StubDom

Stubdoms (or stub domains) are lightweight 'service' or 'driver' domain to run device models and one technique to implement Dom0 Disaggregation. The initial purpose of stub domains were to offload qemu workloads from dom0 into a seperate domain.

So with stub domains, a separate unprivileged stub domain is created per HVM guest. This boosts performance and makes your system more secure.

Xen 4.x : Device Model Stub Domains

For XL and newer versions of Xen, please see Device Model Stub Domains, which contains simple configuration examples for how to set up.

Xen 3.x

For older version of Xen, stub domains are harder to set up.

It is not clear when this example has last been tested. If you try it and find it to work, please remove the warning.

Configuration Example

• /etc/xen/OVM_EL5U1_X86_HVM_4GB:

name = 'OVM_EL5U1_X86_HVM_4GB'
uuid = 'de0edfdc-c6f1-4d2b-a2a3-88c035208c79'
builder = 'hvm'
device_model = '/usr/lib/xen/bin/stubdom-dm'
kernel = '/usr/lib/xen/boot/hvmloader'
memory = '256'
vif = [ 'mac=00:16:3e:6b:66:d7,bridge=xenbr0', ]
disk = [ 'file:/share/vm/OVM_EL5U1_X86_HVM_4GB/system.img,hda,w!', ]
acpi = 1
apic = 1
pae = 1
vcpus = 1
sdl = 0
vnc = 1
vnclisten = '192.168.1.199'
on_crash = 'restart'
on_reboot = 'restart'

Note: you can comment the disk statement here; but if you want use pvhvm drivers, you should leave it uncommented.

• /etc/xen/OVM_EL5U1_X86_HVM_4GB-dm:

disk = [ 'file:/share/vm/OVM_EL5U1_X86_HVM_4GB/system.img,hda,w!', ]
kernel = "/usr/lib/xen/boot/ioemu-stubdom.gz"
#vfb = [ 'type=sdl' ]
#vfb = [ 'type=vnc' ]
vif = [ 'ip=192.168.1.199', 'mac=00:16:3e:6b:66:d7,bridge=xenbr0', ]

• Create the VM:

# mkdir -p /exports/usr/share/xen/qemu
# ln -s /usr/share/xen/qemu/keymaps /exports/usr/share/xen/qemu
# mkdir -p /exports/var/lib
# ln -s /var/lib/xen /exports/var/lib
# /usr/sbin/fs-backend &
# xm create OVM_EL5U1_X86_HVM_4GB

• Connect to the VNC:

$ vncviewer 192.168.1.199

Note: you should make sure the host running vncviewer is in the same subnet 192.168.1.0.

Reference

• 2008 Boston Xen Summit slides on Stubdom.
• Xen 3.3 Feature: Stub Domains.
• Xen 3.3 Feature: HVM Device Model Domain.
• Stubdom README.