Date: Thu, 14 May 2015 19:46:08 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@amacapital.net>, Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86/asm/entry/64: Use shorter MOVs from segmers registers
Message-ID: <20150514174608.GD19338@gmail.com>
References: <1431622519-19718-1-git-send-email-dvlasenk@redhat.com>
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* Denys Vlasenko <dvlasenk@redhat.com> wrote:

> On 05/14/2015 07:08 PM, Linus Torvalds wrote:
> > On Thu, May 14, 2015 at 10:05 AM, Linus Torvalds
> > <torvalds@linux-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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