linux-kernel - Re: [CFT PATCH v2 2/2] KVM: x86: support XSAVES usage in the host

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Date:	Wed, 26 Nov 2014 18:31:02 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	wanpeng.li@...ux.intel.com, namit@...technion.ac.il,
	hpa@...ux.intel.com, Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [CFT PATCH v2 2/2] KVM: x86: support XSAVES usage in the host

2014-11-26 17:26+0100, Paolo Bonzini:
> On 26/11/2014 15:42, Radim Krčmář wrote:
> >> I'm not sure what is more future proof. :)  I wonder if native_xrstor
> >> could be a problem the day XRSTORS actually sets/restores MSRs as the
> >> processor documentation promises.
> > 
> > XRSTORS won't affect the guest in any way, we are just going to use it
> > to convert the xsave, so any side-effects are going to stay in the host.
> > (This could break the host though.)
> 
> Yes, that's the problem. :)

(It would be a bug in Linux's xsave API, if we were using it correctly.)

> > My main presumption is that XSAVE*->XRSTOR*->XSAVE->XRSTOR has the same
> > result as XSAVE->XRSTOR, because we are only interested in the state,
> > not in any metadata.
> > (If it isn't possible to combine intructions, like XSAVE after XRSTORS,
> >  this solution won't work.)
> 
> Yes, that should be right.  But actually what KVM would do it is
> XRSTOR->XSAVE*->XRSTOR*->XSAVE.  The problem here is the side effects of
> doing XRSTORS far from a guest entry...

The entry can be aborted after doing XRSTORS, so we are going to know if
this doesn't work :)

>                                         though that would likely be
> handled by load_guest_fpu/put_guest_fpu.

Yes, I don't see a principal difference between manipulating xsave for
vmentry and this conversion, it can be wrapped in the same way.
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