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

[Xin Li]

On 10/15/2025 11:18 PM, H. Peter Anvin wrote:
> On October 15, 2025 8:08:17 AM PDT, Xin Li <xin@xxxxxxxxx> wrote:
> > On 10/15/2025 6:35 PM, Borislav Petkov wrote:
> > > From: "Borislav Petkov (AMD)" <bp@xxxxxxxxx>
> > >
> > > 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@xxxxxxxxx>
> > > ---
> > >    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@xxxxxxxxx>
>
> 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