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

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Date:   Thu, 21 Sep 2017 10:12:29 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Josh Poimboeuf <jpoimboe@...hat.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 Wed, Sep 20, 2017 at 11:19 PM, Andy Lutomirski <luto@...capital.net> 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.

Thanks.

I guess it must be finite recursion, because we could not handle
infinite with finite stack. I thing that solves it is simply:

sub $256, %rsp
... do stuff ...
add $256, %rsp

Don't know if it's applicable to interrupts or not.