Re: [RFC Patch v1 2/4] irqchip/gic-v3: Add support to handle SGI as pseudo NMI

[Sumit Garg <sumit.garg@linaro.org>]

On Thu, 30 Apr 2020 at 14:43, Marc Zyngier <maz@kernel.org> wrote:
>
> On Thu, 30 Apr 2020 12:50:28 +0530
> Sumit Garg <sumit.garg@linaro.org> wrote:
>
> > Hi Marc,
> >
> > On Wed, 29 Apr 2020 at 13:53, Marc Zyngier <maz@kernel.org> wrote:
>
> [...]
>
> > > What I would like is for the arch code to request these interrupts as
> > > normal interrupts, for which there is one problem to solve: how do you
> > > find out about the Linux IRQ number for a given IPI. Or rather, how
> > > do you get rid of the requirement to have IPI numbers at all and just
> > > say "give me a per-cpu interrupt that I can use as an IPI, and by the
> > > way here's the handler you can call".
> >
> > I think what you are looking for here is something that could be
> > sufficed via enabling "CONFIG_GENERIC_IRQ_IPI" framework for arm64/arm
> > architectures. It's currently used for mips architecture. Looking at
> > its implementation, I think it should be possible to hook up SGIs
> > under new IPI irq_domain for GICv2/v3.
> >
> > So with this framework we should be able to dynamically allocate IPIs
> > and use common APIs such as request_irq()/request_nmi() to tell IPI
> > specific handlers.
> >
> > If this approach looks sane to you then I can start working on a PoC
> > implementation for arm64.
>
> I can't say I'm keen. This IPI framework doesn't really work for the
> GIC:
>
> - it requires a separate irqdomain to be able to guarantee that you
>   allocate the hwirq in the SGI range. What is the point?
> - it creates yet another level of indirection on IPI injection
>
> This framework was created to deal with two cases:
> - systems that can't represent their IPI with a single hwirq spanning
>   all the CPUs
> - "accelerator cores" that don't run Linux
>
> The GIC architecture avoids the first point, and I don't even want to
> think of the second one.
>
> Also, it doesn't solve the initial problem, which is to bootstrap the
> whole thing. The IPI framework relies on an irqdomain to be created the
> first place. This would mean teaching the arch code about the
> intricacies of irqdomains, FW nodes and other terrible things. All
> things which are better hidden in the GIC drivers (not to mention the
> other horror stories that are the RPi-2/3 irqchip and the Huawei GIC
> knock-off).
>
> What I have in mind is to replace the set_smp_cross_call() with
> something that passes the required set of information (interrupt range,
> at the very least). The only thing that I plan to reuse from the IPI
> framework is the chip->ipi_send_mask() callback.
>

Fair enough, I will just pass the allocated interrupt range base
instead of set_smp_cross_call() and use __ipi_send_mask() to invoke a
particular IPI.

And to request an arch specific IPI as NMI, will use
arch_get_ipinr_nmi() and in turn use request_percpu_nmi() to turn that
particular IPI as NMI.

Is there anything that I missed here?

-Sumit

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