linux-kernel - Re: [PATCH MANUALSEL 5.15 8/8] KVM: x86: Report deprecated x87 features in supported CPUID

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Date:   Thu, 10 Feb 2022 17:40:28 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     Jim Mattson <jmattson@...gle.com>,
        Aaron Lewis <aaronlewis@...gle.com>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, kvm@...r.kernel.org
Subject: Re: [PATCH MANUALSEL 5.15 8/8] KVM: x86: Report deprecated x87
 features in supported CPUID

On 2/9/22 19:56, Sasha Levin wrote:
> From: Jim Mattson <jmattson@...gle.com>
> 
> [ Upstream commit e3bcfda012edd3564e12551b212afbd2521a1f68 ]

Acked-by: Paolo Bonzini <pbonzini@...hat.com>

Paolo

> CPUID.(EAX=7,ECX=0):EBX.FDP_EXCPTN_ONLY[bit 6] and
> CPUID.(EAX=7,ECX=0):EBX.ZERO_FCS_FDS[bit 13] are "defeature"
> bits. Unlike most of the other CPUID feature bits, these bits are
> clear if the features are present and set if the features are not
> present. These bits should be reported in KVM_GET_SUPPORTED_CPUID,
> because if these bits are set on hardware, they cannot be cleared in
> the guest CPUID. Doing so would claim guest support for a feature that
> the hardware doesn't support and that can't be efficiently emulated.
> 
> Of course, any software (e.g WIN87EM.DLL) expecting these features to
> be present likely predates these CPUID feature bits and therefore
> doesn't know to check for them anyway.
> 
> Aaron Lewis added the corresponding X86_FEATURE macros in
> commit cbb99c0f5887 ("x86/cpufeatures: Add FDP_EXCPTN_ONLY and
> ZERO_FCS_FDS"), with the intention of reporting these bits in
> KVM_GET_SUPPORTED_CPUID, but I was unable to find a proposed patch on
> the kvm list.
> 
> Opportunistically reordered the CPUID_7_0_EBX capability bits from
> least to most significant.
> 
> Cc: Aaron Lewis <aaronlewis@...gle.com>
> Signed-off-by: Jim Mattson <jmattson@...gle.com>
> Message-Id: <20220204001348.2844660-1-jmattson@...gle.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>   arch/x86/kvm/cpuid.c | 13 +++++++------
>   1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index f666fd79d8ad6..5f1d4a5aa8716 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -421,12 +421,13 @@ void kvm_set_cpu_caps(void)
>   	);
>   
>   	kvm_cpu_cap_mask(CPUID_7_0_EBX,
> -		F(FSGSBASE) | F(SGX) | F(BMI1) | F(HLE) | F(AVX2) | F(SMEP) |
> -		F(BMI2) | F(ERMS) | F(INVPCID) | F(RTM) | 0 /*MPX*/ | F(RDSEED) |
> -		F(ADX) | F(SMAP) | F(AVX512IFMA) | F(AVX512F) | F(AVX512PF) |
> -		F(AVX512ER) | F(AVX512CD) | F(CLFLUSHOPT) | F(CLWB) | F(AVX512DQ) |
> -		F(SHA_NI) | F(AVX512BW) | F(AVX512VL) | 0 /*INTEL_PT*/
> -	);
> +		F(FSGSBASE) | F(SGX) | F(BMI1) | F(HLE) | F(AVX2) |
> +		F(FDP_EXCPTN_ONLY) | F(SMEP) | F(BMI2) | F(ERMS) | F(INVPCID) |
> +		F(RTM) | F(ZERO_FCS_FDS) | 0 /*MPX*/ | F(AVX512F) |
> +		F(AVX512DQ) | F(RDSEED) | F(ADX) | F(SMAP) | F(AVX512IFMA) |
> +		F(CLFLUSHOPT) | F(CLWB) | 0 /*INTEL_PT*/ | F(AVX512PF) |
> +		F(AVX512ER) | F(AVX512CD) | F(SHA_NI) | F(AVX512BW) |
> +		F(AVX512VL));
>   
>   	kvm_cpu_cap_mask(CPUID_7_ECX,
>   		F(AVX512VBMI) | F(LA57) | F(PKU) | 0 /*OSPKE*/ | F(RDPID) |