Re: [PATCH v3 02/13] genirq: Define ack_irq() and eoi_irq() helpers

From: Marc Zyngier <maz@kernel.org>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>
Subject: Re: [PATCH v3 02/13] genirq: Define ack_irq() and eoi_irq() helpers
Date: Thu, 12 Aug 2021 15:45:25 +0100	[thread overview]
Message-ID: <87y29690xm.wl-maz@kernel.org> (raw)
In-Reply-To: <877dgq9450.mognet@arm.com>

On Thu, 12 Aug 2021 14:36:11 +0100,
Valentin Schneider <valentin.schneider@arm.com> wrote:
> 
> On 12/08/21 08:49, Marc Zyngier wrote:
> > On Tue, 29 Jun 2021 13:49:59 +0100,
> > Valentin Schneider <valentin.schneider@arm.com> wrote:
> >> +void eoi_irq(struct irq_desc *desc)
> >> +{
> >> +	desc->irq_data.chip->irq_eoi(&desc->irq_data);
> >> +
> >> +	if (desc->irq_data.chip->flags & IRQCHIP_AUTOMASKS_FLOW)
> >> +		irq_state_clr_flow_masked(desc);
> >
> > I just realised that this has a good chance to result in a mess with
> > KVM, and specially the way we let the vGIC deactivate an interrupt
> > directly from the guest, without any SW intervention (the magic HW bit
> > in the LRs).
> >
> 
> I didn't think to consider those. It can't ever be simple, can it...
> 
> > With this, interrupts routed to a guest (such as the timers) will
> > always have the IRQD_IRQ_FLOW_MASKED flag set, which will never be
> > cleared.
> >
> > I wonder whether this have a chance to interact badly with
> > mask/unmask, or with the rest of the flow...
> >
> 
> Isn't it the other way around? That is, eoi_irq() will clear
> IRQD_IRQ_FLOW_MASKED regardless of what happens within chip->irq_eoi(),
> so we would end up with !IRQD_IRQ_FLOW_MASKED even if the (physical) IRQ
> remains Active (irqd_is_forwarded_to_vcpu() case).

Ah, I missed (again) that we always clear the flag, no matter what.

> This does not entirely match reality (if the IRQ is still Active then it is
> still "flow-masked"), but AFAICT this doesn't impact our handling of
> forwarded IRQs: IRQD_IRQ_FLOW_MASKED is only really relevant from ack_irq()
> to eoi_irq(), and deactivation-from-the-guest (propagated via LR.HW=1)
> happens after that.

Right. So we can have an active interrupt that is not flow-masked.
That's counter-intuitive, but that's the GIC architecture for you...

I'll take the series for a ride in -next. If anything breaks, we
should know pretty soon.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.