[PATCH v2] irqchip/sifive-plic: enable interrupt if needed before EOI

[Nam Cao]

On 20/Mar/2024 Palmer Dabbelt wrote:
> On Tue, 13 Feb 2024 02:26:40 PST (-0800), tglx at linutronix.de wrote:
> > Nam!
> >
> > On Wed, Jan 31 2024 at 09:19, Nam Cao wrote:  
> >> RISC-V PLIC cannot "end-of-interrupt" (EOI) disabled interrupts, as
> >> explained in the description of Interrupt Completion in the PLIC spec:
> >>
> >> "The PLIC signals it has completed executing an interrupt handler by
> >> writing the interrupt ID it received from the claim to the claim/complete
> >> register. The PLIC does not check whether the completion ID is the same
> >> as the last claim ID for that target. If the completion ID does not match
> >> an interrupt source that *is currently enabled* for the target, the
> >> completion is silently ignored."
> >>
> >> Commit 69ea463021be ("irqchip/sifive-plic: Fixup EOI failed when masked")
> >> ensured that EOI is successful by enabling interrupt first, before EOI.
> >>
> >> Commit a1706a1c5062 ("irqchip/sifive-plic: Separate the enable and mask
> >> operations") removed the interrupt enabling code from the previous
> >> commit, because it assumes that interrupt should already be enabled at the
> >> point of EOI. However, this is incorrect: there is a window after a hart
> >> claiming an interrupt and before irq_desc->lock getting acquired,
> >> interrupt can be disabled during this window. Thus, EOI can be invoked
> >> while the interrupt is disabled, effectively nullify this EOI. This
> >> results in the interrupt never gets asserted again, and the device who
> >> uses this interrupt appears frozen.  
> >
> > Nice detective work!
> >  
> >> Make sure that interrupt is really enabled before EOI.
> >>
> >> Fixes: a1706a1c5062 ("irqchip/sifive-plic: Separate the enable and mask operations")
> >> Cc: <stable at vger.kernel.org>
> >> Signed-off-by: Nam Cao <namcao at linutronix.de>
> >> ---
> >> v2:
> >>   - add unlikely() for optimization
> >>   - re-word commit message to make it clearer
> >>
> >>  drivers/irqchip/irq-sifive-plic.c | 8 +++++++-
> >>  1 file changed, 7 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/irqchip/irq-sifive-plic.c b/drivers/irqchip/irq-sifive-plic.c
> >> index e1484905b7bd..0a233e9d9607 100644
> >> --- a/drivers/irqchip/irq-sifive-plic.c
> >> +++ b/drivers/irqchip/irq-sifive-plic.c
> >> @@ -148,7 +148,13 @@ static void plic_irq_eoi(struct irq_data *d)
> >>  {
> >>  	struct plic_handler *handler = this_cpu_ptr(&plic_handlers);
> >>
> >> -	writel(d->hwirq, handler->hart_base + CONTEXT_CLAIM);
> >> +	if (unlikely(irqd_irq_disabled(d))) {
> >> +		plic_toggle(handler, d->hwirq, 1);
> >> +		writel(d->hwirq, handler->hart_base + CONTEXT_CLAIM);
> >> +		plic_toggle(handler, d->hwirq, 0);  
> >
> > It's unfortunate to have this condition in the hotpath, though it should
> > be cache hot, easy to predict and compared to the writel() completely in
> > the noise.  
> 
> Ya, I think it's fine.
> 
> I guess we could try and play some tricks.  Maybe hide the load latency 
> with a relaxed writel and some explict fencing, or claim interrupts when 
                                                     ^ you mean complete?
> enabling them.  Those both seem somewhat race-prone, though, so I'm not 
> even sure if they're sane.

The latter option is what I also have in mind. Just need to make sure the
interrupt is masked and we should be safe. Though there is the question of
whether it's worth the effort.

I may do that one day when I stop being lazy.

Best regards,
Nam