<?xml version="1.0" encoding="ISO-8859-1"?>
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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" /><link rel="stylesheet" type="text/css" href="libvirt.css" /><link rel="SHORTCUT ICON" href="/32favicon.png" /><title>XML Format</title></head><body><div id="container"><div id="intro"><div id="adjustments"></div><div id="pageHeader"></div><div id="content2"><h1 class="style1">XML Format</h1><p>This section describes the XML format used to represent domains, there are
variations on the format based on the kind of domains run and the options
used to launch them:</p><ul><li><a href="#Normal1">Normal paravirtualized Xen domains</a></li>
<li><a href="#Fully1">Fully virtualized Xen domains</a></li>
<li><a href="#KVM1">KVM domains</a></li>
<li><a href="#Net1">Networking options for QEmu and KVM</a></li>
<li><a href="#QEmu1">QEmu domains</a></li>
<li><a href="#Capa1">Discovering virtualization capabilities</a></li>
</ul><p>The formats try as much as possible to follow the same structure and reuse
elements and attributes where it makes sense.</p><h3 id="Normal"><a name="Normal1" id="Normal1">Normal paravirtualized Xen
guests</a>:</h3><p>The library use an XML format to describe domains, as input to <a href="html/libvirt-libvirt.html#virDomainCreateLinux">virDomainCreateLinux()</a>
and as the output of <a href="html/libvirt-libvirt.html#virDomainGetXMLDesc">virDomainGetXMLDesc()</a>,
the following is an example of the format as returned by the shell command
<code>virsh xmldump fc4</code> , where fc4 was one of the running domains:</p><pre><domain type='xen' <span style="color: #0071FF; background-color: #FFFFFF">id='18'</span>>
<name>fc4</name>
<span style="color: #00B200; background-color: #FFFFFF"><os>
<type>linux</type>
<kernel>/boot/vmlinuz-2.6.15-1.43_FC5guest</kernel>
<initrd>/boot/initrd-2.6.15-1.43_FC5guest.img</initrd>
<root>/dev/sda1</root>
<cmdline> ro selinux=0 3</cmdline>
</os></span>
<memory>131072</memory>
<vcpu>1</vcpu>
<devices>
<span style="color: #FF0080; background-color: #FFFFFF"><disk type='file'>
<source file='/u/fc4.img'/>
<target dev='sda1'/>
</disk></span>
<span style="color: #0000FF; background-color: #FFFFFF"><interface type='bridge'>
<source bridge='xenbr0'/>
<mac address='</span><span style="color: #0000FF; background-color: #FFFFFF"></span><span style="color: #0000FF; background-color: #FFFFFF">aa:00:00:00:00:11'/>
<script path='/etc/xen/scripts/vif-bridge'/>
</interface></span>
<span style="color: #FF8000; background-color: #FFFFFF"><console tty='/dev/pts/5'/></span>
</devices>
</domain></pre><p>The root element must be called <code>domain</code> with no namespace, the
<code>type</code> attribute indicates the kind of hypervisor used, 'xen' is
the default value. The <code>id</code> attribute gives the domain id at
runtime (not however that this may change, for example if the domain is saved
to disk and restored). The domain has a few children whose order is not
significant:</p><ul><li>name: the domain name, preferably ASCII based</li>
<li>memory: the maximum memory allocated to the domain in kilobytes</li>
<li>vcpu: the number of virtual cpu configured for the domain</li>
<li>os: a block describing the Operating System, its content will be
dependant on the OS type
<ul><li>type: indicate the OS type, always linux at this point</li>
<li>kernel: path to the kernel on the Domain 0 filesystem</li>
<li>initrd: an optional path for the init ramdisk on the Domain 0
filesystem</li>
<li>cmdline: optional command line to the kernel</li>
<li>root: the root filesystem from the guest viewpoint, it may be
passed as part of the cmdline content too</li>
</ul></li>
<li>devices: a list of <code>disk</code>, <code>interface</code> and
<code>console</code> descriptions in no special order</li>
</ul><p>The format of the devices and their type may grow over time, but the
following should be sufficient for basic use:</p><p>A <code>disk</code> device indicates a block device, it can have two
values for the type attribute either 'file' or 'block' corresponding to the 2
options available at the Xen layer. It has two mandatory children, and one
optional one in no specific order:</p><ul><li>source with a file attribute containing the path in Domain 0 to the
file or a dev attribute if using a block device, containing the device
name ('hda5' or '/dev/hda5')</li>
<li>target indicates in a dev attribute the device where it is mapped in
the guest</li>
<li>readonly an optional empty element indicating the device is
read-only</li>
<li>shareable an optional empty element indicating the device
can be used read/write with other domains</li>
</ul><p>An <code>interface</code> element describes a network device mapped on the
guest, it also has a type whose value is currently 'bridge', it also have a
number of children in no specific order:</p><ul><li>source: indicating the bridge name</li>
<li>mac: the optional mac address provided in the address attribute</li>
<li>ip: the optional IP address provided in the address attribute</li>
<li>script: the script used to bridge the interface in the Domain 0</li>
<li>target: and optional target indicating the device name.</li>
</ul><p>A <code>console</code> element describes a serial console connection to
the guest. It has no children, and a single attribute <code>tty</code> which
provides the path to the Pseudo TTY on which the guest console can be
accessed</p><p>Life cycle actions for the domain can also be expressed in the XML format,
they drive what should be happening if the domain crashes, is rebooted or is
poweroff. There is various actions possible when this happen:</p><ul><li>destroy: The domain is cleaned up (that's the default normal processing
in Xen)</li>
<li>restart: A new domain is started in place of the old one with the same
configuration parameters</li>
<li>preserve: The domain will remain in memory until it is destroyed
manually, it won't be running but allows for post-mortem debugging</li>
<li>rename-restart: a variant of the previous one but where the old domain
is renamed before being saved to allow a restart</li>
</ul><p>The following could be used for a Xen production system:</p><pre><domain>
...
<on_reboot>restart</on_reboot>
<on_poweroff>destroy</on_poweroff>
<on_crash>rename-restart</on_crash>
...
</domain></pre><p>While the format may be extended in various ways as support for more
hypervisor types and features are added, it is expected that this core subset
will remain functional in spite of the evolution of the library.</p><h3 id="Fully"><a name="Fully1" id="Fully1">Fully virtualized guests</a>
(added in 0.1.3):</h3><p>Here is an example of a domain description used to start a fully
virtualized (a.k.a. HVM) Xen domain. This requires hardware virtualization
support at the processor level but allows to run unmodified operating
systems:</p><pre><domain type='xen' id='3'>
<name>fv0</name>
<uuid>4dea22b31d52d8f32516782e98ab3fa0</uuid>
<os>
<span style="color: #0000E5; background-color: #FFFFFF"><type>hvm</type></span>
<span style="color: #0000E5; background-color: #FFFFFF"><loader>/usr/lib/xen/boot/hvmloader</loader></span>
<span style="color: #0000E5; background-color: #FFFFFF"><boot dev='hd'/></span>
</os>
<memory>524288</memory>
<vcpu>1</vcpu>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>restart</on_crash>
<features>
<span style="color: #E50000; background-color: #FFFFFF"><pae/>
<acpi/>
<apic/></span>
</features>
<span style="color: #0000E5; background-color: #FFFFFF"><clock sync="localtime"/></span>
<devices>
<span style="color: #0000E5; background-color: #FFFFFF"><emulator>/usr/lib/xen/bin/qemu-dm</emulator></span>
<interface type='bridge'>
<source bridge='xenbr0'/>
<mac address='00:16:3e:5d:c7:9e'/>
<script path='vif-bridge'/>
</interface>
<disk type='file'>
<source file='/root/fv0'/>
<target <span style="color: #0000E5; background-color: #FFFFFF">dev='hda'</span>/>
</disk>
<disk type='file' <span style="color: #0000E5; background-color: #FFFFFF">device='cdrom'</span>>
<source file='/root/fc5-x86_64-boot.iso'/>
<target <span style="color: #0000E5; background-color: #FFFFFF">dev='hdc'</span>/>
<readonly/>
</disk>
<disk type='file' <span style="color: #0000E5; background-color: #FFFFFF">device='floppy'</span>>
<source file='/root/fd.img'/>
<target <span style="color: #0000E5; background-color: #FFFFFF">dev='fda'</span>/>
</disk>
<span style="color: #0000E5; background-color: #FFFFFF"><graphics type='vnc' port='5904'/></span>
</devices>
</domain></pre><p>There is a few things to notice specifically for HVM domains:</p><ul><li>the optional <code><features></code> block is used to enable
certain guest CPU / system features. For HVM guests the following
features are defined:
<ul><li><code>pae</code> - enable PAE memory addressing</li>
<li><code>apic</code> - enable IO APIC</li>
<li><code>acpi</code> - enable ACPI bios</li>
</ul></li>
<li>the optional <code><clock></code> element is used to specify
whether the emulated BIOS clock in the guest is synced to either
<code>localtime</code> or <code>utc</code>. In general Windows will
want <code>localtime</code> while all other operating systems will
want <code>utc</code>. The default is thus <code>utc</code></li>
<li>the <code><os></code> block description is very different, first
it indicates that the type is 'hvm' for hardware virtualization, then
instead of a kernel, boot and command line arguments, it points to an os
boot loader which will extract the boot informations from the boot device
specified in a separate boot element. The <code>dev</code> attribute on
the <code>boot</code> tag can be one of:
<ul><li><code>fd</code> - boot from first floppy device</li>
<li><code>hd</code> - boot from first harddisk device</li>
<li><code>cdrom</code> - boot from first cdrom device</li>
</ul></li>
<li>the <code><devices></code> section includes an emulator entry
pointing to an additional program in charge of emulating the devices</li>
<li>the disk entry indicates in the dev target section that the emulation
for the drive is the first IDE disk device hda. The list of device names
supported is dependant on the Hypervisor, but for Xen it can be any IDE
device <code>hda</code>-<code>hdd</code>, or a floppy device
<code>fda</code>, <code>fdb</code>. The <code><disk></code> element
also supports a 'device' attribute to indicate what kinda of hardware to
emulate. The following values are supported:
<ul><li><code>floppy</code> - a floppy disk controller</li>
<li><code>disk</code> - a generic hard drive (the default it
omitted)</li>
<li><code>cdrom</code> - a CDROM device</li>
</ul>
For Xen 3.0.2 and earlier a CDROM device can only be emulated on the
<code>hdc</code> channel, while for 3.0.3 and later, it can be emulated
on any IDE channel.</li>
<li>the <code><devices></code> section also include at least one
entry for the graphic device used to render the os. Currently there is
just 2 types possible 'vnc' or 'sdl'. If the type is 'vnc', then an
additional <code>port</code> attribute will be present indicating the TCP
port on which the VNC server is accepting client connections.</li>
</ul><p>It is likely that the HVM description gets additional optional elements
and attributes as the support for fully virtualized domain expands,
especially for the variety of devices emulated and the graphic support
options offered.</p><h3><a name="KVM1" id="KVM1">KVM domain (added in 0.2.0)</a></h3><p>Support for the <a href="http://kvm.qumranet.com/">KVM virtualization</a>
is provided in recent Linux kernels (2.6.20 and onward). This requires
specific hardware with acceleration support and the availability of the
special version of the <a href="http://fabrice.bellard.free.fr/qemu/">QEmu</a> binary. Since this
relies on QEmu for the machine emulation like fully virtualized guests the
XML description is quite similar, here is a simple example:</p><pre><domain <span style="color: #FF0000; background-color: #FFFFFF">type='kvm'</span>>
<name>demo2</name>
<uuid>4dea24b3-1d52-d8f3-2516-782e98a23fa0</uuid>
<memory>131072</memory>
<vcpu>1</vcpu>
<os>
<type>hvm</type>
</os>
<span style="color: #0000E5; background-color: #FFFFFF"><clock sync="localtime"/></span>
<devices>
<span style="color: #FF0000; background-color: #FFFFFF"><emulator>/home/user/usr/kvm-devel/bin/qemu-system-x86_64</emulator></span>
<disk type='file' device='disk'>
<source file='/home/user/fedora/diskboot.img'/>
<target dev='hda'/>
</disk>
<interface <span style="color: #FF0000; background-color: #FFFFFF">type='user'</span>>
<mac address='24:42:53:21:52:45'/>
</interface>
<graphics type='vnc' port='-1'/>
</devices>
</domain></pre><p>The specific points to note if using KVM are:</p><ul><li>the top level domain element carries a type of 'kvm'</li>
<li>the <clock> optional is supported as with Xen HVM</li>
<li>the <devices> emulator points to the special qemu binary required
for KVM</li>
<li>networking interface definitions definitions are somewhat different due
to a different model from Xen see below</li>
</ul><p>except those points the options should be quite similar to Xen HVM
ones.</p><h3><a name="Net1" id="Net1">Networking options for QEmu and KVM (added in 0.2.0)</a></h3><p>The networking support in the QEmu and KVM case is more flexible, and
support a variety of options:</p><ol><li>Userspace SLIRP stack
<p>Provides a virtual LAN with NAT to the outside world. The virtual
network has DHCP & DNS services and will give the guest VM addresses
starting from <code>10.0.2.15</code>. The default router will be
<code>10.0.2.2</code> and the DNS server will be <code>10.0.2.3</code>.
This networking is the only option for unprivileged users who need their
VMs to have outgoing access. Example configs are:</p>
<pre><interface type='user'/></pre>
<pre>
<interface type='user'>
<mac address="11:22:33:44:55:66"/>
</interface>
</pre>
</li>
<li>Virtual network
<p>Provides a virtual network using a bridge device in the host.
Depending on the virtual network configuration, the network may be
totally isolated, NAT'ing to an explicit network device, or NAT'ing to
the default route. DHCP and DNS are provided on the virtual network in
all cases and the IP range can be determined by examining the virtual
network config with '<code>virsh net-dumpxml <network
name></code>'. There is one virtual network called 'default' setup out
of the box which does NAT'ing to the default route and has an IP range of
<code>192.168.22.0/255.255.255.0</code>. Each guest will have an
associated tun device created with a name of vnetN, which can also be
overriden with the <target> element. Example configs are:</p>
<pre><interface type='network'>
<source network='default'/>
</interface>
<interface type='network'>
<source network='default'/>
<target dev='vnet7'/>
<mac address="11:22:33:44:55:66"/>
</interface>
</pre>
</li>
<li>Bridge to to LAN
<p>Provides a bridge from the VM directly onto the LAN. This assumes
there is a bridge device on the host which has one or more of the hosts
physical NICs enslaved. The guest VM will have an associated tun device
created with a name of vnetN, which can also be overriden with the
<target> element. The tun device will be enslaved to the bridge.
The IP range / network configuration is whatever is used on the LAN. This
provides the guest VM full incoming & outgoing net access just like a
physical machine. Examples include:</p>
<pre><interface type='bridge'>
<source dev='br0'/>
</interface>
<interface type='bridge'>
<source dev='br0'/>
<target dev='vnet7'/>
<mac address="11:22:33:44:55:66"/>
</interface></pre>
</li>
<li>Generic connection to LAN
<p>Provides a means for the administrator to execute an arbitrary script
to connect the guest's network to the LAN. The guest will have a tun
device created with a name of vnetN, which can also be overriden with the
<target> element. After creating the tun device a shell script will
be run which is expected to do whatever host network integration is
required. By default this script is called /etc/qemu-ifup but can be
overriden.</p>
<pre><interface type='ethernet'/>
<interface type='ethernet'>
<target dev='vnet7'/>
<script path='/etc/qemu-ifup-mynet'/>
</interface></pre>
</li>
<li>Multicast tunnel
<p>A multicast group is setup to represent a virtual network. Any VMs
whose network devices are in the same multicast group can talk to each
other even across hosts. This mode is also available to unprivileged
users. There is no default DNS or DHCP support and no outgoing network
access. To provide outgoing network access, one of the VMs should have a
2nd NIC which is connected to one of the first 4 network types and do the
appropriate routing. The multicast protocol is compatible with that used
by user mode linux guests too. The source address used must be from the
multicast address block.</p>
<pre><interface type='mcast'>
<source address='230.0.0.1' port='5558'/>
</interface></pre>
</li>
<li>TCP tunnel
<p>A TCP client/server architecture provides a virtual network. One VM
provides the server end of the network, all other VMS are configured as
clients. All network traffic is routed between the VMs via the server.
This mode is also available to unprivileged users. There is no default
DNS or DHCP support and no outgoing network access. To provide outgoing
network access, one of the VMs should have a 2nd NIC which is connected
to one of the first 4 network types and do the appropriate routing.</p>
<p>Example server config:</p>
<pre><interface type='server'>
<source address='192.168.0.1' port='5558'/>
</interface></pre>
<p>Example client config:</p>
<pre><interface type='client'>
<source address='192.168.0.1' port='5558'/>
</interface></pre>
</li>
</ol><p>To be noted, options 2, 3, 4 are also supported by Xen VMs, so it is
possible to use these configs to have networking with both Xen &
QEMU/KVMs connected to each other.</p><h3>Q<a name="QEmu1" id="QEmu1">Emu domain (added in 0.2.0)</a></h3><p>Libvirt support for KVM and QEmu is the same code base with only minor
changes. The configuration is as a result nearly identical, the only changes
are related to QEmu ability to emulate <a href="http://www.qemu.org/status.html">various CPU type and hardware
platforms</a>, and kqemu support (QEmu own kernel accelerator when the
emulated CPU is i686 as well as the target machine):</p><pre><domain <span style="color: #FF0000; background-color: #FFFFFF">type='qemu'</span>>
<name>QEmu-fedora-i686</name>
<uuid>c7a5fdbd-cdaf-9455-926a-d65c16db1809</uuid>
<memory>219200</memory>
<currentMemory>219200</currentMemory>
<vcpu>2</vcpu>
<os>
<span style="color: #FF0000; background-color: #FFFFFF"><type arch='i686' machine='pc'>hvm</type></span>
<boot dev='cdrom'/>
</os>
<devices>
<span style="color: #FF0000; background-color: #FFFFFF"><emulator>/usr/bin/qemu</emulator></span>
<disk type='file' device='cdrom'>
<source file='/home/user/boot.iso'/>
<target dev='hdc'/>
<readonly/>
</disk>
<disk type='file' device='disk'>
<source file='/home/user/fedora.img'/>
<target dev='hda'/>
</disk>
<interface type='network'>
<source name='default'/>
</interface>
<graphics type='vnc' port='-1'/>
</devices>
</domain></pre><p>The difference here are:</p><ul><li>the value of type on top-level domain, it's 'qemu' or kqemu if asking
for <a href="http://www.qemu.org/kqemu-tech.html">kernel assisted
acceleration</a></li>
<li>the os type block defines the architecture to be emulated, and
optionally the machine type, see the discovery API below</li>
<li>the emulator string must point to the right emulator for that
architecture</li>
</ul><h3><a name="Capa1" id="Capa1">Discovering virtualization capabilities (Added in 0.2.1)</a></h3><p>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:</p><pre> char * virConnectGetCapabilities (virConnectPtr conn);</pre><p>The value returned is an XML document listing the virtualization
capabilities of the host and virtualization engine to which
<code>@conn</code> is connected. One can test it using <code>virsh</code>
command line tool command '<code>capabilities</code>', it dumps the XML
associated to the current connection. For example in the case of a 64 bits
machine with hardware virtualization capabilities enabled in the chip and
BIOS you will see</p><pre><capabilities>
<span style="color: #E50000; background-color: #FFFFFF"><host>
<cpu>
<arch>x86_64</arch>
<features>
<vmx/>
</features>
</cpu>
</host></span>
<!-- xen-3.0-x86_64 -->
<span style="color: #0000E5; background-color: #FFFFFF"><guest>
<os_type>xen</os_type>
<arch name="x86_64">
<wordsize>64</wordsize>
<domain type="xen"></domain>
<emulator>/usr/lib64/xen/bin/qemu-dm</emulator>
</arch>
<features>
</features>
</guest></span>
<!-- hvm-3.0-x86_32 -->
<span style="color: #00B200; background-color: #FFFFFF"><guest>
<os_type>hvm</os_type>
<arch name="i686">
<wordsize>32</wordsize>
<domain type="xen"></domain>
<emulator>/usr/lib/xen/bin/qemu-dm</emulator>
<machine>pc</machine>
<machine>isapc</machine>
<loader>/usr/lib/xen/boot/hvmloader</loader>
</arch>
<features>
</features>
</guest></span>
...
</capabilities></pre><p>The first block (in red) indicates the host hardware capbilities, currently
it is limited to the CPU properties but other information may be available,
it shows the CPU architecture, and the features of the chip (the feature
block is similar to what you will find in a Xen fully virtualized domain
description).</p><p>The second block (in blue) indicates the paravirtualization support of the
Xen support, you will see the os_type of xen to indicate a paravirtual
kernel, then architecture informations and potential features.</p><p>The third block (in green) gives similar informations but when running a
32 bit OS fully virtualized with Xen using the hvm support.</p><p>This section is likely to be updated and augmented in the future, see <a href="https://www.redhat.com/archives/libvir-list/2007-March/msg00215.html">the
discussion</a> which led to the capabilities format in the mailing-list
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