Project Links¶

• The Xen hypervisor on Linux and Solaris hosts

Deployment pre-requisites¶

The libvirt libxl driver uses Xen's libxl API, also known as libxenlight, to implement libvirt's hypervisor driver functionality. libxl provides a consolidated interface for managing a Xen host and its virtual machines, unlike old versions of Xen where applications often had to communicate with xend, xenstored, and the hypervisor itself via hypercalls. With libxl the only pre-requisit is a properly installed Xen host with the libxl toolstack running in a service domain (often Domain-0).

Connections to libxl driver¶

The libvirt libxl driver is a single-instance privileged driver, with a driver name of 'xen'. Some example connection URIs for the libxl driver are:

xen:///system                     (local access, direct)
xen+unix:///system                (local access, via daemon)
xen://example.com/system          (remote access, TLS/x509)
xen+tcp://example.com/system      (remote access, SASl/Kerberos)
xen+ssh://root@example.com/system (remote access, SSH tunnelled)

Location of configuration files¶

The libxl driver comes with sane default values. However, during its initialization it reads a configuration file which offers system administrator to override some of that default. The file is located under /etc/libvirt/libxl.conf

Import and export of libvirt domain XML configs¶

The libxl driver currently supports three native config formats. The first, known as xen-xm, is the original Xen virtual machine config format used by the legacy xm/xend toolstack. The second, known as xen-sxpr, is also one of the original formats that was used by xend's legacy HTTP RPC service ( removed in 5.6.0 )

The third format is xen-xl, which is the virtual machine config format supported by modern Xen. The xen-xl format is described in the xl.cfg(5) man page.

$ virsh -c xen:///system domxml-from-native xen-xm rhel5.cfg
<domain type='xen'>
  <name>rhel5pv</name>
  <uuid>8f07fe28-753f-2729-d76d-bdbd892f949a</uuid>
  <memory>2560000</memory>
  <currentMemory>307200</currentMemory>
  <vcpu>4</vcpu>
  <bootloader>/usr/bin/pygrub</bootloader>
  <os>
    <type arch='x86_64' machine='xenpv'>linux</type>
  </os>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <disk type='file' device='disk'>
      <driver name='tap' type='aio'/>
      <source file='/var/lib/xen/images/rhel5pv.img'/>
      <target dev='xvda' bus='xen'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='tap' type='qcow'/>
      <source file='/root/qcow1-xen.img'/>
      <target dev='xvdd' bus='xen'/>
    </disk>
    <interface type='bridge'>
      <mac address='00:16:3e:60:36:ba'/>
      <source bridge='xenbr0'/>
    </interface>
    <console type='pty'>
      <target port='0'/>
    </console>
    <input type='mouse' bus='xen'/>
    <graphics type='vnc' port='-1' autoport='yes' listen='0.0.0.0'/>
  </devices>
</domain>

$ virsh -c xen:///system domxml-to-native xen-xm rhel5pv.xml
name = "rhel5pv"
uuid = "8f07fe28-753f-2729-d76d-bdbd892f949a"
maxmem = 2500
memory = 300
vcpus = 4
bootloader = "/usr/bin/pygrub"
kernel = "/var/lib/xen/boot_kernel.0YK-cS"
ramdisk = "/var/lib/xen/boot_ramdisk.vWgrxK"
extra = "ro root=/dev/VolGroup00/LogVol00 rhgb quiet"
on_poweroff = "destroy"
on_reboot = "restart"
on_crash = "restart"
sdl = 0
vnc = 1
vncunused = 1
vnclisten = "0.0.0.0"
disk = [ "tap:aio:/var/lib/xen/images/rhel5pv.img,xvda,w", "tap:qcow:/root/qcow1-xen.img,xvdd,w" ]
vif = [ "mac=00:16:3e:60:36:ba,bridge=virbr0,script=vif-bridge,vifname=vif5.0" ]

Pass-through of arbitrary command-line arguments to the qemu device model¶

Since 6.7.0, the Xen driver supports passing arbitrary command-line arguments to the qemu device model used by Xen with the <xen:commandline> element under domain. In order to use command-line pass-through, an XML namespace request must be issued that pulls in http://libvirt.org/schemas/domain/xen/1.0. With the namespace in place, it is then possible to add <xen:arg>sub-elements to <xen:commandline> describing each argument passed to the device model when starting the domain.

The following example illustrates passing arguments to the QEMU device model that define a floppy drive, which Xen does not support through its public APIs:

<domain type="xen" xmlns:xen="http://libvirt.org/schemas/domain/xen/1.0">
  ...
  <xen:commandline>
    <xen:arg value='-drive'/>
    <xen:arg value='file=/path/to/image,format=raw,if=none,id=drive-fdc0-0-0'/>
    <xen:arg value='-global'/>
    <xen:arg value='isa-fdc.driveA=drive-fdc0-0-0'/>
  </xen:commandline>
</domain>

Example domain XML config¶

Below are some example XML configurations for Xen guest domains. For full details of the available options, consult the domain XML format guide.

<domain type='xen' >
  <name>fc8</name>
  <bootloader>/usr/bin/pygrub</bootloader>
  <os>
    <type>linux</type>
  </os>
  <memory>131072</memory>
  <vcpu>1</vcpu>
  <devices>
    <disk type='file'>
      <source file='/var/lib/xen/images/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>

<domain type='xen' >
  <name>fc8</name>
  <os>
    <type>linux</type>
    <kernel>/var/lib/xen/install/vmlinuz-fedora8-x86_64</kernel>
    <initrd>/var/lib/xen/install/initrd-vmlinuz-fedora8-x86_64</initrd>
    <cmdline> kickstart=http://example.com/myguest.ks </cmdline>
  </os>
  <memory>131072</memory>
  <vcpu>1</vcpu>
  <devices>
    <disk type='file'>
      <source file='/var/lib/xen/images/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>
    <graphics type='vnc' port='-1'/>
    <console tty='/dev/pts/5'/>
  </devices>
</domain>

<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='/var/lib/xen/images/fv0'/>
      <target dev='hda'/>
    </disk>
    <disk type='file' device='cdrom'>
      <source file='/var/lib/xen/images/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>

<domain type='xen' id='3'>
  <name>fv0</name>
  <uuid>4dea22b31d52d8f32516782e98ab3fa0</uuid>
  <os>
    <type>hvm</type>
    <loader>/usr/lib/xen/boot/hvmloader</loader>
    <kernel>/var/lib/xen/install/vmlinuz-fedora8-x86_64</kernel>
    <initrd>/var/lib/xen/install/initrd-vmlinuz-fedora8-x86_64</initrd>
    <cmdline> kickstart=http://example.com/myguest.ks </cmdline>
  </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='/var/lib/xen/images/fv0'/>
      <target dev='hda'/>
    </disk>
    <disk type='file' device='cdrom'>
      <source file='/var/lib/xen/images/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>