linux-kernel - Re: [PATCH 1/6] x86: move zmm exclusion list into CPU feature flag

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Message-ID: <Z0Q2ali5rvtmj1z9@gmail.com>
Date: Mon, 25 Nov 2024 09:33:46 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Eric Biggers <ebiggers@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
	x86@...nel.org, Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH 1/6] x86: move zmm exclusion list into CPU feature flag


* Eric Biggers <ebiggers@...nel.org> wrote:

> From: Eric Biggers <ebiggers@...gle.com>
> 
> Lift zmm_exclusion_list in aesni-intel_glue.c into the x86 CPU setup
> code, and add a new x86 CPU feature flag X86_FEATURE_PREFER_YMM that is
> set when the CPU is on this list.
> 
> This allows other code in arch/x86/, such as the CRC library code, to
> apply the same exclusion list when deciding whether to execute 256-bit
> or 512-bit optimized functions.
> 
> Note that full AVX512 support including zmm registers is still exposed
> to userspace and is still supported for in-kernel use.  This flag just
> indicates whether in-kernel code should prefer to use ymm registers.
> 
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> ---
>  arch/x86/crypto/aesni-intel_glue.c | 22 +---------------------
>  arch/x86/include/asm/cpufeatures.h |  1 +
>  arch/x86/kernel/cpu/intel.c        | 22 ++++++++++++++++++++++
>  3 files changed, 24 insertions(+), 21 deletions(-)

Acked-by: Ingo Molnar <mingo@...nel.org>

I suppose you'd like to carry this in the crypto tree?

> +/*
> + * This is a list of Intel CPUs that are known to suffer from downclocking when
> + * zmm registers (512-bit vectors) are used.  On these CPUs, when the kernel
> + * executes SIMD-optimized code such as cryptography functions or CRCs, it
> + * should prefer 256-bit (ymm) code to 512-bit (zmm) code.
> + */

One speling nit, could you please do:

  s/ymm/YMM
  s/zmm/ZMM

... to make it consistent with how the rest of the x86 code is 
capitalizing the names of FPU vector register classes. Just like
we are capitalizing CPU and CRC properly ;-)

Thanks,

	Ingo