Fedora Host Installation

This page covers the steps to get a working Xen host system (a.k.a. dom0) running Fedora.

Fedora 16 is the first version of Fedora that comes with a kernel that supports a Xen dom0 out of the box since Fedora 8. This is a result of dom0 support being merged into the mainline kernel. As such, there are very few steps that have to be done to get a functional dom0. Some information about using a Fedora system as a virtualization host, coming directly from them, can be found here.

Installing Fedora

Base System Installation

For the most part, you can follow the Installation steps outlined in the official Fedora installation guides (here they are for F16, F17 and F18). If you're lazy, you probably may just look at the installation quick start guides (F16, F17).

For obtaining install media images, look here.

The only portion you have to watch out is the disk partitioning section. If you're going to use file-backed domUs, you can safely ignore this step. However, if you're going to use LVM (or other block devices, like hard drive partitions)-backed domUs, you're probably going to partition your drive into /boot, and at least one or two volume groups, meaning that you'll be making a custom partitioning scheme. Have a look here for more details about this Host OS Install Considerations.

Updating the System

You probably want to be sure the all the software is as up-to-date as possible. To do that, run

 # yum update

as root (or use any graphical front-end, e.g., find and click on 'Software Update').

Using the virt-preview Repository

People who would like to test the very latest virtualization related Fedora packages (coming from Fedora rawhide) can enable the Virtualization Preview Repository (please, notice that Fedora discourages doing this in 'production' deployments).

In order to make it happen, just do as follows:

# cd /etc/yum.repos.d/
# wget http://fedorapeople.org/groups/virt/virt-preview/fedora-virt-preview.repo
# yum update

Installing and Running Xen

Installing the Packages

Getting the Xen hypervisor is a simple matter - just run

 # yum install xen

as root. Yum will take care of downloading the dependencies and installing them for you automatically.

As part of the download, Yum will create a new grub2 boot menu. As such, don't panic if Yum seems to freeze at "Installing xen-hypervisor" for a while.

Once the installation is complete, you should reboot, and select Xen 4.x.x from the GRUB menu.

Networking deserves some special attention. In fact, it is something that gave a lot of trouble in the past (Xen 3/4.0). However, starting from Xen 4.1, the default behavior is to let the dom0's network scripts (i.e., the ones that came with the distribution, not the ones that come with Xen) handle setting up a network bridge, and let the toolstack just attach the domU's network interface to the it.

Booting Xen by Default

Although a Xen GRUB2 menu entry will be automatically created, it will likely not be the default boot option. If this is what you want, ensure to you have a line like this in /etc/default/grub:

GRUB_DEFAULT="saved"

and then do:

 # grub2-mkconfig -o /boot/grub2/grub.cfg
 # grep ^menuentry /boot/grub2/grub.cfg | cut -d "'" -f2
 # grub2-set-default <menu entry title you want>

Disabling Network Manager

In case of any networking issues, or if you want to manually manage the network configuration, disabling NetworkManager is necessary (as NetworkManager does not support bridges), and that requires manually editing a couple of config files. This is particularly true if you do not plan to install and use the typical Fedora virtualization tools, like libvirt, etc. In fact, libvirt automatically creates a bridge and sets up the VMs created via its interface(s) to use it by default. If not installing libvirt, the bridge has to be created by hand.

For Red Hat-based distros, the networking configuration is stored in /etc/sysconfig/network-scripts/. By default, NetworkManager is included in the default Fedora install. To disable it, do the following (as root):

 # systemctl disable NetworkManager.service || systemctl restart network.service

Also, make sure that the network service is kicked off automatically on subsequent restarts by running:

 # chkconfig network on

Next, create the configuration file for the bridge. The most common bridge name is br0, so let's stick to that (although other Xen documentation suggests using xenbr0 to make it obvious that the bridge is for Xen):

 # touch /etc/sysconfig/network-scripts/ifcfg-br0

open the file in a text editor and put the following lines in it:

DEVICE=br0
TYPE=Bridge
BOOTPROTO=dhcp
ONBOOT=yes
DELAY=0
NM_CONTROLLED=no

Next, you need to find the configuration file for your existing network adaptor. It's most likely named ifcfg-eth0 or ifcfg-em0 (the part after the dash can and does change depending on the actual hardware). Once you know what config file you're using, open it, find the line

NM_CONTROLLED=yes

and change it to

NM_CONTROLLED=no

. Also, do not forget to add the line

BRIDGE=br0

(or whatever you named your bridge) to that file.

Finally, run this:

 # systemctl restart network.service

and your network should be looking like below:

 # ifconfig br0
 br0       Link encap:Ethernet  HWaddr 00:07:E9:06:50:12
           inet addr:192.168.1.122  Bcast:192.168.1.255  Mask:255.255.255.0
           inet6 addr: fe80::207:e9ff:fe06:5012/64 Scope:Link
           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
           RX packets:1858 errors:0 dropped:0 overruns:0 frame:0
           TX packets:678 errors:0 dropped:0 overruns:0 carrier:0
           collisions:0 txqueuelen:0
           RX bytes:329223 (321.5 KiB)  TX bytes:76663 (74.8 KiB)

Rebooting and Installing DomUs

Now that Xen is installed, boot it and check it's working, e.g., by doing by running xl info from the command line. Your screen should then look like the below:

# xl info
host                   : caesium
release                : 3.1.1-2.fc16.i686.PAE
version                : #1 SMP Mon Nov 14 15:57:20 UTC 2011
machine                : i686
nr_cpus                : 4
nr_nodes               : 1
...

If Xen's not working, you'll get something like:

libxl: error: libxl.c:56:libxl_ctx_init Is xenstore daemon running?
failed to stat /var/run/xenstored.pid: No such file or directory
cannot init xl context

By this point, your system should be set up to install domUs. You can install domUs with a variety of ways, e.g., by using xl. Notice that xl is the default toolstack since Xen 4.2, and therefore, xend is not started by default. If wanting to use xm/xend, you have to start the daemon manually, with something like:

 # xend

but that will make it impossible to use xl.

Using libvirt and Typical Fedora Virt-Tools

Installing the Packages

If wanting to use libvirt and/or VirtManager, just do the following (tested on Fedora 18):

 # yum install libvirt-daemon-driver-xen python-virtinst libvirt-daemon-config-network libvirt-daemon-driver-network virt-manager virt-viewer

Most likely, everything will just work, and you are all set for starting creating VMs, e.g., with virt-install as described here or with VirtManager, as described here. That happens because libvirt automatically creates and configures bridging.

libvirt: libxl VS. xend Starting from Xen 4.2, the default toolstack is libxl. libvirt has a libxl driver, but libvirt packages for F18 does not ship the proper version of it. On the other hand, the xend based driver is known to work quite well, so it might be best (for now) to just use that. That happens by making sure that libvirt-daemon-driver-libxl is not installed (again, tested on Fedora 18). There is an on-going effort to fill the gap and render the libvirt libxl driver feature complete and fully functional, although it is yet unclear when that will happen. This page will be kept up-to-date with any progress being made with respect to this, so come back here periodically and check them out.

Installing DomUs

With libvirt, installing VMs, especially in case of Red Hat based distros (like Fedora/CentOS), is extremely easy. You can refer to:

• installing a domU with virt-install (console based) or,
• installing a domU with virt-manager (GUI based).

Known Issues

GRUB2 Menu in Fedora 16

In Fedora 16 there is a known bug that results in GRUB2 generating a number of boot entries for each Xen file that it finds in /boot. Unfortunately, it doesn't check if the file in question is a linked file, and results in generating a host of spurious entries. This is no longer the case from Fedora 17.

SELinux and LVM in Fedora 16 & 17

In Fedora 16 and 17, you may also need to disable SELinux if you're managing LVM storage manually, unless you want to relabel the created LVs. xl will complain that "the disk is not accessible" if you fail to do this and you're using LVs for storage. This does not seem to be an issue in Fedora 18 (if it is for you, report it to fedora-virt and/or fedora-xen).

Kernel Update in Fedora 16, 17 & 18

When a new kernel is installed, as a consequence of running:

 # yum update

grubby is used to update the GRUB2 config file. As reported by this bug, grubby does not handle Xen menu entries very well, and thus it is possible for Xen to no longer be able to boot. We're looking into possible fixes. For now, the solution is to manually run, after each update that install a new kernel, the following command:

# grub2-mkconfig -o /boot/grub2/grub.cfg

That is why, when updating the xen-hypervisor package, it is grub2-mkconfig that is invoked, and hence the issue does not manifest itself in that case.

xend vs. libxl in Fedora 16, 17 & 18

Starting from Xen 4.2, the default toolstack is libxl. libvirt has a libxl driver, but libvirt packages does not ship the proper version of it. On the other hand, the xend based driver is known to work quite well, so it is best (for now) to just use it. That happens by making sure that libvirt-daemon-driver-libxl is not installed (tested recently on Fedora 18).

There is an on-going effort to fill the gap and render the libvirt libxl driver feature complete and fully functional, although it is yet unclear when that will happen.