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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 08:39:31 +0100
Cc: Tim Deegan <Tim.Deegan@xxxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>
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On 01/09/2010 08:26, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx> wrote:

>> 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.

Actually, hang on, Dexuan's patch only enabled XSAVE of FPU/SSE and didn't
make any other new state visible. We already save/restore FPU/SSE of course,
so I don't see why the code we already have is broken for HVM save/restore.
It's the adding of new state that we don't s/r that would be broken --- like
your patch to make AVX visible.

Well that's how it was explained to me at the time. Was that in fact wrong
and we are already broken for save/restore for some subtle unexplained

 -- Keir

> 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
> libxc/xc_cpuid_x86.c?

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