From: Jan Kiszka<jan.kiszka@xxxxxxxxxxx>
PCI 2.3 allows to generically disable IRQ sources at device level. This
enables us to share IRQs of such devices on the host side when passing
them to a guest.
However, IRQ disabling via the PCI config space is more costly than
masking the line via disable_irq. Therefore we register the IRQ in adaptive
mode and switch between line and device level disabling on demand.
This feature is optional, user space has to request it explicitly as it
also has to inform us about its view of PCI_COMMAND_INTX_DISABLE. That
way, we can avoid unmasking the interrupt and signaling it if the guest
masked it via the PCI config space.
+ ret =IRQ_NONE;
+