Re: NMI for ARC

[Andy Lutomirski]

On Sep 28, 2016 1:37 PM, "Peter Zijlstra" <peterz@xxxxxxxxxxxxx> wrote:
>
> 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.
>

The same argument works here, too: an NMI won't set TIF_NEED_RESCHED
without sending an IPI, so we can't miss a wakeup.