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

To: Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] [Patch 2/4] Refining Xsave/Xrestore support
From: Haitao Shan <maillists.shan@xxxxxxxxx>
Date: Thu, 28 Oct 2010 15:55:30 +0800
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Weidong Han <weidong.han@xxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
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In my patch, if processor supports CR4.OSXSAVE, Xen will enable it and
won't clear it as long as we are in ROOT mode.

Shan Haitao

2010/10/28 Jan Beulich <JBeulich@xxxxxxxxxx>:
>>>> On 28.10.10 at 04:57, Haitao Shan <maillists.shan@xxxxxxxxx> wrote:
>> 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.
> Hmm, in our kernels we check the OSXSAVE feature bit instead,
> expecting (just like for FXSAVE) the hypervisor to enable the bit
> in CR4 when it supports the feature.
> I think a mechanism needs to be found in the hypervisor that
> accommodates both current pv-ops and current forward port
> kernels' behaviors.
> Jan
>> 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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