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

[Marc Zyngier]

On 2020-04-25 11:29, Marc Zyngier wrote:
> On Fri, 24 Apr 2020 16:39:12 +0530
> Sumit Garg <sumit.garg@xxxxxxxxxx> wrote:
>
> Hi Sumit,
>
> > With pseudo NMIs enabled, interrupt controller can be configured to
> > deliver SGI as a pseudo NMI. So add corresponding handling for SGIs.
> >
> > Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
> > ---
> >  drivers/irqchip/irq-gic-v3.c | 22 +++++++++++++++++-----
> >  1 file changed, 17 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/irqchip/irq-gic-v3.c 
> > b/drivers/irqchip/irq-gic-v3.c
> > index d7006ef..be361bf 100644
> > --- a/drivers/irqchip/irq-gic-v3.c
> > +++ b/drivers/irqchip/irq-gic-v3.c
> > @@ -609,17 +609,29 @@ static inline void gic_handle_nmi(u32 irqnr, 
> > struct pt_regs *regs)
> >  	if (irqs_enabled)
> >  		nmi_enter();
> >
> > -	if (static_branch_likely(&supports_deactivate_key))
> > -		gic_write_eoir(irqnr);
> >  	/*
> >  	 * Leave the PSR.I bit set to prevent other NMIs to be
> >  	 * received while handling this one.
> >  	 * PSR.I will be restored when we ERET to the
> >  	 * interrupted context.
> >  	 */
> > -	err = handle_domain_nmi(gic_data.domain, irqnr, regs);
> > -	if (err)
> > -		gic_deactivate_unhandled(irqnr);
> > +	if (likely(irqnr > 15)) {
> > +		if (static_branch_likely(&supports_deactivate_key))
> > +			gic_write_eoir(irqnr);
> > +
> > +		err = handle_domain_nmi(gic_data.domain, irqnr, regs);
> > +		if (err)
> > +			gic_deactivate_unhandled(irqnr);
> > +	} else {
> > +		gic_write_eoir(irqnr);
> > +		if (static_branch_likely(&supports_deactivate_key))
> > +			gic_write_dir(irqnr);
> > +#ifdef CONFIG_SMP
> > +		handle_IPI(irqnr, regs);
> > +#else
> > +		WARN_ONCE(true, "Unexpected SGI received!\n");
> > +#endif
> > +	}
> >
> >  	if (irqs_enabled)
> >  		nmi_exit();
>
> If there is one thing I would like to avoid, it is to add more ugly
> hacks to the way we handle SGIs. There is very little reason why SGIs
> should be handled differently from all other interrupts. They have the
> same properties, and it is only because of the 32bit legacy that we 
> deal
> with them in such a cumbersome way. Nothing that we cannot fix though.
>
> What I would really like to see is first a conversion of the SGIs to
> normal, full fat interrupts. These interrupts can then be configured as
> NMI using the normal API.
>
> I think Julien had something along these lines (or was that limited to
> the PMU?). Otherwise, I'll happily help you with that.

OK, to give you an idea of what I am after, here's a small series[1] 
that
can be used as a base (it has been booted exactly *once* on a model, and
is thus absolutely perfect ;-).

There is still a bit of work to be able to actually request a SGI (they
are hard-wired as chained interrupts so far, as this otherwise changes
the output of /proc/interrupts, among other things), but you will
hopefully see what I'm aiming for.

Thanks,

        M.

[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=irq/gic-sgi
--
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