linux-kernel - Re: [PATCH v2 2/3] x86/cpufeatures: Introduce X86_FEATURE_NO

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Message-ID: <Yys2r0QYWjeEXLe0@zn.tnic>
Date:   Wed, 21 Sep 2022 18:07:11 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 2/3] x86/cpufeatures: Introduce X86_FEATURE_NO_LMSLE

On Tue, Sep 20, 2022 at 01:59:21PM -0700, Jim Mattson wrote:
> When AMD introduced "Long Mode Segment Limit Enable" (a.k.a. "VMware
> mode"), the feature was not enumerated by a CPUID bit. Now that VMware
> has abandoned binary translation, AMD has deprecated EFER.LMSLE.
> 
> The absence of the feature *is* now enumerated by a CPUID bit (a la
> Intel's X86_FEATURE_ZERO_FCS_DCS and X86_FEATURE_FDP_EXCPTN_ONLY).
> 
> This defeature bit is already present in feature word 13, but it was
> previously anonymous. Name it X86_FEATURE_NO_LMSLE, so that KVM can
> reference it when constructing a supported guest CPUID table.
> 
> Since this bit indicates the absence of a feature, don't enumerate it
> in /proc/cpuinfo.
> 
> Signed-off-by: Jim Mattson <jmattson@...gle.com>
> ---
>  arch/x86/include/asm/cpufeatures.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index ef4775c6db01..0f5a3285d8d8 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -319,6 +319,7 @@
>  #define X86_FEATURE_AMD_IBRS		(13*32+14) /* "" Indirect Branch Restricted Speculation */
>  #define X86_FEATURE_AMD_STIBP		(13*32+15) /* "" Single Thread Indirect Branch Predictors */
>  #define X86_FEATURE_AMD_STIBP_ALWAYS_ON	(13*32+17) /* "" Single Thread Indirect Branch Predictors always-on preferred */
> +#define X86_FEATURE_NO_LMSLE		(13*32+20) /* "" EFER_LMSLE is unsupported */
>  #define X86_FEATURE_AMD_PPIN		(13*32+23) /* Protected Processor Inventory Number */
>  #define X86_FEATURE_AMD_SSBD		(13*32+24) /* "" Speculative Store Bypass Disable */
>  #define X86_FEATURE_VIRT_SSBD		(13*32+25) /* Virtualized Speculative Store Bypass Disable */
> -- 

Acked-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

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