xen/arm: inflight irqs during migration

xen/arm: inflight irqs during migration

We need to take special care when migrating irqs that are already
inflight from one vcpu to another. See "The effect of changes to an
GICD_ITARGETSR", part of chapter 4.3.12 of the ARM Generic Interrupt
Controller Architecture Specification, IHI 0048B.

The main issue from the Xen point of view is that the lr_pending and
inflight lists are per-vcpu. The lock we take to protect them is also
per-vcpu.

In order to avoid issues, if the irq is still lr_pending, we can
immediately move it to the new vcpu for injection.

Otherwise if it is in a GICH_LR register, set a new flag
GIC_IRQ_GUEST_MIGRATING.  If GIC_IRQ_GUEST_MIGRATING is set we'll change
the affinity of the physical irq when clearing the LR (in a following
commit). Therefore if the irq is inflight in an LR when the guest writes
to itarget, we wait until the LR is cleared before changing physical
irq affinity. This guarantees that the old vcpu is going to be
interrupted and clear the LR, either because it has been descheduled
after EOIing the interrupt or because it is interrupted by the second
physical irq coming.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>