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 13:45:29 -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 11:28:30AM -0700, Linus Torvalds wrote:
> On Thu, Sep 14, 2017 at 10:33 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >>
> >>  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.
> 
> Exactly. Do something like
> 
>    #ifdef CONFIG_FRAME_POINTER
>    # define EXTRA_ASM_CLOBBERS "rsp"
>    #else
>    # define EXTRA_ASM_CLOBBERS
>    #endif
> 
> and then replace the nasty
> 
>         register void *__sp asm(_ASM_SP);
>         ..
>         "+r" (__sp)
> 
> games with just that EXTRA_ASM_CLOBBERS thing at the end of the clobbers.
> 
> Yes, you'd probably have to document that the alternative_call_2()
> thing doesn't take a "input" argument, but a input_and_clobbers, but
> all users do that anyway.
> 
> I dunno.

There's also alternative_call(), which doesn't yet have the '__rsp'
annotation, but it probably should.  It has some callers which pass
clobbers and some which don't, so its conversion would be trickier.

So my plan is to keep patch 3 of this series, which clarifies those
alternative macro interfaces, and also separates the inputs from the
clobbers.  That'll make it really easy to add something like
EXTRA_ASM_CLOBBERS above.

In fact I'll probably keep patches 1-3, because they're all
improvements.  Then I'll replace the original patch 4 (ASM_CALL) with
the "sp" clobbers thing.

-- 
Josh