linux-kernel - Re: [tip:x86/urgent] x86/cpufeature: Add AVX512_4VNNIW and AVX512

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Date:   Sun, 16 Oct 2016 16:22:33 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Ingo Molnar <mingo@...nel.org>, peterz@...radead.org,
        tglx@...utronix.de, dave.hansen@...el.com,
        torvalds@...ux-foundation.org, piotr.luc@...el.com,
        luto@...nel.org, brgerst@...il.com, hpa@...or.com,
        linux-kernel@...r.kernel.org, dvlasenk@...hat.com,
        jpoimboe@...hat.com
Cc:     linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/urgent] x86/cpufeature: Add AVX512_4VNNIW and
 AVX512_4FMAPS features

On Sun, Oct 16, 2016 at 04:21:49AM -0700, tip-bot for Piotr Luc wrote:
> Commit-ID:  a518dcc82b6162009c8ca3d169fe61c81536ff17
> Gitweb:     http://git.kernel.org/tip/a518dcc82b6162009c8ca3d169fe61c81536ff17
> Author:     Piotr Luc <piotr.luc@...el.com>
> AuthorDate: Wed, 12 Oct 2016 19:57:31 +0200
> Committer:  Ingo Molnar <mingo@...nel.org>
> CommitDate: Sun, 16 Oct 2016 11:32:11 +0200
> 
> x86/cpufeature: Add AVX512_4VNNIW and AVX512_4FMAPS features
> 
> AVX512_4VNNIW  - Vector instructions for deep learning enhanced word
> variable precision.
> AVX512_4FMAPS - Vector instructions for deep learning floating-point
> single precision.
> 
> The new instructions are to be used in future Intel Xeon & Xeon Phi
> processors.
> 
> The spec can be found in Intel Software Developer Manual or in
> Instruction Set Extensions Programming Reference. See
> https://software.intel.com/sites/default/files/managed/69/78/319433-025.pdf.
> 
> Signed-off-by: Piotr Luc <piotr.luc@...el.com>
> Reviewed-by: Dave Hansen <dave.hansen@...el.com>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Brian Gerst <brgerst@...il.com>
> Cc: Denys Vlasenko <dvlasenk@...hat.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Link: http://lkml.kernel.org/r/20161012175731.29619-1-piotr.luc@intel.com
> Signed-off-by: Ingo Molnar <mingo@...nel.org>

...

> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 1188bc8..6697b75 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -12,7 +12,7 @@
>  /*
>   * Defines x86 CPU feature bits
>   */
> -#define NCAPINTS	18	/* N 32-bit words worth of info */
> +#define NCAPINTS	19	/* N 32-bit words worth of info */
>  #define NBUGINTS	1	/* N 32-bit bug flags */
>  
>  /*
> @@ -285,6 +285,10 @@
>  #define X86_FEATURE_SUCCOR	(17*32+1) /* Uncorrectable error containment and recovery */
>  #define X86_FEATURE_SMCA	(17*32+3) /* Scalable MCA */
>  
> +/* Intel-defined CPU features, CPUID level 0x00000007:0 (edx), word 18 */
> +#define X86_FEATURE_AVX512_4VNNIW  (18*32+2) /* AVX-512 Neural Network Instructions */
> +#define X86_FEATURE_AVX512_4FMAPS  (18*32+3) /* AVX-512 Multiply Accumulation Single precision */

This is getting ridiculous: we keep adding new leafs to
->x86_capability, thus bloating cpuinfo_x86 but then it is not even
worth it - this patch defines only two bits.

I know, I know, it is a CPUID leaf of features, we will need them, yadda
yadda but until we do, I'd suggest these all new feature bits to to
init_scattered_cpuid_features() and be carved out to a leaf of their
own *only* when we really, actually add them and fill up that leaf.
Otherwise, we have one fat and sparse x86_capability array.

-- 
Regards/Gruss,
    Boris.

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