linux-kernel - Re: [PATCH 1/2] x86/asm/entry/32: Open-code CLEAR

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Date:	Tue, 02 Jun 2015 22:25:03 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.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 1/2] x86/asm/entry/32: Open-code CLEAR_RREGS. No code
 changes.

On 06/02/2015 09:34 PM, Steven Rostedt wrote:
> On Tue,  2 Jun 2015 21:04:01 +0200
> Denys Vlasenko <dvlasenk@...hat.com> wrote:
> 
>> This macro is small, has only four callsites, and one of them is
>> slightly different using a conditional parameter.
>>
>> A few saved lines aren't worth the resulting obfuscation.
> 
> I'm curious, why? Did someone recommend this change?

I'm proposing to do this. Of course, I don't expect that
any my patch must be accepted.


> I don't see it as obfuscation at all.

Riddle me this, looking at the current code. What's up with that
strange manipulations of %r9 register? Why the code in
SYSCALL and SYSENTER entry points is not the same,
why "r9 dance" is done only in one of these entry points?

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