From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1753797AbbCYSEu (ORCPT <rfc822;w@1wt.eu>);
	Wed, 25 Mar 2015 14:04:50 -0400
Received: from mail-wi0-f173.google.com ([209.85.212.173]:36803 "EHLO
	mail-wi0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1752235AbbCYSEq (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 25 Mar 2015 14:04:46 -0400
Date: Wed, 25 Mar 2015 19:04:41 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@amacapital.net>, Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/4] x86/asm/entry/64: do not TRACE_IRQS fast SYSRET64
 path
Message-ID: <20150325180441.GB7321@gmail.com>
References: <1427303896-24023-1-git-send-email-dvlasenk@redhat.com>
 <1427303896-24023-2-git-send-email-dvlasenk@redhat.com>
 <20150325172922.GA7321@gmail.com>
 <5512F339.8060209@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <5512F339.8060209@redhat.com>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org


* Denys Vlasenko <dvlasenk@redhat.com> wrote:

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

Why not something like 'jumps come here from the irqs-off SYSRET 
path'?

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

I'd also put a comment to the actual sysret IRQ-disablement that we 
are skipping with the annotation. Explain that it's an optimization 
for a visible irqs-off path that needs no annotation - and that the 
moment something complex is done in that path, this optimization loses 
its validity.

Thanks,

	Ingo