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

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Message-ID: <20241125180814.GA1242949@google.com>
Date: Mon, 25 Nov 2024 18:08:14 +0000
From: Eric Biggers <ebiggers@...nel.org>
To: Ingo Molnar <mingo@...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

On Mon, Nov 25, 2024 at 09:33:46AM +0100, Ingo Molnar wrote:
> 
> * 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?

I am planning to carry CRC-related patches myself
(https://lore.kernel.org/lkml/20241117002244.105200-12-ebiggers@kernel.org/).

> 
> > +/*
> > + * 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 ;-)
> 

Will do, thanks.

- Eric