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 11:28:30 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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 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.

               Linus