linux-kernel - Re: [PATCH] KVM: SVM: Add xsetbv intercept

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Message-ID: <20101207161211.GD13125@amd.com>
Date:	Tue, 7 Dec 2010 17:12:11 +0100
From:	"Roedel, Joerg" <Joerg.Roedel@....com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: SVM: Add xsetbv intercept

On Tue, Dec 07, 2010 at 04:34:33AM -0500, Avi Kivity wrote:
> On 12/06/2010 07:48 PM, Roedel, Joerg wrote:
> > On Mon, Dec 06, 2010 at 11:10:46AM -0500, Avi Kivity wrote:
> > >  On 12/03/2010 06:42 PM, Joerg Roedel wrote:
> > >  >  This patch implements the xsetbv intercept to the AMD part
> > >  >  of KVM. This makes AVX usable in a save way for the guest on
> > >  >  AVX capable AMD hardware.
> > >  >  The patch is tested by using AVX in the guest and host in
> > >  >  parallel and checking for data corruption. I also used the
> > >  >  KVM xsave unit-tests and they all pass.
> > >  >
> > >
> > >  That is really strange.  You didn't need to do anything to get cpuid.avx
> > >  recognized.  So running an older kvm on newer hardware will happily
> > >  expose avx even though it's not supported.
> > >
> > >  We screwed up - we should have made cpuid.avx dependent on vendor support.
> >
> > Hmm, right. The set_supported_cpuid arch-callback should basically
> > disable xsave on AMD for all KVM versions which do not handle the xsetbv
> > intercept.
> >
> 
> Please post a patch to do that, and update this patch to undo the 
> change.  We'll backport the first patch to -stable so that people 
> running older kernels don't get a nasty surprise when they upgrade their 
> hardware (and use -cpu host).

Ok, I'll post them soon.

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