Xen Notes



Not meant to be complete.

Term Explanation
Type I Hypervisor Runs directly on hardware. Virtual machines don’t know they’re virtualized.
Type II Hypervisor Hypervisor (Xen) runs in OS (RHEL/CentOS). The virtual machines ‘‘know’’ they’re being run in a virtual environment
HVM (“Hardware Virtual Machine” or “Hardware-assisted Virtualization”) Not entirely sure about this. Certain processor technology (e.g. Intel VT-x) allows “complete simulation of underlying hardware.” VMs don’t know they’re virtualized.
dom0 The hypervisor itself
domU A single virtual instance
xm Xen-provided tool to manage domU’s
virsh A Red Hat-designed shell to manage VM’s. Differs from xm in that it can manage QEMU and HVM-based domU’s as well since it’s based on the libvirt API.
virt-install and virt-manager Management and provisioning tools based on libvirt/


yum groupinstall Xen  
yum install python-virtinst qemu*

The first installs the Xen-enabled kernel, Xen daemon, virtualization libraries, etc. Make sure that (a) SELinux is disabled, and (b) that you reboot into the Xen kernel before doing anything else.

The First VM

Preparing the dom0

Creating the VM

virt-manager is the easiest way to do things. You can do a command-line install via virt-install. Here’s a sample command that creates a 64-bit VM called “devel1” running CentOS 6 with two virtual CPUs and 1.2GB of RAM. Observe that I explicitly specify the MAC address.

virt-install \  
--name=devel1 \  
--arch=x86_64 \  
--vcpus=2 --check-cpu \  
--ram=1200 \  
--disk path=/dev/xenspace/devel1 \  
--mac=00:0C:29:1A:98:D5 \  
--os-type=linux \  
--os-variant=rhel6 \  
--location=http://hypervisor.example.com/install/6/x86_64/ \  
--debug \  

Once the VM is installed, it’s a good idea to save the kickstart files. Here’s a sample:

# Modified by Nikhil Anand 
url --url http://hypervisor.example.com/install/6/x86_64/
lang en_US.UTF-8
keyboard us
network --device eth0 --bootproto dhcp
rootpw --iscrypted $1$9P2b0WZe$CSd.fBGCVjjUfzlZ6m5Rk1
firewall --enabled --port=22:tcp
authconfig --enableshadow --enablemd5
selinux --enforcing
timezone --utc America/Chicago
bootloader --location=mbr --driveorder=xvda
# The following is the partition information you requested
# Note that any partitions you deleted are not expressed
# here so unless you clear all partitions first, this is
# not guaranteed to work
clearpart --linux --drives=xvda
part /boot --fstype ext3 --size=100 --ondisk=xvda
part pv.6 --size=0 --grow --ondisk=xvda
volgroup VolGroup00 --pesize=32768 pv.6
logvol / --fstype ext3 --name=LogVol00 --vgname=VolGroup00 --size=1024 --grow
logvol swap --fstype swap --name=LogVol01 --vgname=VolGroup00 --size=528 --grow --maxsize=1056


If you ever wanted to reinstall the VM, you can now append a flag with the (HTTP downloadable) path to the kickstart file:

-x "ks=http://hypervisor.example.com/kickstarts/centos-6.ks"

HVM Support

You can find if your processor supports HVM by issuing

egrep '^flags.*(vmx|svm)' /proc/cpuinfo

Network Topologies

Xen offers the following:

It’s unusual (and crazy) to use all three on a given dom0 instance. The default is bridged networking. The brctl command is used to manage network bridges.

In our case, the router hands out DHCP leases depending on MAC addresses. This is why I didn’t have to do anything other than specify the MAC address in a domU’s config:

vif = [ "mac=00:50:56:78:0a:1b,bridge=xenbr0,script=vif-bridge" ]

More exotic configurations are possible. You can, for example, specify two virtual interfaces (vif’s), with public and private IPs. In this case, the route and iptables commands become important, since you’ll have to set up routes and masquerading.

Edit /etc/xen/xend-config.sxp to set up these configs. For instance, if you only had a routed config, you’d comment out every other network-script and vif-script other than these:

#(network-script network-route)  
#(vif-script     vif-route)


virt-install removes the kernel and ramdisk lines from a domU’s config file and adds this instead:

bootloader = "/usr/bin/pygrub"

PyGRUB itself will look for the first partition or LVM container that contain the kernel and init image.

I made an error of using the CentOS project-supplied kernel and ramdisk, which were good for an install, but useless when the domU was rebooted. They’re built specifically for installation :)

“Could not connect to localhost:8000”

You may see this when using virt-install or virt-manager. Edit /etc/xen/xend-config.sxp and make sure these lines are uncommented:

(xend-http-server yes)  
(xend-port 8000)  
(xend-address localhost)

And restart the Xen daemon.


You’re supposed to be able to edit /etc/sysconfig/xend, uncomment this line and see logs in /var/log/xen/console


Didn’t work for me.


“Guest name already in use”

virsh undefine <guestname>