Xen devel - Dec 2006 - Why "Mask cmpxchg8b capability in cpuid for non-pae guests"?

Hi Tim, 
I noticed you "Mask cmpxchg8b capability in cpuid for non-pae guests"
in
changeset 13107/13108: clear_bit(X86_FEATURE_CX8 & 31, edx). May I know
the reason for the change?

We found an issue caused by the change: we can NOT install or boot 32bit
HVM Windows 2003 guest on 32bit host.
When we try to install 32bit HVM Windows 2003 guest on 32bit host, the
windows installer complains:
Windows requires certain processor features that are available on the
processor in this computer.   Specifically, Windows requires the
following instructions.
CPUID 
CMPXCHG8B 
Setup cannot continue. Press any key to exit.

Thanks!
 -- Dexuan

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On 28/12/06 9:55 am, "Cui, Dexuan" <dexuan.cui@intel.com> wrote:
> I noticed you "Mask cmpxchg8b capability in cpuid for non-pae
guests" in
> changeset 13107/13108: clear_bit(X86_FEATURE_CX8 & 31, edx). May I know
> the reason for the change?
It''s needed to successfully install NT4, which has a bug in older
service
packs where it barfs on the CMPXCHG8B feature bit in CPUID. The right
''fix''
is to allow arbitrary masking of CPUID features in the guest config file.
We''ll revert this workaround.

 Thanks,
 Keir



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