On Mon, 25 Nov 2019 10:55:01 +0000
Marc Zyngier <m...@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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