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

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Message-ID: <87v80c19ez.ffs@tglx>
Date: Wed, 07 Aug 2024 20:21:08 +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 Wed, Aug 07 2024 at 20:08, Thomas Gleixner wrote:
> 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.

Also spare the extra indentation level and do:

static inline void arch_exit_work(unsigned long ti_work)
{
	if (ti_work & _TIF_USER_RETURN_NOTIFY)
		fire_user_return_notifiers();

	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();
}

static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
						  unsigned long ti_work)
{
	if (IS_ENABLED(CONFIG_X86_DEBUG_FPU) || unlikely(ti_work))
		arch_exit_work(ti_work);
        ...

Thanks,

        tglx