System *boots* with one of:
- maxcpus=0 (equivalent nosmp)
- nolapic (with e1000 warning about missing MSI-X
System does *not* boot with one of:
I thought SMP was generally not compatible with PIC IRQ routing (which noapic enforces?) and this would explain case 2.
As for case 1, maybe I missed some detail with my commit  when switching from LAPIC to TSC timers. Like leaving LAPIC timers running at different rate or generally having the timer counters too much out-of-sync across CPU #0 and #1. You coud try if that one is the commit with regression.