linux-kernel - Re: [PATCH v2 2/2] x86/cpufeatures: Enumerate new AVX512 BFLOAT16 instructions

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Message-ID: <20190619173140.GH9574@zn.tnic>
Date:   Wed, 19 Jun 2019 19:31:40 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Christopherson Sean J <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v2 2/2] x86/cpufeatures: Enumerate new AVX512 BFLOAT16
 instructions

On Mon, Jun 17, 2019 at 11:00:16AM -0700, Fenghua Yu wrote:
> AVX512 Vector Neural Network Instructions (VNNI) in Intel Deep Learning
> Boost support BFLOAT16 format (BF16).

That sentence is a mouthful and I have no clue what it means. Marketing
junk? If so, either rewrite it for mere mortals or kill it.

> BF16 is a short version of FP32

FP32?

Please write out.

> and has several advantages over FP16.

Ditto.

> BF16 offers more than enough range for
> deep learning training tasks and doesn't need to handle hardware exception
> as this is a performance optimization. FP32 accumulation after the
> multiply is essential to achieve sufficient numerical behavior on an
> application level.
> 
> AVX512 BFLOAT16 instructions can be enumerated by:
> 	CPUID.7.1:EAX[bit 5] AVX512_BF16
> 
> Use word 12, which is empty now, to hold features in CPUID.7.1:EAX
> including AVX512_BF16.

... because that leaf is features only, right?

> Leaf CPUID_DUMMY is renamed as CPUID_7_1_EAX.

That's obvious from the patch, ain't it?

> Detailed information of the CPUID bit and AVX512 BFLOAT16 instructions
> can be found in the latest Intel Architecture Instruction Set Extensions
> and Future Features Programming Reference.

-- 
Regards/Gruss,
    Boris.

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