kern/47153: Plugging in USB keyboard at "root device:" prompt crashes 6.0

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

>Number:         47153
>Category:       kern
>Synopsis:       Plugging in USB keyboard at "root device:" prompt crashes 6.0
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Nov 02 15:40:01 +0000 2012
>Originator:     Andreas Gustafsson
>Release:        NetBSD 6.0
>Organization:
>Environment:
System: NetBSD guava.gson.org 6.0
Architecture: amd64
Machine: amd64
>Description:

I successfully installed NetBSD 6.0/amd64 on a SATA disk in one
machine, then moved the disk to a second machine and attempted
to boot it there.

This resulted in the kernel not finding the root file system and
displaying a "root device:" prompt.  I guess that's to be expected
given the disk transplant; so far so good.

I then plugged in a USB keyboard to enter the root device name.
When I did, the kernel printed some usb attach messages, followed
by

  This port is broken, it does not call cnpollc() before calling cngetc().
  This should be fixed, but it will work anyway (for now).
  uvm_fault(0xffffffff80e11b20, 0x0, 1) -> e
  fatal page fault in supervisor mode
  [...some lines omitted..]
  Stopped in pid 0.59 (system) at netbsd:usb_transfer_complete+0x124: movq 
78(%rax),%rax
  db{0}>

Just in case it's relevant, I should mention that when I previously
attempted to boot 6.0_RC1 on the same (second) machine, I ran into
kern/46696.

The kernel messages in kern/46285 also show evidence of a crash
triggered by plugging in a USB keyboard at the root device prompt, but
since the subject of that PR is the disk error triggering the "root
device" prompt and not the crash following it, I'm considering it a
separate problem and filing a separate PR.

>How-To-Repeat:

Boot an amd64 machine in such a way as to trigger the "root device:"
prompt, and then plug in an USB keyboard.

>Fix:

Please.