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Re: [Qemu-devel] [PATCH] target-i386: kvm: cache KVM_GET_SUPPORTED_CPUID

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH] target-i386: kvm: cache KVM_GET_SUPPORTED_CPUID data
Date: Tue, 14 Jun 2016 10:21:41 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.0

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.

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


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