port-macppc/55462: 9.0 MP kernel does not boot on dual-cpu PowerMac G4

[ky0ko%disroot.org@localhost]

>Number:         55462
>Category:       port-macppc
>Synopsis:       9.0 MP kernel does not boot on dual-cpu PowerMac G4
>Confidential:   no
>Severity:       critical
>Priority:       medium
>Responsible:    port-macppc-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Jul 05 22:20:00 +0000 2020
>Originator:     ky0ko
>Release:        NetBSD 9.0
>Organization:
>Environment:
NetBSD ringoen 9.0 NetBSD 9.0 (GENERIC) #0: Fri Feb 14 00:06:28 UTC 2020  mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/macppc/compile/GENERIC macppc

>Description:
after upgrading to 9.0, i found my machine no longer able to boot correctly. it hangs during the kernel initialization, after the display and usb devices and such have been initialized (but immediately before software watchdog has been initialized - disabling swwdog in userconf does not change this). the kernel is still responsive to removing and re-adding usb devices, and to pressing the power button, but never gets to the point of mounting the root filesystem, even if left for hours. booting the non-MP kernel works around this issue, but leaves me with only one of my two processors usable, which is non-ideal.

the computer in question is a PowerMac G4 (FW800) 1.25DP. Under NetBSD 8.1 and 8.2 i am able to boot the machine with the MP kernel without any issue. 

>How-To-Repeat:
Boot an MP-capable PowerMac with the NetBSD 9.0 MP kernel. (unknown if specific to this model, or if others will show similar issues)
>Fix: