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SubjectRe: [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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