linux-kernel - Re: [RFC PATCH 3/4] x86/asm: Make alternative macro interfaces more clear and consistent

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Date:   Sat, 2 Sep 2017 12:32:21 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH 3/4] x86/asm: Make alternative macro interfaces more
 clear and consistent


* Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> On Thu, Aug 31, 2017 at 12:25:42PM -0500, Josh Poimboeuf wrote:
> > 2) Put "sp" in the clobbers list instead of as an i/o constraint.  This
> >    mostly works for GCC, and doesn't break clang.  However, it causes
> >    GCC to insert a "lea -0x10(%rbp),%rsp" in the epilogue of every
> >    affected function.
> 
> And maybe this extra instruction is negligible for performance and not a
> big deal?  I might look at this one after the holiday too.

Please do statistics of how many functions are affected, on a defconfig-ish 
kernel.

Thanks,

	Ingo