linux-kernel - Re: [PATCH v2 1/6] x86/cpufeature: add cpu feature bit for LKGS

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Date:   Tue, 11 Oct 2022 18:40:26 -0700
From:   "Chang S. Bae" <chang.seok.bae@...el.com>
To:     Xin Li <xin3.li@...el.com>, <linux-kernel@...r.kernel.org>,
        <x86@...nel.org>
CC:     <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
        <dave.hansen@...ux.intel.com>, <hpa@...or.com>,
        <peterz@...radead.org>
Subject: Re: [PATCH v2 1/6] x86/cpufeature: add cpu feature bit for LKGS

On 10/10/2022 12:01 PM, Xin Li wrote:
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index ef4775c6db01..459fb0c21dd4 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -308,6 +308,7 @@
>   /* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
>   #define X86_FEATURE_AVX_VNNI		(12*32+ 4) /* AVX VNNI instructions */
>   #define X86_FEATURE_AVX512_BF16		(12*32+ 5) /* AVX512 BFLOAT16 instructions */
> +#define X86_FEATURE_LKGS		(12*32+ 18) /* Load "kernel" (userspace) gs */

The spec says [1]:
     "Execution of LKGS causes an invalid-opcode exception (#UD) if CPL >
      0."

Perhaps userspace has no interest in this. Then, we can add "" not to 
show "lkgs" in /proc/cpuinfo:
     +#define X86_FEATURE_LKGS		(12*32+ 18) /* "" Load "kernel" 
(userspace) gs */

Thanks,
Chang

[1] https://cdrdv2.intel.com/v1/dl/getContent/678938