Sam Stelfox

Thoughts from a software engineer, systems hacker and Linux gubernāre.

Xen

Full Xen dom0 support was added in the Linux 3.0 kernel. This means there's no longer a need to drag patches forward from old kernels and work from special branches and git repositories when building a kernel for dom0.

Something else you might not have noticed is that the Fedora kernel team has quietly slipped Linux 3.0 into Fedora 15's update channels in disguise. Click that link, scroll down, and you'll see "Rebase to 3.0. Version reports as 2.6.40 for compatibility with older userspace." Although I'm not a fan of calling something what it isn't (2.6.40 doesn't exist on kernel.org), I can understand some of the reasoning behind the choice.

This change makes the Xen installation on Fedora 15 pretty trivial. To get started, update your kernel to the latest if you're not already on Fedora's 2.6.40 kernels:

yum -y upgrade kernel

We need three more packages (quite a few dependencies will roll in with them):

yum -y install xen libvirt python-virtinst

The xen package reels in the hypervisor itself along with libraries and command line tools (like xl and xm). Libvirt gives us easy access to VM management with the virsh command and python-virtinst gives us the handy virt-install command to make OS installations easy.

Once those packages are installed, we need to make some adjustments in your grub configuration. Open /boot/grub/menu.lst in your text editor of choice and add something like this at the bottom:

title Fedora + Xen (2.6.40-4.fc15.x86_64)
        root (hd0,1)
  kernel /boot/xen.gz
        module /boot/vmlinuz-2.6.40-4.fc15.x86_64 ro root=/dev/sda1
        module /boot/initramfs-2.6.40-4.fc15.x86_64.img

Ensure that the root (hd0,1) is applicable to your system (adjust it if it isn't). Also, check the kernel version to ensure it matches your installed kernel and adjust the root= portion to match your root volume. Flip the default line to a value which will boot your new grub entry and ensure the timeout is set to a reasonable number if you need to temporarily switch back to your original grub entry at boot time. (Hey, we all make mistakes.)

I take one extra precaution and change the UPDATEDEFAULT=yes line to no in /etc/sysconfig/kernel. This ensures that future kernel updates don't trample the entry you've just made. Keep in mind that you'll need to manually update your grub configuration when you do kernel upgrades later.

Cross your fingers and reboot. If your system doesn't reboot properly, reboot it again and choose your old kernel from the grub menu. Double-check your configuration for fat-fingering and give it another try. If your system boots and pings but you have no output via a monitor, don't fret. There's a patch for the problem which should appear soon in Linux 3.0. The impatient can snag a kernel source RPM, add the patch file, and build a local kernel (or you can download my local build from when I did it).

Log in and verify that you booted into the dom0:

[[email protected] ~]# xm dmesg | head -n 5
 __  __            _  _    _   _   ____     __      _ ____  
 \ \/ /___ _ __   | || |  / | / | |___ \   / _| ___/ | ___| 
  \  // _ \ '_ \  | || |_ | | | |__ __) | | |_ / __| |___ \ 
  /  \  __/ | | | |__   _|| |_| |__/ __/ _|  _| (__| |___) |
 /_/\_\___|_| |_|    |_|(_)_(_)_| |_____(_)_|  \___|_|____/

Once you're done with that, make sure libvirtd is running:

/etc/init.d/libvirtd start
chkconfig libvirtd on

Try installing a VM:

virt-install \
  --paravirt \
  --name=testvm \
  --ram=512 \
  --vcpus=4 \
  --file /dev/vmstorage/testvm \
  --graphics vnc,port=5905 --noautoconsole \
  --autostart --noreboot \
  --location=http://mirrors.kernel.org/debian/dists/squeeze/main/installer-amd64/

You should have a VM installation underway pretty quickly and it will be visible via port 5905 on the local host. Enjoy the power and freedom of your brand new type 1 hypervisor.

Additional notes about dom0's CPU and memory