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

On Tue, 20 Oct 2020 at 18:02, Marc Zyngier [off-list ref] wrote:

On 2020-10-20 13:25, Daniel Thompson wrote:

quoted

On Tue, Oct 20, 2020 at 04:52:43PM +0530, Sumit Garg wrote:

[...]

quoted

quoted

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...

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