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

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Date:	Thu, 4 Dec 2014 14:14:55 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Wanpeng Li <wanpeng.li@...ux.intel.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	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

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.)

> 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].)

> Signed-off-by: Wanpeng Li <wanpeng.li@...ux.intel.com>
> ---
> v1 -> v2:
>  * add F(XSAVEC) check
> 
>  arch/x86/kvm/cpuid.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 646e6e8..5b78e9b 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -477,7 +477,10 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>  			do_cpuid_1_ent(&entry[i], function, idx);
>  			if (idx == 1) {
>  				entry[i].eax &= kvm_supported_word10_x86_features;
> -				entry[i].ebx = 0;
> +				if (entry[i].eax & (F(XSAVES) | F(XSAVEC)))
> +					entry[i].ebx = xstate_required_size(supported, true);
> +				else
> +					entry[i].ebx = xstate_required_size(supported, false);
>  			} else {
>  				if (entry[i].eax == 0 || !(supported & mask))
>  					continue;
> -- 
> 1.9.1
> 
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