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

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Date:   Wed, 13 Oct 2021 17:36:14 -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 27, 2021 at 1:02 PM Noah Goldstein <goldstein.w.n@...il.com> wrote:
>
> 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
> >
> Ping
>
> (sorry if this has shown up multiple times for anyone,
> was accidentally including HTML earlier)

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