linux-kernel - Re: [PATCH] x86/asm/entry/64: Use shorter MOVs from segmers registers

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Message-ID: <20150514174608.GD19338@gmail.com>
Date:	Thu, 14 May 2015 19:46:08 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	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>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/asm/entry/64: Use shorter MOVs from segmers registers


* Denys Vlasenko <dvlasenk@...hat.com> wrote:

> On 05/14/2015 07:08 PM, Linus Torvalds wrote:
> > On Thu, May 14, 2015 at 10:05 AM, Linus Torvalds
> > <torvalds@...ux-foundation.org> wrote:
> >>
> >> I don't object to the patch, but did we actually confirm that it
> >> always overwrites all of %ecx?
> > 
> > Just to clarify: I don't object to the patch because the code 
> > doesn't actually end up *depending* on the high bits anyway, and 
> > does word-sized compares etc. And the instruction size and speed 
> > things I don't doubt. So it's just the commit message I wanted to 
> > check wrt that whole "always overwrites all of %ecx". Because 
> > older CPU's didn't necessarily (things like partial register 
> > writes are much less of an issue when you're in-order and stupid 
> > ;)
> 
> This is 64-bit code, and all 64-bit CPUs zero-extend moves from 
> segment registers. As you said, in this particular code it wouldn't 
> matter anyway since subsequent code doesn't care about high bits of 
> %ecx...

Mind updating the changelog with all that information? It wasn't 
obvious to me either, as most of the mnemonics 'look' 32-bit.

I'd also say that a changelog is not complete, by definition, if Linus 
has to ask about it ;-)

Thanks,

	Ingo
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