linux-kernel - Re: [PATCH 2/2] x86/asm/entry/32: Remove unnecessary optimization in stub32

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Message-ID: <20150604155813.GA7829@x>
Date:	Thu, 4 Jun 2015 08:58:13 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	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 2/2] x86/asm/entry/32: Remove unnecessary optimization in
 stub32_clone

On Thu, Jun 04, 2015 at 12:07:31PM +0200, Denys Vlasenko wrote:
> On 06/03/2015 06:38 PM, Josh Triplett wrote:
> > On Wed, Jun 03, 2015 at 03:58:50PM +0200, Denys Vlasenko wrote:
> >> Really swap arguments #4 and #5 in stub32_clone instead of "optimizing"
> >> it into a move.
> >>
> >> Yes, tls_val is currently unused. Yes, on some CPUs XCHG is a little bit
> >> more expensive than MOV. But a cycle or two on an expensive syscall like
> >> clone() is way below noise floor, and this optimization is simply not worth
> >> the obfuscation of logic.
> > [...]
> >> This is a resend.
> >>
> >> There was a patch by Josh Triplett
> >> "x86: Opt into HAVE_COPY_THREAD_TLS, for both 32-bit and 64-bit"
> >> sent on May 11,
> >> which does the same thing as part of a bigger cleanup.
> >> He was supportive of this patch because of comments.
> >> He will simply have to drop one hunk from his patch.
> > 
> > Strictly speaking, nothing needs this until clone starts paying
> > attention to its tls argument, which only happens in my cleanup series
> > that includes this change.  So what's the purpose of driving this patch
> > separately?
> 
> You wanted my comments in this patch to go in:
> 
> On 04/22/2015 07:10 PM, Josh Triplett wrote:
> > I do think my two-patch HAVE_COPY_THREAD_TLS series should go in fixing
> > this, but I'd like to see the final version of Denys' comment added on
> > top of it (with an update to the type and name of the tls argument to
> > match the changes to sys_clone).
> 
> If your patch will go in first, I'll send a patch adding only the comment.
> 
> Since for now your patch did not make it yet, I'm submitting
> a patch which adds both a comment and the insn change.

Ah, I see.

My two-patch series is currently sitting in -mm; would you consider
providing a version of the patch that adds the comment for Andrew to
apply on top of those?

- Josh Triplett
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