Note that using the existing data connection (PCI-e is point to point, not a bus anymore) helps solve some race conditions. DMA writes from a device to memory may be buffered by bridges, but interrupt pins are likely not; so if the device wrote to memory and signals that to the host via an interrupt pin, the interrupt may arrive before the write. A message signaled interrupt uses the same mechanism as the DMA write, if the DMA write is buffered, the interrupt write will wait behind it.