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

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Date:	Wed, 25 Mar 2015 18:29:22 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
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


* 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 ...

Thanks,

	Ingo
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