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

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Date:   Thu, 14 Sep 2017 09:48:20 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>
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

On Sat, Sep 02, 2017 at 12:32:21PM +0200, Ingo Molnar wrote:
> 
> * 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.

As it turns out, the real problem with this option is that it imposes a
penalty for CONFIG_FRAME_POINTER=n: even with frame pointers disabled,
it forces the frame pointer to be saved for each function which uses the
inline asm "call" statements.  Our current solution doesn't do that.

- On a defconfig-based kernel, this adds +6k of .text (+0.06%).

- On a Fedora distro-based config, it adds +27k of .text (+0.3%).
  (I think the difference from defconfig is mostly caused by
  CONFIG_PARAVIRT.)

I'll try a few more experiments, but I'll probably end up engaging the
compiler people as Linus suggested.

-- 
Josh