>
<dd>The full path to the emulator binary.</dd>
<dt>
<code>domain</code>
</dt>
<dd>Describes the <a href="formatdomain.html#elements">virtualization
type</a> (or so called domain type).</dd>
<dt>
<code>machine</code>
</dt>
<dd>The domain's <a href="formatdomain.html#elementsOSBIOS">machine
type</a>. Since not every hypervisor has a sense of machine types
this element might be omitted in such drivers.</dd>
<dt>
<code>arch</code>
</dt>
<dd>The domain's <a href="formatdomain.html#elementsOSBIOS">
architecture</a>.</dd>
</dl>
<h3>
<a id="elementsCPUAllocation">CPU Allocation</a>
<a class="headerlink" href="#elementsCPUAllocation" title="Permalink to this headline">¶</a>
</h3>
<p>Before any devices capability occurs, there might be info on domain
wide capabilities, e.g. virtual CPUs:</p>
<pre>
<domainCapabilities>
...
<vcpu max='255'/>
...
</domainCapabilities>
</pre>
<dl>
<dt>
<code>vcpu</code>
</dt>
<dd>The maximum number of supported virtual CPUs</dd>
</dl>
<h3>
<a id="elementsOSBIOS">BIOS bootloader</a>
<a class="headerlink" href="#elementsOSBIOS" title="Permalink to this headline">¶</a>
</h3>
<p>Sometimes users might want to tweak some BIOS knobs or use
UEFI. For cases like that, <a href="formatdomain.html#elementsOSBIOS"><code>os</code></a>
element exposes what values can be passed to its children.</p>
<pre>
<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>
</pre>
<p>The <code>firmware</code> enum corresponds to
<code>firmware</code> attribute of the <code>os</code> element.
Plain presence of this enum means that libvirt is capable of so
called firmware auto selection. The listed values then represent
accepted values for the domain attribute. Only values for which
there exists a firmware descriptor that matches machine type and
architecture are listed, i.e. those which won't cause a failure
on domain startup.
</p>
<p>For the <code>loader</code> element, the following can occur:</p>
<dl>
<dt>
<code>value</code>
</dt>
<dd>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.</dd>
<dt>
<code>type</code>
</dt>
<dd>Whether loader is a typical BIOS (<code>rom</code>) or
an UEFI binary (<code>pflash</code>). This refers to
<code>type</code> attribute of the <loader/>
element.</dd>
<dt>
<code>readonly</code>
</dt>
<dd>Options for the <code>readonly</code> attribute of the
<loader/> element.</dd>
<dt>
<code>secure</code>
</dt>
<dd>Options for the <code>secure</code> attribute of the
<loader/> element. Note, that <code>yes</code> is listed
only if there is a firmware that supports it.</dd>
</dl>
<h3>
<a id="elementsCPU">CPU configuration</a>
<a class="headerlink" href="#elementsCPU" title="Permalink to this headline">¶</a>
</h3>
<p>
The <code>cpu</code> element exposes options usable for configuring
<a href="formatdomain.html#elementsCPU">guest CPUs</a>.
</p>
<pre>
<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>
</pre>
<p>
Each CPU mode understood by libvirt is described with a
<code>mode</code> element which tells whether the particular mode
is supported and provides (when applicable) more details about it:
</p>
<dl>
<dt>
<code>host-passthrough</code>
</dt>
<dd>No mode specific details are provided.</dd>
<dt>
<code>host-model</code>
</dt>
<dd>
If <code>host-model</code> is supported by the hypervisor, the
<code>mode</code> describes the guest CPU which will be used when
starting a domain with <code>host-model</code> 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 <code>fallback</code> attribute of the
<code>model</code> sub element: <code>allow</code> means not all
specifics were accounted for and thus the CPU a guest will see may
be different; <code>forbid</code> indicates that the CPU a guest will
see should match this CPU definition.
</dd>
<dt>
<code>custom</code>
</dt>
<dd>
The <code>mode</code> element contains a list of supported CPU
models, each described by a dedicated <code>model</code> element.
The <code>usable</code> attribute specifies whether the model can
be used on the host. A special value <code>unknown</code> indicates
libvirt does not have enough information to provide the usability
data.
</dd>
</dl>
<h3>
<a id="elementsIothreads">I/O Threads</a>
<a class="headerlink" href="#elementsIothreads" title="Permalink to this headline">¶</a>
</h3>
<p>
The <code>iothread</code> elements indicates whether or not
<a href="formatdomain.html#elementsIOThreadsAllocation">I/O threads</a>
are supported.
</p>
<pre>
<domainCapabilities>
...
<iothread supported='yes'/>
...
<domainCapabilities>
</pre>
<h3>
<a id="elementsDevices">Devices</a>
<a class="headerlink" href="#elementsDevices" title="Permalink to this headline">¶</a>
</h3>
<p>
Another set of XML elements describe the supported devices and their
capabilities. All devices occur as children of the main
<code>devices</code> element.
</p>
<pre>
<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>
</pre>
<p>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 <code>device</code> which can
support the values <code>disk</code>, <code>cdrom</code>,
<code>floppy</code>, or <code>lun</code>.</p>
<h4>
<a id="elementsDisks">Hard drives, floppy disks, CDROMs</a>
<a class="headerlink" href="#elementsDisks" title="Permalink to this headline">¶</a>
</h4>
<p>Disk capabilities are exposed under the <code>disk</code> element. For
instance:</p>
<pre>
<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>
</pre>
<dl>
<dt>
<code>diskDevice</code>
</dt>
<dd>Options for the <code>device</code> attribute of the <disk/>
element.</dd>
<dt>
<code>bus</code>
</dt>
<dd>Options for the <code>bus</code> attribute of the <target/>
element for a <disk/>.</dd>
</dl>
<h4>
<a id="elementsGraphics">Graphical framebuffers</a>
<a class="headerlink" href="#elementsGraphics" title="Permalink to this headline">¶</a>
</h4>
<p>Graphics device capabilities are exposed under the
<code>graphics</code> element. For instance:</p>
<pre>
<domainCapabilities>
...
<devices>
<graphics supported='yes'>
<enum name='type'>
<value>sdl</value>
<value>vnc</value>
<value>spice</value>
</enum>
</graphics>
...
</devices>
</domainCapabilities>
</pre>
<dl>
<dt>
<code>type</code>
</dt>
<dd>Options for the <code>type</code> attribute of the <graphics/>
element.</dd>
</dl>
<h4>
<a id="elementsVideo">Video device</a>
<a class="headerlink" href="#elementsVideo" title="Permalink to this headline">¶</a>
</h4>
<p>Video device capabilities are exposed under the
<code>video</code> element. For instance:</p>
<pre>
<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>
</pre>
<dl>
<dt>
<code>modelType</code>
</dt>
<dd>Options for the <code>type</code> attribute of the
<video><model> element.</dd>
</dl>
<h4>
<a id="elementsHostDev">Host device assignment</a>
<a class="headerlink" href="#elementsHostDev" title="Permalink to this headline">¶</a>
</h4>
<p>Some host devices can be passed through to a guest (e.g. USB, PCI and
SCSI). Well, only if the following is enabled:</p>
<pre>
<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>
</pre>
<dl>
<dt>
<code>mode</code>
</dt>
<dd>Options for the <code>mode</code> attribute of the <hostdev/>
element.</dd>
<dt>
<code>startupPolicy</code>
</dt>
<dd>Options for the <code>startupPolicy</code> attribute of the
<hostdev/> element.</dd>
<dt>
<code>subsysType</code>
</dt>
<dd>Options for the <code>type</code> attribute of the <hostdev/>
element in case of <code>mode="subsystem"</code>.</dd>
<dt>
<code>capsType</code>
</dt>
<dd>Options for the <code>type</code> attribute of the <hostdev/>
element in case of <code>mode="capabilities"</code>.</dd>
<dt>
<code>pciBackend</code>
</dt>
<dd>Options for the <code>name</code> attribute of the <driver/>
element.</dd>
</dl>
<h3>
<a id="elementsFeatures">Features</a>
<a class="headerlink" href="#elementsFeatures" title="Permalink to this headline">¶</a>
</h3>
<p>One more set of XML elements describe the supported features and
their capabilities. All features occur as children of the main
<code>features</code> element.</p>
<pre>
<domainCapabilities>
...
<features>
<gic supported='yes'>
<enum name='version'>
<value>2</value>
<value>3</value>
</enum>
</gic>
<vmcoreinfo supported='yes'/>
<genid supported='yes'/>
<sev>
<cbitpos>47</cbitpos>
<reduced-phys-bits>1</reduced-phys-bits>
</sev>
</features>
</domainCapabilities>
</pre>
<p>Reported capabilities are expressed as an enumerated list of
possible values for each of the elements or attributes. For example, the
<code>gic</code> element has an attribute <code>version</code> which can
support the values <code>2</code> or <code>3</code>.</p>
<p>For information about the purpose of each feature, see the
<a href="formatdomain.html#elementsFeatures">relevant section</a> in
the domain XML documentation.
</p>
<h4>
<a id="elementsGIC">GIC capabilities</a>
<a class="headerlink" href="#elementsGIC" title="Permalink to this headline">¶</a>
</h4>
<p>GIC capabilities are exposed under the <code>gic</code> element.</p>
<dl>
<dt>
<code>version</code>
</dt>
<dd>Options for the <code>version</code> attribute of the
<code>gic</code> element.</dd>
</dl>
<h4>
<a id="elementsvmcoreinfo">vmcoreinfo</a>
<a class="headerlink" href="#elementsvmcoreinfo" title="Permalink to this headline">¶</a>
</h4>
<p>Reports whether the vmcoreinfo feature can be enabled.</p>
<h4>
<a id="elementsgenid">genid</a>
<a class="headerlink" href="#elementsgenid" title="Permalink to this headline">¶</a>
</h4>
<p>Reports whether the genid feature can be used by the domain.</p>
<h4>
<a id="elementsSEV">SEV capabilities</a>
<a class="headerlink" href="#elementsSEV" title="Permalink to this headline">¶</a>
</h4>
<p>AMD Secure Encrypted Virtualization (SEV) capabilities are exposed under
the <code>sev</code> 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.</p>
<p>
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 <a href="formatdomain.html#launchSecurity">SEV in domain XML</a>
</p>
<dl>
<dt>
<code>cbitpos</code>
</dt>
<dd>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.</dd>
<dt>
<code>reducedPhysBits</code>
</dt>
<dd>When memory encryption is enabled, we lose certain bits in physical
address space. The number of bits we lose is hypervisor dependent.</dd>
</dl>
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