MIME-Version: 1.0
In-Reply-To: <eddc6128-9ae8-a5ea-0174-98dfadf7d07b@zytor.com>
References: <cover.1505846562.git.jpoimboe@redhat.com> <31e96e6bcfcb47725e15a093b9c31660dfaad430.1505846562.git.jpoimboe@redhat.com>
 <eddc6128-9ae8-a5ea-0174-98dfadf7d07b@zytor.com>
From: Dmitry Vyukov <dvyukov@google.com>
Date: Wed, 20 Sep 2017 19:38:09 +0200
Message-ID: <CACT4Y+YrgBgKSR1ViWCuw2rxe9jrk3YNnGXXo1MNL+3rsFPaMw@mail.gmail.com>
Subject: Re: [PATCH 2/2] x86/asm: Fix inline asm call constraints for clang
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
        "x86@kernel.org" <x86@kernel.org>, LKML <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>,
        Matthias Kaehlcke <mka@chromium.org>, Arnd Bergmann <arnd@arndb.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1338
Lines: 32

On Wed, Sep 20, 2017 at 7:32 PM, H. Peter Anvin <hpa@zytor.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.