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Re: [Xen-devel] [PATCH 0/3 v2] XSAVE/XRSTOR fixes and enhancements

To: Weidong Han <weidong.han@xxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 0/3 v2] XSAVE/XRSTOR fixes and enhancements
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Wed, 1 Sep 2010 08:26:01 +0100
Cc: Tim Deegan <Tim.Deegan@xxxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>
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On 01/09/2010 02:53, "Weidong Han" <weidong.han@xxxxxxxxx> wrote:

>> Performance overhead of this fix? Is there no other lazy save technique that
>> can work?
> I think the cost of set_xcr0 which just changes some bits in XCR0
> register should be little.  I don't have any optimization for it now.

I obviously don't mean that. What about the increased cost of XSAVE and
XRSTOR s/r'ing more state unconditionally? At least it is conditional on
v->fpu_dirtied I suppose?

>>> Patch 3/3. Enable guest AVX
>>> This patch enables Intel(R) Advanced Vector Extension (AVX) for guest.
>> If we enable this but don't implement save/restore then don't guests lose
>> state across s/r with unpredictable results?
> Yes. As I said in another email, actually it already breaks hvm guests
> save/restore on platforms which supports XSAVE/XRSTOR.

Wow, so the last couple of Xen releases are broken for the latest Intel
platforms unless you specify no-xsave. Handy to know I guess.

Why is the feature flag stuff all stuffed in Xen itself rather than
xc_cpuid_x86.c, by the way? Shouldn't your change also be in the same place,
or (much preferably) all XSAVE related stuff be moved out into

 -- Keir

> Regards,
> Weidong

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