linux-kernel - Re: [PATCH] x86/fpu: Correct AVX512 state tracking

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Date:   Tue, 16 Nov 2021 10:49:16 -0600
From:   Noah Goldstein <goldstein.w.n@...il.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     tglx@...utronix.de, mingo@...hat.com, X86 ML <x86@...nel.org>,
        hpa@...or.com, Andy Lutomirski <luto@...nel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/fpu: Correct AVX512 state tracking

On Tue, Nov 16, 2021 at 10:21 AM Borislav Petkov <bp@...en8.de> wrote:
>
> Does that still work with your test case? (Ontop of -rc1)

This should work.

>
> ---
> From: Noah Goldstein <goldstein.w.n@...il.com>
> Date: Tue, 16 Nov 2021 17:14:21 +0100
> Subject: [PATCH] x86/fpu: Correct AVX512 state tracking
>
> Add a separate, local mask for tracking AVX512 usage which does not
> include the opmask xfeature set. Opmask registers usage does not cause
> frequency throttling so it is a completely unnecessary false positive.
>
> While at it, carve it out into a separate function to keep that
> abomination extracted out.
>
>  [ bp: Rediff and cleanup ontop of 5.16-rc1. ]
>
> Signed-off-by: Noah Goldstein <goldstein.w.n@...il.com>
> Signed-off-by: Borislav Petkov <bp@...e.de>
> Link: https://lore.kernel.org/r/20210920053951.4093668-1-goldstein.w.n@gmail.com
> ---
>  arch/x86/kernel/fpu/core.c | 21 ++++++++++++++-------
>  1 file changed, 14 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index 8ea306b1bf8e..dd3777ac0443 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -98,6 +98,19 @@ bool irq_fpu_usable(void)
>  }
>  EXPORT_SYMBOL(irq_fpu_usable);
>
> +/*
> + * Track AVX512 state use because it is known to slow the max clock
> + * speed of the core.
> + */
> +static void update_avx_timestamp(struct fpu *fpu)
> +{
> +
> +#define AVX512_TRACKING_MASK   (XFEATURE_MASK_ZMM_Hi256 | XFEATURE_MASK_Hi16_ZMM)
> +
> +       if (fpu->fpstate->regs.xsave.header.xfeatures & AVX512_TRACKING_MASK)
> +               fpu->avx512_timestamp = jiffies;
> +}
> +
>  /*
>   * Save the FPU register state in fpu->fpstate->regs. The register state is
>   * preserved.
> @@ -116,13 +129,7 @@ void save_fpregs_to_fpstate(struct fpu *fpu)
>  {
>         if (likely(use_xsave())) {
>                 os_xsave(fpu->fpstate);
> -
> -               /*
> -                * AVX512 state is tracked here because its use is
> -                * known to slow the max clock speed of the core.
> -                */
> -               if (fpu->fpstate->regs.xsave.header.xfeatures & XFEATURE_MASK_AVX512)
> -                       fpu->avx512_timestamp = jiffies;
> +               update_avx_timestamp(fpu);
>                 return;
>         }
>
> --
> 2.29.2
>
>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette