From: Denys Vlasenko <dvlasenk@redhat.com>
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
Date: Thu, 14 May 2015 19:41:42 +0200 [thread overview]
Message-ID: <5554DE56.5030202@redhat.com> (raw)
In-Reply-To: <CA+55aFwe4ODQo=VzM+hbM+NJamF7MvMhFAvfUiyTdBFtA5yzLg@mail.gmail.com>
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...
next prev parent reply other threads:[~2015-05-14 17:41 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-14 16:55 Denys Vlasenko
2015-05-14 17:05 ` Linus Torvalds
2015-05-14 17:08 ` Linus Torvalds
2015-05-14 17:41 ` Denys Vlasenko [this message]
2015-05-14 17:46 ` Ingo Molnar
2015-05-15 18:17 ` Thiago Farina
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