Re: [PATCH v5 1/5] arm64: Add framework to turn IPI as NMI

[Marc Zyngier]

On 2020-10-20 12:22, Sumit Garg wrote:
> On Tue, 20 Oct 2020 at 15:38, Marc Zyngier <maz@xxxxxxxxxx> wrote:
> > On 2020-10-20 07:43, Sumit Garg wrote:
> > > On Mon, 19 Oct 2020 at 17:07, Marc Zyngier <maz@xxxxxxxxxx> wrote:
> >
> > [...]
> >
> > >> > +{
> > >> > +     if (!ipi_desc)
> > >> > +             return;
> > >> > +
> > >> > +     if (is_nmi) {
> > >> > +             if (!prepare_percpu_nmi(ipi_id))
> > >> > +                     enable_percpu_nmi(ipi_id, IRQ_TYPE_NONE);
> > >> > +     } else {
> > >> > +             enable_percpu_irq(ipi_id, IRQ_TYPE_NONE);
> > >>
> > >> I'm not keen on this. Normal IRQs can't reliably work, so why do you
> > >> even bother with this?
> > >
> > > Yeah I agree but we need to support existing functionality for kgdb
> > > roundup and sysrq backtrace using normal IRQs as well.
> >
> > When has this become a requirement? I don't really see the point in
> > implementing something that is known not to work.
>
> For kgdb:
>
> Default implementation [1] uses smp_call_function_single_async() which
> in turn will invoke IPI as a normal IRQ to roundup CPUs.
>
> [1] 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/debug/debug_core.c#n244
>
> For sysrq backtrace:
>
> Default implementation [2] fallbacks to smp_call_function() (IPI as a
> normal IRQ) to print backtrace in case architecture doesn't provide
> arch_trigger_cpumask_backtrace() hook.
>
> [2] 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/tty/sysrq.c#n250
>
> So in general, IPI as a normal IRQ is still useful for debugging but
> it can't debug a core which is stuck in deadlock with interrupts
> disabled.

And that's not something we implement today for good reasons:
it *cannot* work reliably. What changed that we all of a sudden need it?

> And since we choose override default implementations for pseudo NMI
> support, we need to be backwards compatible for platforms which don't
> possess pseudo NMI support.

No. There is nothing to be "backward compatible" with, because
- this isn't a userspace visible feature
- *it doesn't work*

So please drop this non-feature from this series.

        M.
--
Jazz is not dead. It just smells funny...