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

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Date:   Wed, 21 Oct 2020 10:52:10 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Marc Zyngier <maz@...nel.org>,
        Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Mark Rutland <mark.rutland@....com>,
        julien.thierry.kdev@...il.com,
        Douglas Anderson <dianders@...omium.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        Masayoshi Mizuma <msys.mizuma@...il.com>,
        ito-yuichi@...itsu.com, kgdb-bugreport@...ts.sourceforge.net,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 1/5] arm64: Add framework to turn IPI as NMI

On Tue, 20 Oct 2020 at 18:02, Marc Zyngier <maz@...nel.org> 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...