Level-triggered mapped IRQs are special because we only observe rising
edges as input to the VGIC, and we don't set the EOI flag and therefore
are not told when the level goes down, so that we can re-queue a new
interrupt when the level goes up.

One way to solve this problem is to side-step the logic of the VGIC and
special case the validation in the injection path, but it has the
unfortunate drawback of having to peak into the physical GIC state
whenever we want to know if the interrupt is pending on the virtual
distributor.

Instead, we can maintain the current semantics of a level triggered
interrupt by sort of treating it as an edge-triggered interrupt,
following from the fact that we only observe an asserting edge.  This
requires us to be a bit careful when populating the LRs and when folding
the state back in though:

 * We lower the line level when populating the LR, so that when
   subsequently observing an asserting edge, the VGIC will do the right
   thing.

 * If the guest never acked the interrupt while running (for example if
   it had masked interrupts at the CPU level while running), we have
   to preserve the pending state of the LR and move it back to the
   line_level field of the struct irq when folding LR state.

   If the guest never acked the interrupt while running, but changed the
   device state and lowered the line (again with interrupts masked) then
   we need to observe this change in the line_level.

   Both of the above situations are solved by sampling the physical line
   and set the line level when folding the LR back.

 * Finally, if the guest never acked the interrupt while running and
   sampling the line reveals that the device state has changed and the
   line has been lowered, we must clear the physical active state, since
   we will otherwise never be told when the interrupt becomes asserted
   again.

This has the added benefit of making the timer optimization patches
(https://lists.cs.columbia.edu/pipermail/kvmarm/2017-July/026343.html) a
bit simpler, because the timer code doesn't have to clear the active
state on the sync anymore.  It also potentially improves the performance
of the timer implementation because the GIC knows the state or the LR
and only needs to clear the
active state when the pending bit in the LR is still set, where the
timer has to always clear it when returning from running the guest with
an injected timer interrupt.
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>