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

[Palmer Dabbelt]

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 
enabling them.  Those both seem somewhat race-prone, though, so I'm not 
even sure if they're sane.

Anything with a PLIC is going to have pretty poor interrupt latency 
already, so I doubt it's worth the headache.

>> +	} else {
>> +		writel(d->hwirq, handler->hart_base + CONTEXT_CLAIM);
>> +	}
>>  }
>
> Can the RISCV folks please have a look at this?

Sorry I missed this.

Acked-by: Palmer Dabbelt <palmer at rivosinc.com>

in case anyone was worried, though I saw it got merged so I think we're 
safe there.  I'm always a bit lost with the IRQ stuff, I didn't even 
know that race condition was posisble.

Thanks for the fix!

>
> Thanks,
>
>         tglx