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

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Message-ID: <5554DE56.5030202@redhat.com>
Date:	Thu, 14 May 2015 19:41:42 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...nel.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

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...
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