linux-kernel - Re: [PATCH] x86/cpufeatures: Correct LKGS feature flag description

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Message-ID: <329BCA65-5152-4892-AFEA-DF72DCCE80B6@zytor.com>
Date: Wed, 15 Oct 2025 08:18:45 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Xin Li <xin@...or.com>, Borislav Petkov <bp@...nel.org>,
        Xin Li <xin3.li@...el.com>
CC: X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        "Borislav Petkov (AMD)" <bp@...en8.de>
Subject: Re: [PATCH] x86/cpufeatures: Correct LKGS feature flag description

On October 15, 2025 8:08:17 AM PDT, Xin Li <xin@...or.com> wrote:
>On 10/15/2025 6:35 PM, Borislav Petkov wrote:
>> From: "Borislav Petkov (AMD)" <bp@...en8.de>
>> 
>> Quotation marks in cpufeatures.h comments are special and when the
>> comment begins with a quoted string, that string lands in /proc/cpuinfo,
>> turning it into a user-visible one.
>> 
>> The LKGS comment doesn't begin with a quoted string but just in case
>> drop the quoted "kernel" in there to avoid confusion. And while at it,
>> simply change the description into what the LKGS instruction does for
>> more clarity.
>> 
>> No functional changes.
>> 
>> Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
>> ---
>>   arch/x86/include/asm/cpufeatures.h       | 2 +-
>>   tools/arch/x86/include/asm/cpufeatures.h | 2 +-
>>   2 files changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
>> index 80b68f4726e7..4fb5e12dbdbf 100644
>> --- a/arch/x86/include/asm/cpufeatures.h
>> +++ b/arch/x86/include/asm/cpufeatures.h
>> @@ -320,7 +320,7 @@
>>   #define X86_FEATURE_FSRS		(12*32+11) /* Fast short REP STOSB */
>>   #define X86_FEATURE_FSRC		(12*32+12) /* Fast short REP {CMPSB,SCASB} */
>>   #define X86_FEATURE_FRED		(12*32+17) /* "fred" Flexible Return and Event Delivery */
>> -#define X86_FEATURE_LKGS		(12*32+18) /* Load "kernel" (userspace) GS */
>> +#define X86_FEATURE_LKGS		(12*32+18) /* MSR_KERNEL_GS_BASE = GS.base */
>
>Yes, the assignment is more clearer to us programmers.
>
>I'm just not sure if "correct" in the shortlog is accurate; it sounds the
>existing one is wrong.  Otherwise,
>
>Reviewed-by: Xin Li (Intel) <xin@...or.com>
>
>
>
>
>
>
>>   #define X86_FEATURE_WRMSRNS		(12*32+19) /* Non-serializing WRMSR */
>>   #define X86_FEATURE_AMX_FP16		(12*32+21) /* AMX fp16 Support */
>>   #define X86_FEATURE_AVX_IFMA            (12*32+23) /* Support for VPMADD52[H,L]UQ */
>> diff --git a/tools/arch/x86/include/asm/cpufeatures.h b/tools/arch/x86/include/asm/cpufeatures.h
>> index bd655e100395..d5f091babf96 100644
>> --- a/tools/arch/x86/include/asm/cpufeatures.h
>> +++ b/tools/arch/x86/include/asm/cpufeatures.h
>> @@ -320,7 +320,7 @@
>>   #define X86_FEATURE_FSRS		(12*32+11) /* Fast short REP STOSB */
>>   #define X86_FEATURE_FSRC		(12*32+12) /* Fast short REP {CMPSB,SCASB} */
>>   #define X86_FEATURE_FRED		(12*32+17) /* "fred" Flexible Return and Event Delivery */
>> -#define X86_FEATURE_LKGS		(12*32+18) /* Load "kernel" (userspace) GS */
>> +#define X86_FEATURE_LKGS		(12*32+18) /* MSR_KERNEL_GS_BASE = GS.base */
>>   #define X86_FEATURE_WRMSRNS		(12*32+19) /* Non-serializing WRMSR */
>>   #define X86_FEATURE_AMX_FP16		(12*32+21) /* AMX fp16 Support */
>>   #define X86_FEATURE_AVX_IFMA            (12*32+23) /* Support for VPMADD52[H,L]UQ */
>

That "assignment" is rather wrong, though; it implies that the two are identical, which they are not; nor does it imply the relationship is fixed (that is provided by FRED, not LKGS). Perhaps just call it "Load user space GS".