There already is a multiprocessing (as defined by having more than one hardware processor running concurrently) precedent on amigaos. It surprises me how quickly people forget it.
Very good point - but actually this is
asymmetric multiprocessing rather than SMP. Just like the old 'coprocessor architecture' with Copper and Blitter it shows the possibilities of offloading special tasks or subroutines to other hardware. With the PPC and more advanced 68ks, cache coherency becomes an issue - which can be solved at some cost as you've stated.
A somewhat similar approach could be used to utilize a 2nd core, but it'd be very troublesome to get the kernel itself run multithreaded. Novell Netware 4 SMP used something similar where only special programs could make use of additional CPUs.
Real SMP however makes no assumptions to where the code is actually running - this implies to the OS itself as well. Apart from the Forbid/Permit workarounds, you're looking at a complete redesign of the kernel. Remember the Windows 95/98/ME line? These relatively simple kernel designs stood no chance of running SMP, so MS didn't even bother trying.
