There is a problem with the through-8259A mode for IRQ 0 on I/O APIC
systems.  Depending on correctness of an MP table, IRQ 0 routing is either
not registered at all or registered at a wrong pin.  As a result the 8254
timer IRQ only works by an accident (it's edge-triggered and never
disabled/enabled so it happens to survive this incorrect configuration).
A visible effect is you can't change the affinity for IRQ 0.

 Following is a patch that fixes both cases referred to above.  The code
looks obvious but it was additionally run-time tested just in case.  The
issue is serious -- please apply the patch ASAP.  As no changes were done
to io_apic.c since the development fork, the patch applies cleanly both to
2.4 and to 2.5.

 Credit goes to Joe for discovering the affinity problem and providing a
fix proposal (incorporated in the final one).

  Maciej