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Date: Tue, 14 Jun 2016 16:31:37 +0800
From: Chao Peng <chao.p.peng@linux.intel.com>
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Subject: Re: [Qemu-devel] [PATCH] target-i386: kvm: cache
 KVM_GET_SUPPORTED_CPUID data
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org, kvm@vger.kernel.org, Eduardo Habkost <ehabkost@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>, Richard Henderson <rth@twiddle.net>

On Tue, Jun 14, 2016 at 10:21:41AM +0200, Paolo Bonzini wrote:
> 
> 
> On 14/06/2016 07:01, Chao Peng wrote:
> >> > 
> >> > Which are the CPUID leaves for which KVM_GET_SUPPORTED_CPUID is not
> >> > stateless?  I cannot find any.
> > I have though leaf 0xd, sub leaf 1 is not stateless, as the size of
> > xsave buffer(EBX) is based on XCR0 | IA32_XSS. But after looking KVM
> > code more carefully, seems I was wrong. The code calculates EBX with the
> > host xcr0 but not guest xcr0, nor guest IA32_XSS (not sure if this is
> > the correct behavior), so it can always returns constant data on a
> > certain machine.
> 
> Indeed, KVM computes the correct value at runtime, but
> KVM_GET_SUPPORTED_CPUID runs before there is a value for guest XCR0 or
> guest IA32_XSS.

Yes, this is the point.

> 
> Thanks, I've queued the patch for QEMU 2.7.

Thanks :)

Chao