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

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Message-ID: <530c3c1a-5fab-4798-830b-1fe0e8522bc9@zytor.com>
Date: Wed, 15 Oct 2025 23:41:03 +0800
From: Xin Li <xin@...or.com>
To: "H. Peter Anvin" <hpa@...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 10/15/2025 11:36 PM, H. Peter Anvin wrote:
> On October 15, 2025 8:34:00 AM PDT, Xin Li <xin@...or.com> wrote:
>> On 10/15/2025 11:18 PM, H. Peter Anvin wrote:
>>> 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>
>>>>
>>>
>>> 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".
>>
>> I see your point, is the following assignment better?
>>
>>     MSR_KERNEL_GS_BASE = gs_sel->base
>>
> 
> Except that that is also wrong, because that's not all of what the instruction does. "Load user space GS" is really what it does, despite the initiate historical naming.
> 

Do you mean the load of GS attributes by LKGS is still missing?