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Date: Wed, 20 Sep 2017 12:51:25 -0500
From: Josh Poimboeuf <jpoimboe@redhat.com>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Alexander Potapenko <glider@google.com>,
        Dmitriy Vyukov <dvyukov@google.com>,
        Matthias Kaehlcke <mka@chromium.org>, Arnd Bergmann <arnd@arndb.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>
Subject: Re: [PATCH 2/2] x86/asm: Fix inline asm call constraints for clang
Message-ID: <20170920175125.2quikhfdy2okubsw@treble>
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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