<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
dependent 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>
<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 information 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 dependent 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
