Finally

I just booted into Haiku working on an SMP machine. Unfortunately, I am not really sure what change exactly triggered this - I've tried so much and all of a sudden it started to work, after I disabled setting up the APIC (the advanced programmable interrupt controller) to use ExtINT delivery mode - that shouldn't tell you anything, I know, but it's still remarkably that this code was originally disabled as well.
It took me quite a number of hours to get it working, so it's a bit frustrating not to know what was actually responsible for the hickup, but it still didn't make me that curious to start an investigation on this topic for now...

Anyway, our SMP configuration is pretty weak right now - it only supports virtual-wire mode which is only one of two possible modes every compatible IA-32 MP system should support. We don't yet support to run the system in so called symmetrical MP mode - that would require us to do some more APIC programming for interrupt redirection, which I obviously didn't need to do to get my machine up and running. Bad for Haiku, but good for me :-)

I even provided a screen shot, but Blogger.com seems to convert anything into JPEGs which just isn't nice for screen shorts (compare to this version, which is even 10 kB smaller).

Next on the list are some more SMP related changes, as some things like call_all_cpus() are not yet working. I expect to finish the remaining SMP work tomorrow, and that's when all the testing can begin on your end. A serial debugging cable (with a second machine) would be very helpful, though, in order to get me useful information about what went wrong. Your effort is surely appreciated!
Older SMP machines could or even should work now, but I would be surprised if the same could be said for current SMP machines - but as I don't have access to such a machine, it's not on me to find out about that now.