RE: [PATCH 1/4] irqchip/sifive-plic: Setup cpuhp once after current handler is present

[Anup Patel]

> -----Original Message-----
> From: Marc Zyngier <maz@xxxxxxxxxx>
> Sent: 16 May 2020 17:42
> To: Anup Patel <Anup.Patel@xxxxxxx>
> Cc: Palmer Dabbelt <palmer@xxxxxxxxxxx>; Paul Walmsley
> <paul.walmsley@xxxxxxxxxx>; Thomas Gleixner <tglx@xxxxxxxxxxxxx>; Jason
> Cooper <jason@xxxxxxxxxxxxxx>; Atish Patra <Atish.Patra@xxxxxxx>; Alistair
> Francis <Alistair.Francis@xxxxxxx>; Anup Patel <anup@xxxxxxxxxxxxxx>; linux-
> riscv@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH 1/4] irqchip/sifive-plic: Setup cpuhp once after current
> handler is present
> 
> Hi Anup,
> 
> On 2020-05-16 07:38, Anup Patel wrote:
> > For multiple PLIC instances, the plic_init() is called once for each
> > PLIC instance. Due to this we have two issues:
> > 1. cpuhp_setup_state() is called multiple times 2. plic_starting_cpu()
> > can crash for boot CPU if cpuhp_setup_state()
> >    is called before boot CPU PLIC handler is available.
> >
> > This patch fixes both above issues.
> >
> > Signed-off-by: Anup Patel <anup.patel@xxxxxxx>
> > ---
> >  drivers/irqchip/irq-sifive-plic.c | 14 ++++++++++++--
> >  1 file changed, 12 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/irqchip/irq-sifive-plic.c
> > b/drivers/irqchip/irq-sifive-plic.c
> > index 822e074c0600..7dc23edb3267 100644
> > --- a/drivers/irqchip/irq-sifive-plic.c
> > +++ b/drivers/irqchip/irq-sifive-plic.c
> > @@ -76,6 +76,7 @@ struct plic_handler {
> >  	void __iomem		*enable_base;
> >  	struct plic_priv	*priv;
> >  };
> > +static bool plic_cpuhp_setup_done;
> >  static DEFINE_PER_CPU(struct plic_handler, plic_handlers);
> >
> >  static inline void plic_toggle(struct plic_handler *handler, @@
> > -282,6 +283,7 @@ static int __init plic_init(struct device_node *node,
> >  	int error = 0, nr_contexts, nr_handlers = 0, i;
> >  	u32 nr_irqs;
> >  	struct plic_priv *priv;
> > +	struct plic_handler *handler;
> >
> >  	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
> >  	if (!priv)
> > @@ -310,7 +312,6 @@ static int __init plic_init(struct device_node
> > *node,
> >
> >  	for (i = 0; i < nr_contexts; i++) {
> >  		struct of_phandle_args parent;
> > -		struct plic_handler *handler;
> >  		irq_hw_number_t hwirq;
> >  		int cpu, hartid;
> >
> > @@ -364,9 +365,18 @@ static int __init plic_init(struct device_node
> > *node,
> >  		nr_handlers++;
> >  	}
> >
> > -	cpuhp_setup_state(CPUHP_AP_IRQ_SIFIVE_PLIC_STARTING,
> > +	/*
> > +	 * We can have multiple PLIC instances so setup cpuhp state only
> > +	 * when context handler for current/boot CPU is present.
> > +	 */
> > +	handler = this_cpu_ptr(&plic_handlers);
> > +	if (handler->present && !plic_cpuhp_setup_done) {
> 
> If there is no context handler for the boot CPU, the system is doomed, right? It
> isn't able to get any interrupt, and you don't register the hotplug notifier that
> could allow secondary CPUs to boot.
> 
> So what is the point? It feels like you should just give up here.
> 
> Also, the boot CPU is always CPU 0. So checking that you only register the
> hotplug notifier from CPU 0 should be enough.

The boot CPU is not fixed in RISC-V, the logical id of the boot CPU will always
be zero but physical id (or HART id) can be something totally different.

On RISC-V NUMA system, we will have a separate PLIC in each NUMA node.

Let's say we have a system with 2 NUMA nodes, each NUMA node having
4 CPUs (or 4 HARTs).  In this case, the DTB passed to Linux will have two PLIC
DT nodes where each PLIC device targets only 4 CPUs (or 4 HARTs). The
plic_init() functions will setup handlers for only 4 CPUs (or 4 HARTs). In other
words, plic_init() for "PLIC0" will setup handler for HART id 0 to 3 and
plic_init() for "PLIC1" will setup handler for HART id 4 to 7. Now, any CPU
can be boot CPU so it is possible that CPU with HART id 4 is boot CPU and
when plic_init() is first called for "PLIC0" the handler for HART id 4 is not
setup because it will be setup later when plic_init() is called for "PLIC1".
This cause plic_starting_cpu() to crash when plic_init() is called for "PLIC0".

I hope above example helps understanding the issue.

I encounter this issue randomly when booting Linux on QEMU RISC-V
with multiple NUMA nodes.

Regards,
Anup

> 
> > +		cpuhp_setup_state(CPUHP_AP_IRQ_SIFIVE_PLIC_STARTING,
> >  				  "irqchip/sifive/plic:starting",
> >  				  plic_starting_cpu, plic_dying_cpu);
> > +		plic_cpuhp_setup_done = true;
> > +	}
> > +
> >  	pr_info("mapped %d interrupts with %d handlers for %d contexts.\n",
> >  		nr_irqs, nr_handlers, nr_contexts);
> >  	set_handle_irq(plic_handle_irq);
> 
> Thanks,
> 
>          M.
> --
> Jazz is not dead. It just smells funny...