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

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Date:   Wed, 20 Sep 2017 19:38:09 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        LKML <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>,
        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 7:32 PM, H. Peter Anvin <hpa@...or.com> 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?

I think we need just the frame itself and RSP pointing below this
frame. If we don't have a frame, CALL instruction will smash whatever
RSP happens to point to. Compiler doesn't have to setup RSP to point
below used part of stack in leaf functions.