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From: Andre Przywara <andre.przywara@arm.com>
To: Marc Zyngier <maz@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org,
	kvmarm@lists.cs.columbia.edu
Subject: Re: [PATCH v2] kvm: arm: VGIC: Fix interrupt group enablement
Date: Fri, 14 Feb 2020 11:36:07 +0000
Message-ID: <20200214113607.39854c81@donnerap.cambridge.arm.com> (raw)
In-Reply-To: <e2426986ebc9be4e14eb99028b28a43e@www.loen.fr>

On Mon, 25 Nov 2019 10:55:01 +0000
Marc Zyngier <maz@kernel.org> wrote:

Hi Marc,

dug this out of my inbox, sorry for warming this up.

> On 2019-11-22 18:51, Andre Przywara wrote:
> > Hi Marc,
> >
> > this is still a bit rough, and only briefly tested, but I wanted to
> > hear your opinion on the general approach (using a second list in
> > addition to the ap_list). Some ugly bits come from the fact that the
> > two lists are not that different, so we have to consider both of them
> > at times. This is what I wanted to avoid with just one list that gets
> > filtered on the fly.
> > Or I am just stupid and don't see how it can be done properly ;-)  
> 
> I don't know about that, but I think there is a better way.
> 
> You have essentially two sets of pending interrupts:
> 
> 1) those that are enabled and group-enabled, that end up in the AP list
> 2) those that are either disabled and/or group-disabled
> 
> Today, (2) are not on any list.

For a reason: because we don't really care about them. And so far they would only become interesting on an *individual* interrupt base, and our VGIC routines can deal very well with that.

> What I'm suggesting is that we create
> a list for these interrupts that cannot be forwarded.

So the problem with that is that a list would need a list lock, and this is where things get hairy:
- Either we introduce a separate disabled_list lock, adding to the nightmare of lock hierarchy we already have. I don't think that's really justifiable just because of group0 IRQs.
- We piggy-back on an existing lock, like the ap_list_lock. The problem with that is that vgic_queue_irq_unlock takes and drops that lock, so we can't just iterate over this disabled list while holding that lock, and feed each IRQ to vgic_queue_irq_unlock() easily.
One solution I was thinking about was something like:
while (!list_empty(disabled_list)) {
	spin_lock(ap_list_lock);
	irq = remove_first_entry(disabled_list);
	spin_unlock(ap_list_lock);

	lock_irq(irq);
	/* re-check? */
	vgic_queue_irq_unlock(irq);
}

Does that sound feasible? It's not really efficient nor nice, but I am not sure we care so much about this since we assume group enablement is rather rare.

Cheers,
Andre

> Then enabling an interrupt or a group is a matter of moving pending
> interrupts from one list to another. And I think most of the logic
> can be hidden in vgic_queue_irq_unlock().
> 
>          M.
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Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-22 18:51 Andre Przywara
2019-11-25 10:55 ` Marc Zyngier
2020-02-14 11:36   ` Andre Przywara [this message]

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