Re: KVM causes #GP on XRSTORS

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Nov 21, 2014 at 03:46:55PM +0100, Paolo Bonzini wrote:
> 
> 
> On 20/11/2014 17:34, Nadav Amit wrote:
> > Fenghua,
> > 
> > I got KVM (v3.17) crashing on a machine that supports XRSTORS - It appears to get a #GP when it is trying to load the guest FPU.
> > One reason for the #GP is that XCOMP_BV[63] is zeroed on the guest_fpu, but I am not sure it is the only problem.
> > Was KVM ever tested with XRSTORS?
> 
> What is the content of the CPUID[EAX=13,ECX=0] and CPUID[EAX=13,ECX=1]
> leaves on the host?
> 
> Fenghua, which processors have XSAVEC, which have XGETBV with ECX=1, and
> which have XSAVES?  We need to expose this in QEMU, for which I can send
> a patch later today or next week (CCing Eduardo for this).
> 
> We will also have to uncompact the XSAVE area either in KVM_GET_XSAVE or
> in QEMU.  It's probably not hard to do it in the kernel.

Oh, now I see that KVM uses xsave_state(), which uses XSAVEOPT and
XSAVES depending on the host CPU features. I didn't know that.

QEMU already expects the standard format to be part of the KVM_GET_XSAVE
ABI. So this would already cause problems in host CPUs supporting
XSAVES, even if XSAVES is not exposed to guests yet.

-- 
Eduardo