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

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Message-ID: <20170920011823.wbkn4qagiovuzkow@treble>
Date:   Tue, 19 Sep 2017 20:18:23 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        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 Tue, Sep 19, 2017 at 01:45:28PM -0500, 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.
> 
> Here are the vmlinux .text size differences with the following configs
> on GCC:
> 
> - defconfig
> - defconfig without frame pointers
> - Fedora distro config
> - Fedora distro config without frame pointers
> 
> 	defconfig	defconfig-nofp	distro		distro-nofp
> before	9796300		9466764		9076191		8789745
> after	9796941		9462859		9076381		8785325
> 
> With frame pointers, the text size increases slightly.  Without frame
> pointers, the text size decreases, and a little more significantly.
> 
> Reported-by: Matthias Kaehlcke <mka@...omium.org>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>

NAK - kbuild bot is reporting some cases where this patch doesn't force
the frame pointer setup.

-- 
Josh