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

[Josh Poimboeuf <jpoimboe@redhat.com>]

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?

-- 
Josh