linux-kernel - Re: [PATCH 2/4] x86/asm/entry/64: do not TRACE

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Date:	Wed, 25 Mar 2015 18:41:13 +0100
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] x86/asm/entry/64: do not TRACE_IRQS fast SYSRET64
 path

On 03/25/2015 06:29 PM, Ingo Molnar wrote:
> 
> * Denys Vlasenko <dvlasenk@...hat.com> 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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