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 12:33:48 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Ingo Molnar <mingo@...nel.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 Thu, Sep 14, 2017 at 12:26:27PM -0500, Josh Poimboeuf wrote:
> On Thu, Sep 14, 2017 at 10:16:08AM -0700, Linus Torvalds wrote:
> > On Thu, Sep 14, 2017 at 7:48 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > >
> > > 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.
> > 
> > But couldn't we make the whole stack pointer clobber be dependent on
> > CONFIG_FRAME_POINTER?
> > 
> > The only reason we do it is to make sure the frame pointer is set up
> > before the inline asm is emitted, but with frame pointers disabled we
> > don't need to.
> 
> We could, but then that would mean either:
> 
>  a) uglifying the 15 or so relevant inline asm locations with ifdefs; or

Actually I guess we could put the "sp" in a macro...  I'll try it.

-- 
Josh