[Fedora-xen] Fedora 12 Xen guests (domU) and hosts (dom0)

[Pasi Kärkkäinen]

Hello,

I thought of writing some information about running Fedora 12 Xen guests
and also Fedora 12 Xen hosts/dom0.

Fedora 12 includes the upstream Linux pv_ops Xen domU support in the default kernel.
By using virt-install or virt-manager you can install Fedora 12 Xen PV 
(paravirtual) guests directly from network, for example on RHEL 5.4, 
CentOS 5.4, or Fedora Xen dom0/host.

If you want to run Fedora 12 Xen dom0 (host), there are some extra steps
needed. Fedora 12 ships with Xen hypervisor and management tools 
(Xen 3.4.1, and Xen 3.4.2 in the F12 updates), but the required Xen dom0 
capable host kernel is not included in Fedora atm/yet.

General information about Xen dom0 status in Fedora: 
http://fedoraproject.org/wiki/Features/XenPvopsDom0

There are a couple of different ways to get and install a Xen dom0
capable kernel to Fedora 12 host:

- By using pre-packaged pv_ops xendom0 kernel rpms by M A Young. his repository:
  http://fedorapeople.org/~myoung/dom0/

- Compiling and installing Xen dom0 capable kernel yourself/manually.
  
  There are many options to choose from, full list of the available Xen
  dom0 capable kernels is here:
  http://wiki.xensource.com/xenwiki/XenDom0Kernels

  The recommended dom0 kernel is the pv_ops kernel, which is in the
  process of being cleaned up for upstream/mainline Linux inclusion.

  More information about pv_ops dom0 kernel, including the status
  reports:
  http://wiki.xensource.com/xenwiki/XenParavirtOps

  Xen developers are interested of both the success and failure reports
  when using the Xen pv_ops dom0 kernel.


Tips for running Xen with Fedora 12:

	- Make sure you install all the latest Fedora 12 updates, since they
	  have an updated Xen version (3.4.2), and also fix a bug in
	  python-virtinst tool to make Xen guest console keymaps work properly:
	  https://bugzilla.redhat.com/show_bug.cgi?id=533707

	- Recent versions of Xen pv_ops dom0 kernel renamed some xen
	  backend driver modules to have "xen-" prefix in them, for
	  example evtchn module became xen-evtchn. Fedora Xen 3.4.2 init
	  scripts take care of this, and load the correct modules, but
	  Xen 3.4.1 doesn't do this automatically, causing xend fail to
	  start in dom0/host before the modules are loaded manually.

	- There are some upcoming apic-related changes coming in pv_ops 
	  dom0 kernel, which will require a patch to Xen hypervisor. 
	  This patch is not yet included in the Fedora Xen rpms. 
	  The patch will be added to Fedora Xen rpms when the upstream 
	  pv_ops kernel starts to require/use it.

	- pv_ops dom0 kernel currently lacks blktap2 support, so using
	  tap:aio: backend disk image files is not yet possible. 
	  Xen file: backend image files work though. 

	  At the moment it's recommended to use LVM volumes for guest
	  disks (Xen phy: backend).

	- virt-manager seems to work OK on Fedora 12 Xen dom0.
	
	- If you experience problems related to Fedora 12 guests (domU)
	  kernel crashing (especially related to save/restore/migration), 
	  install the latest Fedora updates to the guest. There has been 
	  a lot of Xen guest related fixes in the upstream Linux recently. 
	  These fixes will appear in Fedora when the kernel gets updated 
	  to include the latest stable upstream fixes.


Hopefully that helps :)

-- Pasi