Re: [PATCH v2 2/2] irqchip/gic-v2, v3: Prevent SW resends entirely

From: Marc Zyngier <maz@kernel.org>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@arm.com>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	Jason Cooper <jason@lakedaemon.net>
Subject: Re: [PATCH v2 2/2] irqchip/gic-v2, v3: Prevent SW resends entirely
Date: Fri, 31 Jul 2020 09:08:41 +0100	[thread overview]
Message-ID: <10da73b8a8937b08b0993513d6c20e98@kernel.org> (raw)
In-Reply-To: <jhjmu3gim0g.mognet@arm.com>

Hi Valentin,

On 2020-07-31 01:08, Valentin Schneider wrote:
> Hi Marc,
> 
> On 30/07/20 19:10, Marc Zyngier wrote:
> [...]
>>> diff --git a/drivers/irqchip/irq-gic.c b/drivers/irqchip/irq-gic.c
>>> index e2b4cae88bce..a91ce1e73bd2 100644
>>> --- a/drivers/irqchip/irq-gic.c
>>> +++ b/drivers/irqchip/irq-gic.c
>>> @@ -983,6 +983,7 @@ static int gic_irq_domain_map(struct irq_domain
>>> *d, unsigned int irq,
>>>                              irq_hw_number_t hw)
>>>  {
>>>      struct gic_chip_data *gic = d->host_data;
>>> +	struct irq_data *irqd = irq_desc_get_irq_data(irq_to_desc(irq));
>>> 
>>>      if (hw < 32) {
>>>              irq_set_percpu_devid(irq);
>>> @@ -992,8 +993,11 @@ static int gic_irq_domain_map(struct irq_domain
>>> *d, unsigned int irq,
>>>              irq_domain_set_info(d, irq, hw, &gic->chip, 
>>> d->host_data,
>>>                                  handle_fasteoi_irq, NULL, NULL);
>>>              irq_set_probe(irq);
>>> -		irqd_set_single_target(irq_desc_get_irq_data(irq_to_desc(irq)));
>>> +		irqd_set_single_target(irqd);
>>>      }
>>> +
>>> +	/* Prevents SW retriggers which mess up the ACK/EOI ordering */
>>> +	irqd_set_handle_enforce_irqctx(irqd);
>>>      return 0;
>>>  }
>> 
>> I'm OK with this in principle, but this requires additional changes
>> in the rest of the GIC universe. The ITS driver needs to provide its 
>> own
>> retrigger function for LPIs (queuing an INT command), and any of the
>> SPI generating widgets that can be stacked on top of a GIC (GICv3-MBI,
>> GICv2m, and all the other Annapurna/Marvell/NVDIA wonders need to gain
>> directly or indirectly a call to irq_chip_retrigger_hierarchy().
>> 
> 
> Eep, yes indeed... I didn't see that can was full of worms, though even 
> if
> it only really matters for eoimode=0 I think it might still be worth it
> (if only to respect the spec).

Well, given that we are using EOImode=0 for all guests at the moment,
there is some value it getting it right! ;-)

> 
>> We can probably avoid changing the MSI widgets by teaching the MSI
>> code about the HW retrigger, but a number of other non-MSI drivers
>> will need some help...
>> 
>> I'll have a look tomorrow.
>> 
> 
> For LPIs AFAICT we could directly reuse its_irq_set_irqchip_state() - I 
> see
> the VPE side of things already has a HW retrigger callback.

Yes, that's the idea (in general, if you implement the PENDING side of
irq_set_irqchip_state(), retrigger comes for free).

> For gicv2m, I *think* we'd want irq_chip_retrigger_hierarchy() on both 
> MSI
> domains (which IIUC you suggest might be doable by adding the retrigger 
> as
> a default MSI chip op).

Yes, that was my idea.

> I'm not very familiar with the rest of the fauna, so I'll have to do 
> some
> reading tomorrow as well; it's probably high time for me to actually 
> read
> up on LPIs & ITS while I'm at it...

Look for anything that performs an interrupt allocation by calling
into the parent with a 3 cell (DT case) fwspec. There is a bunch
of them.

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

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