Re: [PATCH 2/4] x86/asm/entry/64: do not TRACE_IRQS fast SYSRET64 path

[Denys Vlasenko]

On 03/25/2015 06:29 PM, Ingo Molnar wrote:
> 
> * Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> 
>> SYSRET code path has a small irq-off block.
>> On this code path, TRACE_IRQS_ON can't be called right before interrupts
>> are enabled for real, we can't clobber registers there.
>> So current code does it earlier, in a safe place.
>>
>> But with this, TRACE_IRQS_OFF/ON frames just two fast instructions,
>> which is ridiculous: now most of irq-off block is _outside_ of the framing.
>>
>> Do the same thing that we do on SYSCALL entry: do not track this irq-off block,
>> it is very small to ever cause noticeable irq latency.
>>
>> Be careful: make sure that "jnz int_ret_from_sys_call_irqs_off" now does
>> invoke TRACE_IRQS_OFF - move int_ret_from_sys_call_irqs_off label before
>> TRACE_IRQS_OFF.
> 
>> @@ -345,8 +346,8 @@ tracesys_phase2:
>>   */
>>  GLOBAL(int_ret_from_sys_call)
>>  	DISABLE_INTERRUPTS(CLBR_NONE)
>> -	TRACE_IRQS_OFF
>>  int_ret_from_sys_call_irqs_off:
>> +	TRACE_IRQS_OFF
>>  	movl $_TIF_ALLWORK_MASK,%edi
>>  	/* edi:	mask to check */
> 
> This latter trick absolutely needs a comment, to keep future lockdep 
> developers from wondering about the mismatch and the weird label 
> placement ...

Unsure how to format it.

How about:


	DISABLE_INTERRUPTS(CLBR_NONE)
int_ret_from_sys_call_irqs_off: /* jumps come here with irqs off */
	TRACE_IRQS_OFF



(In truth, there is only one jump as of now, but using pliral
"jumps" if that would change)
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