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

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Message-ID: <CACT4Y+b1EagSNhwsVkgt4sH8PHrLPO7Mp+Z+JNBWo+qxO+LMSg@mail.gmail.com>
Date:   Wed, 20 Sep 2017 20:01:02 +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 7:46 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 09/20/17 10:38, Dmitry Vyukov wrote:
>>
>> 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.
>>
>
> In the kernel it does.  Redzoning is not allowed in the kernel, because
> interrupts or exceptions would also smash the redzone.

I see... But it's the same for user-space signals, the first thing a
signal should do is to skip the redzone. I guess interrupt handlers
should switch to interrupt stack which avoids smashing redzone
altogether. Do you mean nested interrupts/exceptions in interrupts?
In my experience frames in leaf functions can have pretty large
performance penalty. Wonder if we have we considered changing
interrupt/exception handlers to avoid smashing redzones and disable
leaf frames?