We recently found that FreeBSD 8.0 guest failed to install and boot on 
Xen. The reason was that FreeBSD detected clflush feature and invoked 
this instruction to flush MMIO space. This caused a page fault; but 
x86_emulate.c failed to emulate this instruction (not supported). As a 
result, a page fault was detected inside FreeBSD. A similar issue was 
reported earlier.

http://lists.xensource.com/archives/html/xen-devel/2010-03/msg00362.html

I created a patch which enables clflush emulation. I have verified that 
this patch solves FreeBSD issue. This patch returns immediately without 
doing anything. Note that we don't have to intercept clflush in SVM. So 
the only time we have page_fault for clflush instruction is when guest 
VM flushes MMIO space. In this case, do we need to send this command 
over to QEMU? I didn't do it in this patch anyway.

Best,
-Wei