linux-kernel - Re: [PATCH v2 2/4] kvm: cpuid: fix the size of xsaves area

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Date:	Thu, 04 Dec 2014 16:49:10 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>,
	Wanpeng Li <wanpeng.li@...ux.intel.com>
CC:	Nadav Amit <nadav.amit@...il.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/4] kvm: cpuid: fix the size of xsaves area



On 04/12/2014 14:14, Radim Krčmář wrote:
> 2014-12-04 19:11+0800, Wanpeng Li:
>> The section of CPUID(EAX=0xd, ECX=1) in the spec which commit
>> f5c2290cd01e (KVM: cpuid: mask more bits in leaf 0xd and subleaves)
>> mentioned is older than SDM.
>>
>> EBX: Bits 31-00: The size in bytes of the XSAVE area containing all
>> states enabled by XCR0|IA32_XSS.
> 
> Well, CPUs without XSAVES return 0 there, so we would emulate them
> incorrectly ... (I don't mind much, it is reserved.)

I agree it should stay to 0 if !XSAVES && !XSAVEC.

For !XSAVEC && XSAVES there's no silicon, so we have some leeway.

>> The the value of EBX should represent the size of XCR0 related XSAVE
>> area since IA32_XSS is not used currently.
> 
> True, but 'supported' is not the state of XCR0, just its supremum.
> EBX should be set in kvm_update_cpuid(), like [3/4] does.
> (We can safely drop [2/4].)

Still, it's nice to be consistent and return a plausible value to
userspace for KVM_GET_SUPPORTED_CPUID.

Paolo
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