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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 18:11:40 -0700
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, "Han, Weidong" <weidong.han@xxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
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 On 10/28/2010 05:57 PM, Haitao Shan wrote:
> 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.

Sure, if CR4.OSXSAVE is set iff xsave is available, then it should work
fine if we test that.


> 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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