Xen 3.4 – 4.0 Bug On AMD 6100 Series (Magny-Cours) Opteron
Xen 3.4 through 4.0 will not boot on the new AMD 6100 series (Magny-Cours) Opteron CPU (Socket G34). There seems to be a bug in the Machine Check Exception (MCE) handling code that causes Xen to panic. The 'nomce' (3.4) and 'no-mce' (4.0) boot options do not properly turn off the MCE code, so this cannot be used to avoid the issue.
There is a patch in Xen 4.0 unstable that fixes the 'nomce' boot option so that it works properly. Unfortunately this change has not been back-ported to Xen 3.4 by most of the Linux distros yet.
I have applied the fix to Xen 3.4.1 for OpenSUSE 11.2 and built RPMs for the affected packages. This did the trick and my AMD 6100 series systems are now running Xen just fine with the 'nomce' boot option. Read on for more details and the link to the RPMs with the fix.
Xen 3.2 Bridged Networking Explained by Experiment
There exists a lot of documentation explaining how to setup specific network configurations, but nothing that went through a complicated configuration in step-by-step detail. After a day of experimentation I figured out the exact symantics of the bridging configuration. This is still pretty non-advanced stuff, but I am attempting to explain it in detail to help make it clear to new users right from the start.