linux-kernel - Re: [PATCH v1] x86/fpu: Remove opmask state from avx512

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Date:   Mon, 27 Sep 2021 13:02:34 -0500
From:   Noah Goldstein <goldstein.w.n@...il.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        X86 ML <x86@...nel.org>, hpa@...or.com,
        Andy Lutomirski <luto@...nel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] x86/fpu: Remove opmask state from avx512_timestamp check

On Mon, Sep 20, 2021 at 12:40 AM Noah Goldstein <goldstein.w.n@...il.com> wrote:
>
> This commit creates a new mask, XFEATURE_MASK_ZMM, to test against
> xfeatures for conditionally updating the axx512_timestamp.
>
> Based on the comments, the avx512 state is meant to track when the
> state would cause frequencey throttling. The opmasks (k0-k7) do not
> cause frequency throttling, so they don't make sense to include.
>
> The current implementation, as well as the old, still does have a
> false positive on ymm16-ymm31 and xmm16-31 because
> XFEATURE_MASK_Hi16_ZMM includes them.
>
> Signed-off-by: Noah Goldstein <goldstein.w.n@...il.com>
> ---
> Issue is reproducible with the following code on x86_64:
>
> ```
>         .global _start
>         .text
> _start:
>         korq    %k0, %k0, %k0
>
> loop:
>         jmp     loop
>
>
>         movl    $60, %eax
>         xorl    %edi, %edi
>         syscall
> ```
>
> Pretending run as executable named "foo":
>
> $> cat /proc/$(pidof foo)/arch_status
>
>
> This should yield -1 as no frequency changing AVX512 instructions
> where used but instead tracks the process.
>
> Note there still is a false positive with ymm16-ymm31 and xmm16-xmm31
> but since there is no state to distinguish between there use and
> zmm16-31 that seems inevitable.
>
>
>  arch/x86/include/asm/fpu/types.h | 2 ++
>  arch/x86/kernel/fpu/core.c       | 4 +++-
>  2 files changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
> index f5a38a5f3ae1..a4816fa7d541 100644
> --- a/arch/x86/include/asm/fpu/types.h
> +++ b/arch/x86/include/asm/fpu/types.h
> @@ -141,6 +141,8 @@ enum xfeature {
>  #define XFEATURE_MASK_AVX512           (XFEATURE_MASK_OPMASK \
>                                          | XFEATURE_MASK_ZMM_Hi256 \
>                                          | XFEATURE_MASK_Hi16_ZMM)
> +#define XFEATURE_MASK_ZMM              (XFEATURE_MASK_ZMM_Hi256 \
> +                                        | XFEATURE_MASK_Hi16_ZMM)
>
>  #define FIRST_EXTENDED_XFEATURE        XFEATURE_YMM
>
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index 7ada7bd03a32..342620a2e8ef 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -104,8 +104,10 @@ void save_fpregs_to_fpstate(struct fpu *fpu)
>                 /*
>                  * AVX512 state is tracked here because its use is
>                  * known to slow the max clock speed of the core.
> +                * Note: This has a false positive on Hi16 ymm and
> +                * xmm registers.
>                  */
> -               if (fpu->state.xsave.header.xfeatures & XFEATURE_MASK_AVX512)
> +               if (fpu->state.xsave.header.xfeatures & XFEATURE_MASK_ZMM)
>                         fpu->avx512_timestamp = jiffies;
>                 return;
>         }
> --
> 2.25.1
>
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