NMI for ARC

[Peter Zijlstra]

On Wed, Sep 28, 2016 at 12:25:11PM -0700, Andy Lutomirski wrote:
> > Yes. If the NMI returns to kernel space you must not attempt preemption
> > for reasons you found :-),
> 
> Last time I looked at this, I decided that there was no reason that
> NMIs would ever need to handle preemption.  Even if the NMI hit
> interruptible kernel code, anything that would cause preemption to be
> needed would either send an IPI (and thus cause preemption) right
> after the NMI fiinished.  NMI handlers themselves have no business
> setting TIF_NEED_RESCHED or similar.

Good point, they don't and therefore you need not bother.

> > if the NMI returns to userspace you should do
> > the normal return to user bits, I think.
> 
> x86 does this for simplicity.  There was a really nasty corner case
> that I could only figure out how to solve by special casing NMIs from
> user space.  I'm not sure that it's actually necessary from a
> non-arch-specific POV to handle all the usual return-to-userspace work
> on NMI.  But maybe perf NMIs can send signals?

No it cannot. It uses irq_work (which sends a self-IPI) when it wants to
do signals.

> >> 2. The low level return code, resume_user_mode_begin and/or resume_kernel_mode
> >> require interrupt safety, does that need to be NMI safe as well. We ofcourse want
> >> the very late register restore parts to be non-interruptible, but is this required
> >> before we call prrempt_schedule_irq() off of asm code.
> >
> > Urgh, I'm never quite sure on the details here, I've Cc'ed Andy who
> > might actually know this off the top of his head. I'll try and dig
> > through x86 to see what it does.
> 
> On x86, it's quite simple.  IRQs are *always* off during the final
> register restore, and we don't re-check for preemption there.  x86
> handles preemption after turning off IRQs, and IRQs are guaranteed to
> stay off until we actually return to userspace.
> 
> The code is almost entirely in C in arch/x86/entry/common.c.  There
> isn't anything particularly x86-speficic in there.

Right, so what I think Vineet is asking is if we need to disable NMIs as
well, we cannot on x86 disable NMIs so no.