XML Format

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:

Normal paravirtualized Xen domains
Fully virtualized Xen domains
KVM domains
Networking options for QEmu and KVM
QEmu domains
Discovering virtualization capabilities

The formats try as much as possible to follow the same structure and reuse elements and attributes where it makes sense.

Normal paravirtualized Xen guests:

The library use an XML format to describe domains, as input to virDomainCreateLinux() and as the output of virDomainGetXMLDesc(), the following is an example of the format as returned by the shell command virsh xmldump fc4 , where fc4 was one of the running domains:

<domain type='xen' id='18'>
  <name>fc4</name>
  <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>
  <memory>131072</memory>
  <vcpu>1</vcpu>
  <devices>
    <disk type='file'>
      <source file='/u/fc4.img'/>
      <target dev='sda1'/>
    </disk>
    <interface type='bridge'>
      <source bridge='xenbr0'/>
      <mac address='aa:00:00:00:00:11'/>
      <script path='/etc/xen/scripts/vif-bridge'/>
    </interface>
    <console tty='/dev/pts/5'/>
  </devices>
</domain>

The root element must be called domain with no namespace, the type attribute indicates the kind of hypervisor used, 'xen' is the default value. The id 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:

name: the domain name, preferably ASCII based
memory: the maximum memory allocated to the domain in kilobytes
vcpu: the number of virtual cpu configured for the domain
os: a block describing the Operating System, its content will be dependent on the OS type
- type: indicate the OS type, always linux at this point
- kernel: path to the kernel on the Domain 0 filesystem
- initrd: an optional path for the init ramdisk on the Domain 0 filesystem
- cmdline: optional command line to the kernel
- root: the root filesystem from the guest viewpoint, it may be passed as part of the cmdline content too
devices: a list of disk, interface and console descriptions in no special order

The format of the devices and their type may grow over time, but the following should be sufficient for basic use:

A disk 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:

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')
target indicates in a dev attribute the device where it is mapped in the guest
readonly an optional empty element indicating the device is read-only
shareable an optional empty element indicating the device can be used read/write with other domains

An interface 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:

source: indicating the bridge name
mac: the optional mac address provided in the address attribute
ip: the optional IP address provided in the address attribute
script: the script used to bridge the interface in the Domain 0
target: and optional target indicating the device name.

A console element describes a serial console connection to the guest. It has no children, and a single attribute tty which provides the path to the Pseudo TTY on which the guest console can be accessed

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:

destroy: The domain is cleaned up (that's the default normal processing in Xen)
restart: A new domain is started in place of the old one with the same configuration parameters
preserve: The domain will remain in memory until it is destroyed manually, it won't be running but allows for post-mortem debugging
rename-restart: a variant of the previous one but where the old domain is renamed before being saved to allow a restart

The following could be used for a Xen production system:

<domain>
  ...
  <on_reboot>restart</on_reboot>
  <on_poweroff>destroy</on_poweroff>
  <on_crash>rename-restart</on_crash>
  ...
</domain>

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.

Fully virtualized guests (added in 0.1.3):

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:

<domain type='xen' id='3'>
  <name>fv0</name>
  <uuid>4dea22b31d52d8f32516782e98ab3fa0</uuid>
  <os>
    <type>hvm</type>
    <loader>/usr/lib/xen/boot/hvmloader</loader>
    <boot dev='hd'/>
  </os>
  <memory>524288</memory>
  <vcpu>1</vcpu>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <features>
     <pae/>
     <acpi/>
     <apic/>
  </features>
  <clock sync="localtime"/>
  <devices>
    <emulator>/usr/lib/xen/bin/qemu-dm</emulator>
    <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 dev='hda'/>
    </disk>
    <disk type='file' device='cdrom'>
      <source file='/root/fc5-x86_64-boot.iso'/>
      <target dev='hdc'/>
      <readonly/>
    </disk>
    <disk type='file' device='floppy'>
      <source file='/root/fd.img'/>
      <target dev='fda'/>
    </disk>
    <graphics type='vnc' port='5904'/>
  </devices>
</domain>

There is a few things to notice specifically for HVM domains:

the optional <features> block is used to enable certain guest CPU / system features. For HVM guests the following features are defined:
- pae - enable PAE memory addressing
- apic - enable IO APIC
- acpi - enable ACPI bios
the optional <clock> element is used to specify whether the emulated BIOS clock in the guest is synced to either localtime or utc. In general Windows will want localtime while all other operating systems will want utc. The default is thus utc
the <os> 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 dev attribute on the boot tag can be one of:
- fd - boot from first floppy device
- hd - boot from first harddisk device
- cdrom - boot from first cdrom device
the <devices> section includes an emulator entry pointing to an additional program in charge of emulating the devices
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 hda-hdd, or a floppy device fda, fdb. The <disk> element also supports a 'device' attribute to indicate what kinda of hardware to emulate. The following values are supported:
- floppy - a floppy disk controller
- disk - a generic hard drive (the default it omitted)
- cdrom - a CDROM device
For Xen 3.0.2 and earlier a CDROM device can only be emulated on the hdc channel, while for 3.0.3 and later, it can be emulated on any IDE channel.
the <devices> 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 port attribute will be present indicating the TCP port on which the VNC server is accepting client connections.

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.

KVM domain (added in 0.2.0)

Support for the KVM virtualization 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 QEmu 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:

<domain type='kvm'>
  <name>demo2</name>
  <uuid>4dea24b3-1d52-d8f3-2516-782e98a23fa0</uuid>
  <memory>131072</memory>
  <vcpu>1</vcpu>
  <os>
    <type>hvm</type>
  </os>
  <clock sync="localtime"/>
  <devices>
    <emulator>/home/user/usr/kvm-devel/bin/qemu-system-x86_64</emulator>
    <disk type='file' device='disk'>
      <source file='/home/user/fedora/diskboot.img'/>
      <target dev='hda'/>
    </disk>
    <interface type='user'>
      <mac address='24:42:53:21:52:45'/>
    </interface>
    <graphics type='vnc' port='-1'/>
  </devices>
</domain>

The specific points to note if using KVM are:

the top level domain element carries a type of 'kvm'
the <clock> optional is supported as with Xen HVM
the <devices> emulator points to the special qemu binary required for KVM
networking interface definitions definitions are somewhat different due to a different model from Xen see below

except those points the options should be quite similar to Xen HVM ones.

Networking options for QEmu and KVM (added in 0.2.0)

The networking support in the QEmu and KVM case is more flexible, and support a variety of options:

Userspace SLIRP stack
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 10.0.2.15. The default router will be 10.0.2.2 and the DNS server will be 10.0.2.3. This networking is the only option for unprivileged users who need their VMs to have outgoing access. Example configs are:
```
<interface type='user'/>
```
```
<interface type='user'>
  <mac address="11:22:33:44:55:66"/>
</interface>
    
```
Virtual network
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 'virsh net-dumpxml <network name>'. 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 192.168.22.0/255.255.255.0. Each guest will have an associated tun device created with a name of vnetN, which can also be overridden with the <target> element. Example configs are:
```
<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>
    
```
Bridge to to LAN
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 overridden 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:
```
<interface type='bridge'>
 <source bridge='br0'/>
</interface>

<interface type='bridge'>
  <source bridge='br0'/>
  <target dev='vnet7'/>
  <mac address="11:22:33:44:55:66"/>
</interface>
```
Generic connection to LAN
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 overridden 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 overridden.
```
<interface type='ethernet'/>

<interface type='ethernet'>
  <target dev='vnet7'/>
  <script path='/etc/qemu-ifup-mynet'/>
</interface>
```
Multicast tunnel
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.
```
<interface type='mcast'>
  <source address='230.0.0.1' port='5558'/>
</interface>
```
TCP tunnel
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.

Example server config:
```
<interface type='server'>
  <source address='192.168.0.1' port='5558'/>
</interface>
```
Example client config:
```
<interface type='client'>
  <source address='192.168.0.1' port='5558'/>
</interface>
```

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.

QEmu domain (added in 0.2.0)

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 various CPU type and hardware platforms, and kqemu support (QEmu own kernel accelerator when the emulated CPU is i686 as well as the target machine):

<domain type='qemu'>
  <name>QEmu-fedora-i686</name>
  <uuid>c7a5fdbd-cdaf-9455-926a-d65c16db1809</uuid>
  <memory>219200</memory>
  <currentMemory>219200</currentMemory>
  <vcpu>2</vcpu>
  <os>
    <type arch='i686' machine='pc'>hvm</type>
    <boot dev='cdrom'/>
  </os>
  <devices>
    <emulator>/usr/bin/qemu</emulator>
    <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>

The difference here are:

the value of type on top-level domain, it's 'qemu' or kqemu if asking for kernel assisted acceleration
the os type block defines the architecture to be emulated, and optionally the machine type, see the discovery API below
the emulator string must point to the right emulator for that architecture

Discovering virtualization capabilities (Added in 0.2.1)

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. For example in the case of a 64 bits machine with hardware virtualization capabilities enabled in the chip and BIOS you will see

<capabilities>
  <host>
    <cpu>
      <arch>x86_64</arch>
      <features>
        <vmx/>
      </features>
    </cpu>
  </host>

  <!-- xen-3.0-x86_64 -->
  <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>

  <!-- hvm-3.0-x86_32 -->
  <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>
  ...
</capabilities>

The first block (in red) indicates the host hardware capabilities, 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).

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.

The third block (in green) gives similar informations but when running a 32 bit OS fully virtualized with Xen using the hvm support.

This section is likely to be updated and augmented in the future, see the discussion which led to the capabilities format in the mailing-list archives.