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

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Message-Id: <C46A8B37-7249-412D-866F-53EE400C73AC@amacapital.net>
Date:   Wed, 20 Sep 2017 14:19:07 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        "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 Sep 20, 2017, at 2:07 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
>> 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?
> 

Because IST is awful due to recursion issues.  We immediately switch to an IRQ stack, though.

If the kernel wanted a redzone, it would have to use IST for everything, which would entail a bunch of unpleasant hackery.

> -- 
> Josh