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

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Message-ID: <9B0843ED-5207-4A42-A95B-1DB5314DE747@zytor.com>
Date: Wed, 15 Oct 2025 08:44:55 -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 8:41:03 AM PDT, Xin Li <xin@...or.com> wrote:
>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?

Yes. It's really a modified MOV GS,... instruction that loads the base, limit, selector, and attributes; it just loads the base into a different place.