port-amd64/53789: amd64-install.img boot hangs on HP Compaq 8200 Elite

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         53789
>Category:       port-amd64
>Synopsis:       amd64-install.img boot hangs on HP Compaq 8200 Elite
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    port-amd64-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Dec 15 16:00:00 +0000 2018
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date 2018.12.13.13.11.28
>Organization:

>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

Booting NetBSD-8.99.27-amd64-install.img from a USB stick on a
HP Compaq 8200 Elite Small Form Factor desktop, the kernel hangs
after printing the line

  attimer1: attached to pcppi1

I am unable to break into DDB, even with a PS/2 keyboard.

An i386 install image from a few days ago boots successfully
on the same hardware.

The same image on the same USB stick boots successfully on a HP DL360
G7 (modulo PR 51503).  There, the next device attached after attimer1
is pckbd0, so perhaps that's where the hang happens on the 8200.

Please no suggestions for work-arounds; this is a bug report, not a
support request.

>How-To-Repeat:

>Fix: