Linux did the same thing 18 years ago and their reasons for the change
are similar to ours - https://github.com/torvalds/linux/commit/7d12e78
Most interrupt handlers (i.e. IRQ handlers) never used the register
state reference anywhere so there's simply no need of passing it around.
I didn't measure the performance boost but surely this change can't make
things worse anyway.
PCIIRQHandler is a generic IRQ handler that the device driver can
inherit to use either Pin or MSI(x) based interrupt mechanism.
The PCIIRQHandler can do what the existing IRQHandler can do for pin
based interrupts but also deal with MSI based interrupts. We can
hopefully convert all the PCI based devices to use this handler so that
MSI(x) can be used.
