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

[Sumit Garg]

On Tue, 20 Oct 2020 at 18:02, Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On 2020-10-20 13:25, Daniel Thompson wrote:
> > On Tue, Oct 20, 2020 at 04:52:43PM +0530, Sumit Garg wrote:
>
> [...]
>
> >> 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.
> >
> > Do the fallback implementations require a new IPI? The fallbacks
> > could rely on existing mechanisms such as the smp_call_function
> > family.
>
> Indeed. I'd be worried of using that mechanism for NMIs, but normal
> IPIs should stick to the normal cross-call stuff.

Yes, the fallback implementations could rely on existing cross-call
stuff but current framework only allows this fallback choice at
compile time and to make this choice at runtime, we need additional
framework changes like allowing arch_trigger_cpumask_backtrace() to
return a boolean in order to switch to fallback mode, similar would be
the case for kgdb.

I think this should be doable but I am still not sure regarding the
advantages of using existing IPI vs new IPI (which we will already use
in case of pseudo NMI support) as normal IRQ.

>
> The jury is still out on why this is a good idea, given that it
> doesn't work in the only interesting case (deadlocked CPUs).

I think CPU soft-lockups (interrupts enabled) is an interesting case
to debug as well.

-Sumit

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