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

From: Andy Lutomirski
Date: Wed Sep 20 2017 - 17:19:15 EST

> On Sep 20, 2017, at 2:07 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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.

> --
> Josh