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Date: Wed, 22 Apr 2015 20:04:34 +0200
From: Denys Vlasenko <dvlasenk@redhat.com>
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To: Josh Triplett <josh@joshtriplett.org>,
        Andy Lutomirski <luto@amacapital.net>
CC: Ingo Molnar <mingo@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, 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>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2] x86/asm/entry/32: Remove unnecessary optimization
 in stub32_clone
References: <1429720808-7173-1-git-send-email-dvlasenk@redhat.com> <1429720808-7173-2-git-send-email-dvlasenk@redhat.com> <CALCETrV1f8oTqeKfnzpCxAHX-d_CwqdrZrZsV+TX=Y1OSd6qdQ@mail.gmail.com> <20150422171005.GA1020@jtriplet-mobl1>
In-Reply-To: <20150422171005.GA1020@jtriplet-mobl1>
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On 04/22/2015 07:10 PM, Josh Triplett wrote:
> On Wed, Apr 22, 2015 at 09:54:24AM -0700, Andy Lutomirski wrote:
>> On Wed, Apr 22, 2015 at 9:40 AM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
>>> Really swap arguments #4 and #5 in stub32_clone instead of "optimizing"
>>> it into a move.
>>>
>>> Yes, tls_val is currently unused. Yes, on some CPUs XCHG is a little bit
>>> more expensive than MOV. But a cycle or two on an expensive syscall like
>>> clone() is way below noise floor, and obfuscation of logic introduced
>>> by this optimization is simply not worth it.
>>
>> Ditto re: Josh's patch.
> 
> I do think my two-patch HAVE_COPY_THREAD_TLS series should go in fixing
> this, but I'd like to see the final version of Denys' comment added on
> top of it (with an update to the type and name of the tls argument to
> match the changes to sys_clone).
> 
> Denys, would you consider submitting a patch adding your comment on top
> of the two-patch series I just sent?

Okay.