Re: [Xen-devel] [Patch 2/4] Refining Xsave/Xrestore support

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

 On 10/27/2010 07:57 PM, Haitao Shan wrote:
> Hi, Jeremy,
>
> My approach is based an old PV-OPS kernel source. Kernel tries to set
> CR4.OSXSAVE and read it back to determine whether Xsave is actually
> available.
> Could you still use that?

No, because some older versions of Xen kill the domain when they try to
set unknown bits in CR4.

    J

> Shan Haitao
>
> 2010/10/28 Jeremy Fitzhardinge <jeremy@xxxxxxxx>:
>>  On 10/27/2010 12:04 AM, Haitao Shan wrote:
>>> Hi, Keir,
>>>
>>> This is patch #2, which adds PV guest Xsave support.
>> How does a PV guest know whether Xsave support is available?  Previous
>> versions of Xen left the xsave cpu feature flag set even though xsave
>> wasn't usable by the domain, so I had to forceably mask it from the
>> cpuid features within the domain.  Given that a PV domain can't rely on
>> X86_FEATURE_XSAVE, how can it tell that the feature is actually usable?
>>
>> Thanks,
>>    J
>>


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