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

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Message-ID: <BN6PR1101MB2161F6E79A6B123D9B0B53E9A8259@BN6PR1101MB2161.namprd11.prod.outlook.com>
Date:   Thu, 13 Oct 2022 19:35:23 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "peterz@...radead.org" <peterz@...radead.org>
Subject: RE: [PATCH v2 1/6] x86/cpufeature: add cpu feature bit for LKGS


> > 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 */

Good point!

> 
> Thanks,
> Chang
> 
> [1] https://cdrdv2.intel.com/v1/dl/getContent/678938