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

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Date:   Wed, 21 Oct 2020 11:27:56 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Sumit Garg <sumit.garg@...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>,
        Daniel Thompson <daniel.thompson@...aro.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 2020-10-20 12:22, Sumit Garg wrote:
> On Tue, 20 Oct 2020 at 15:38, Marc Zyngier <maz@...nel.org> wrote:
>> 
>> On 2020-10-20 07:43, Sumit Garg wrote:
>> > On Mon, 19 Oct 2020 at 17:07, Marc Zyngier <maz@...nel.org> 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...