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Date:   Sat, 16 May 2020 14:30:46 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Anup Patel <Anup.Patel@....com>
Cc:     Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Atish Patra <Atish.Patra@....com>,
        Alistair Francis <Alistair.Francis@....com>,
        Anup Patel <anup@...infault.org>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] irqchip/sifive-plic: Setup cpuhp once after current
 handler is present

On 2020-05-16 13:52, Anup Patel wrote:
>> -----Original Message-----
>> From: Marc Zyngier <maz@...nel.org>
>> Sent: 16 May 2020 17:42
>> To: Anup Patel <Anup.Patel@....com>
>> Cc: Palmer Dabbelt <palmer@...belt.com>; Paul Walmsley
>> <paul.walmsley@...ive.com>; Thomas Gleixner <tglx@...utronix.de>; 
>> Jason
>> Cooper <jason@...edaemon.net>; Atish Patra <Atish.Patra@....com>; 
>> Alistair
>> Francis <Alistair.Francis@....com>; Anup Patel <anup@...infault.org>; 
>> linux-
>> riscv@...ts.infradead.org; linux-kernel@...r.kernel.org
>> 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@....com>
>> > ---
>> >  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.

So on riscv, smp_processor_id() can return a non-zero value on the
the boot CPU? Interesting... :-/

> 
> 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.

It does, thanks. This pseudo NUMA thing really is a terrible hack...

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

Then why don't you defer the probing of the PLIC you can't initialize
from this CPU? If you're on CPU4-7, only initialize the PLIC that
matters to you, and not the the others. It would certainly make a lot
more sense, and be more robust.

         M.
-- 
Jazz is not dead. It just smells funny...

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