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

[Sumit Garg]

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 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.

-Sumit

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