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

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Date:	Thu, 04 Jun 2015 12:07:31 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Josh Triplett <josh@...htriplett.org>
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 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.

-- 
vda

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