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

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Message-ID: <20170920210731.kbcibdmbd4b3ppfi@treble>
Date:   Wed, 20 Sep 2017 16:07:31 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     "H. Peter Anvin" <hpa@...or.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 08:01:02PM +0200, Dmitry Vyukov wrote:
> 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?

Currently, on x86-64, I believe all exceptions have their own dedicated
stacks in the kernel, but IRQs still come in on the task's kernel stack.

Andy, do you know if there's a reason why IRQs don't use a dedicated IST
stack?

-- 
Josh