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

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Message-ID: <874j7w2oju.ffs@tglx>
Date: Wed, 07 Aug 2024 20:08:53 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: "Xin Li (Intel)" <xin@...or.com>, linux-kernel@...r.kernel.org
Cc: 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 Tue, Aug 06 2024 at 22:47, Xin Li 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();

Please keep this unconditional and independent of ti_work. It's a debug
feature and you kill coverage with making it conditional on ti_work.

Thanks,

        tglx