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

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Message-ID: <eddc6128-9ae8-a5ea-0174-98dfadf7d07b@zytor.com>
Date:   Wed, 20 Sep 2017 10:32:43 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org
Cc:     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 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?

	-hpa