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

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Date:	Wed, 3 Jun 2015 09:04:41 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	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.


* Steven Rostedt <rostedt@...dmis.org> 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 don't see it as 
> obfuscation at all.

So here are a few easy questions, I'm wondering how many minutes it takes for you 
to answer them correctly:

 - What does the CLEAR_RREGS name stand for?

 - What is this macro's purpose?

 - In a single case CLEAR_RREGS takes a 'r9' argument:

    arch/x86/ia32/ia32entry.S:	CLEAR_RREGS
    arch/x86/ia32/ia32entry.S:	CLEAR_RREGS
    arch/x86/ia32/ia32entry.S:	CLEAR_RREGS r9
    arch/x86/ia32/ia32entry.S:	CLEAR_RREGS
    arch/x86/ia32/ia32entry.S:	CLEAR_RREGS

   What is the 'r9' argument's purpose and why is activated in the place where
   it's activated?

The CLEAR_RREGS macro has zero comments. If it takes more than a quick glance to 
determine all these three first-order questions from the source code, then it's an 
obvious code cleanliness fail which needs to be improved.

Thanks,

	Ingo
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