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

To: Haitao Shan <maillists.shan@xxxxxxxxx>
Subject: Re: [Xen-devel] [Patch 2/4] Refining Xsave/Xrestore support
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Thu, 28 Oct 2010 09:18:10 -0700
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, "Han, Weidong" <weidong.han@xxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
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 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.


> 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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