linux-kernel - Re: [PATCH v1 1/3] x86/entry: Test ti_work for zero before processing individual bits

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Message-ID: <CAMzpN2iS076ysZ37gjrz6MGWc62sD9uw0ODTJtzOO1U4kp309A@mail.gmail.com>
Date: Wed, 7 Aug 2024 12:21:27 -0400
From: Brian Gerst <brgerst@...il.com>
To: "Xin Li (Intel)" <xin@...or.com>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com, 
	bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	peterz@...radead.org, andrew.cooper3@...rix.com, seanjc@...gle.com
Subject: Re: [PATCH v1 1/3] x86/entry: Test ti_work for zero before processing
 individual bits

On Wed, Aug 7, 2024 at 1:51 AM Xin Li (Intel) <xin@...or.com> wrote:
>
> In most cases, ti_work values passed to arch_exit_to_user_mode_prepare()
> are zeros, e.g., 99% in kernel build tests.  So an obvious optimization
> is to test ti_work for zero before processing individual bits in it.
>
> In addition, Intel 0day tests find no perf regression with this change.
>
> Suggested-by: H. Peter Anvin (Intel) <hpa@...or.com>
> Signed-off-by: Xin Li (Intel) <xin@...or.com>
> ---
>  arch/x86/include/asm/entry-common.h | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
> index fb2809b20b0a..4c78b99060b5 100644
> --- a/arch/x86/include/asm/entry-common.h
> +++ b/arch/x86/include/asm/entry-common.h
> @@ -47,15 +47,17 @@ static __always_inline void arch_enter_from_user_mode(struct pt_regs *regs)
>  static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
>                                                   unsigned long ti_work)
>  {
> -       if (ti_work & _TIF_USER_RETURN_NOTIFY)
> -               fire_user_return_notifiers();
> +       if (unlikely(ti_work)) {
> +               if (ti_work & _TIF_USER_RETURN_NOTIFY)
> +                       fire_user_return_notifiers();
>
> -       if (unlikely(ti_work & _TIF_IO_BITMAP))
> -               tss_update_io_bitmap();
> +               if (unlikely(ti_work & _TIF_IO_BITMAP))
> +                       tss_update_io_bitmap();
>
> -       fpregs_assert_state_consistent();
> -       if (unlikely(ti_work & _TIF_NEED_FPU_LOAD))
> -               switch_fpu_return();
> +               fpregs_assert_state_consistent();

This call was originally unconditional, and does nothing if
TIF_NEED_FPU_LOAD is set.

> +               if (unlikely(ti_work & _TIF_NEED_FPU_LOAD))
> +                       switch_fpu_return();
> +       }
>
>  #ifdef CONFIG_COMPAT
>         /*
> --
> 2.45.2
>
>

Brian Gerst