linux-kernel - Re: [PATCH 2/2] x86/asm: Fix inline asm call constraints for clang

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Date:   Wed, 20 Sep 2017 12:51:25 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.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>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>
Subject: Re: [PATCH 2/2] x86/asm: Fix inline asm call constraints for clang

On Wed, Sep 20, 2017 at 10:32:43AM -0700, H. Peter Anvin wrote:
> On 09/19/17 11:45, Josh Poimboeuf wrote:
> > For inline asm statements which have a CALL instruction, we list the
> > stack pointer as a constraint to convince GCC to ensure the frame
> > pointer is set up first:
> > 
> >   static inline void foo()
> >   {
> >   	register void *__sp asm(_ASM_SP);
> >   	asm("call bar" : "+r" (__sp))
> >   }
> > 
> > Unfortunately, that pattern causes clang to corrupt the stack pointer.
> > 
> > There's actually an easier way to achieve the same goal in GCC, without
> > causing trouble for clang.  If we declare the stack pointer register
> > variable as a global variable, and remove the constraint altogether,
> > that convinces GCC to always set up the frame pointer before inserting
> > *any* inline asm.
> > 
> > It basically acts as if *every* inline asm statement has a CALL
> > instruction.  It's a bit overkill, but the performance impact should be
> > negligible.
> > 
> 
> Again, probably negligible, but why do we need a frame pointer just
> because we have a call assembly instruction?

It's frame pointer convention.  Without it, if dumping the stack from
the called function, a function will get skipped in the stack trace.

-- 
Josh