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