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From: Dmitry Vyukov <dvyukov@google.com>
Date: Thu, 21 Sep 2017 10:12:29 +0200
Message-ID: <CACT4Y+YYgP4EP0rR27my=JGxRrM70eOH1RtYA8Nz3GUGQFDY2A@mail.gmail.com>
Subject: Re: [PATCH 2/2] x86/asm: Fix inline asm call constraints for clang
To: Andy Lutomirski <luto@amacapital.net>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>, "H. Peter Anvin" <hpa@zytor.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>
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On Wed, Sep 20, 2017 at 11:19 PM, Andy Lutomirski <luto@amacapital.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@zytor.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.