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

From: Dmitry Vyukov
Date: Thu Sep 21 2017 - 04:12:54 EST

On Wed, Sep 20, 2017 at 11:19 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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@xxxxxxxxx> 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.


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.