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

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Message-ID: <5537E2B2.5050006@redhat.com>
Date:	Wed, 22 Apr 2015 20:04:34 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Josh Triplett <josh@...htriplett.org>,
	Andy Lutomirski <luto@...capital.net>
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>, 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 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] x86/asm/entry/32: Remove unnecessary optimization
 in stub32_clone

On 04/22/2015 07:10 PM, Josh Triplett wrote:
> On Wed, Apr 22, 2015 at 09:54:24AM -0700, Andy Lutomirski wrote:
>> On Wed, Apr 22, 2015 at 9:40 AM, Denys Vlasenko <dvlasenk@...hat.com> 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 obfuscation of logic introduced
>>> by this optimization is simply not worth it.
>>
>> Ditto re: Josh's patch.
> 
> 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).
> 
> Denys, would you consider submitting a patch adding your comment on top
> of the two-patch series I just sent?

Okay.

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