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<h1>Domain capabilities XML format</h1>
<ul id="toc"></ul>
<h2><a name="Overview">Overview</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>
<h2><a name="elements">Element and attribute overview</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>path</dt>
<dd>The full path to the emulator binary.</dd>
<dt>domain</dt>
<dd>Describes the <a href="formatdomain.html#elements">virtualization
type</a> (or so called domain type).</dd>
<dt>machine</dt>
<dd>The domain's <a href="formatdomain.html#elementsOSBIOS">machine
type</a>.</dd>
<dt>arch</dt>
<dd>The domain's <a href="formatdomain.html#elementsOSBIOS">
architecture</a>.</dd>
</dl>
<h3><a name="elementsCPUAllocation">CPU Allocation</a></h3>
<p>Before any devices capability occurs, there might be a info on domain
wide capabilities, e.g. virtual CPUs:</p>
<pre>
<domainCapabilities>
...
<vcpu max='255'/>
...
</domainCapabilities>
</pre>
<dl>
<dt>vcpu</dt>
<dd>The maximum number of supported virtual CPUs</dd>
</dl>
<h3><a name="elementsOSBIOS">BIOS bootloader</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'>
<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>
</loader>
</os>
...
<domainCapabilities>
</pre>
<p>For the <code>loader</code> element, the following can occur:</p>
<dl>
<dt>value</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>type</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>readonly</dt>
<dd>Options for the <code>readonly</code> attribute of the
<loader/> element.</dd>
</dl>
<h3><a name="elementsDevices">Devices</a></h3>
<p>
The final 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 name="elementsDisks">Hard drives, floppy disks, CDROMs</a></h4>
<p>Disk capabilities are exposed under <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>uml</value>
<value>sata</value>
<value>sd</value>
</enum>
</disk>
...
</devices>
</domainCapabilities>
</pre>
<dl>
<dt>diskDevice</dt>
<dd>Options for the <code>device</code> attribute of the <disk/>
element.</dd>
<dt>bus</dt>
<dd>Options for the <code>bus</code> attribute of the <target/>
element for a <disk/>.</dd>
</dl>
<h4><a name="elementsHostDev">Host device assignment</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>mode</dt>
<dd>Options for the <code>mode</code> attribute of the <hostdev/>
element.</dd>
<dt>startupPolicy</dt>
<dd>Options for the <code>startupPolicy</code> attribute of the
<hostdev/> element.</dd>
<dt>subsysType</dt>
<dd>Options for the <code>type</code> attribute of the <hostdev/>
element in case of <code>mode="subsystem"</code>.</dd>
<dt>capsType</dt>
<dd>Options for the <code>type</code> attribute of the <hostdev/>
element in case of <code>mode="capabilities"</code>.</dd>
<dt>pciBackend</dt>
<dd>Options for the <code>name</code> attribute of the <driver/>
element.</dd>
</dl>
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