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

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: Re: [Xen-devel] [Patch 2/4] Refining Xsave/Xrestore support
From: Haitao Shan <maillists.shan@xxxxxxxxx>
Date: Fri, 29 Oct 2010 08:57:00 +0800
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, "Han, Weidong" <weidong.han@xxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
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Then what about the approach that Jan uses in 2.6.27 tree, which is
reading CPUID-OSXSAVE? If supported, Xen will always set CR4.OSXSAVE.

Shan Haitao

2010/10/29 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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