> So interrupt-handling can be per CPU (really, per device instance: if
> you have more CPUs than SMP-safe drivers, the extra CPUs cant help
> interrupts).  Whereas raising SPLs will acquire a (global) lock.

can you please explain why do you think raising SPLs will acquire a lock?

if you mean something like the following, please don't.
please keep splxxx() functions cpu local.

	int
	splbio()
	{ 
		int s = _current_implementation_of_splbio();
		spinlock(&splbio_lock);
		return s;
	}

YAMAMOTO Takashi