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Re: [Xen-devel] Windows SMP

On 29/12/2008 02:59, "Venefax" <venefax@xxxxxxxxx> wrote:

> I had to disable both, and PAE. Only APIC=0 would not make any difference. I
> will some further testing with Citrix Xenserver 5, using the same virtual
> machine and another copy with their vmpd drivers. I bet that there is no
> difference in performance. It seems to be a Xen architectural issue. Any
> ideas?

The problem is almost certainly APIC related. APIC=0 actually has no effect
for a multi-processor HVM guest, since APICs are architecturally absolutely
required in x86 multi-processor systems.

The problem is most likely lots of emulated APIC TPR writes slowing things
down. Possible fixes:
 1. Run a Windows guest with the 'lazy TPR' optimisation -- w2k3sp2+, w2k8,
vista. Or run 64-bit Windows which will write TPR in a different way which
most Intel/AMD CPUs can virtualise efficiently.
 2. Run a new enough Intel processor which has automatic TPR handling even
for 32-bit Windows guests.
 3. Run Citrix drivers which patch Windows to avoid TPR writes.

 -- Keir

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