kern/51743: Kernel with DEBUG fails to boot up on KVM since introducing uvm_hotplug(9)

[ozaki-r%netbsd.org@localhost]

>Number:         51743
>Category:       kern
>Synopsis:       Kernel with DEBUG fails to boot up on KVM since introducing uvm_hotplug(9)
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Dec 26 05:30:00 +0000 2016
>Originator:     Ryota Ozaki
>Release:        -current
>Organization:
>Environment:
NetBSD kvm 7.99.52 NetBSD 7.99.52 (KVM) #527: Mon Dec 26 13:58:06 JST 2016  ozaki-r@rangeley:(hidden) amd64
qemu-kvm-2.8.0-1.fc25.x86_64
>Description:
A kernel with DEBUG fails to boot up on a KVM since the commit
"Introduce uvm_hotplug(9) to the kernel.".

Before introducing the feature, a kernel boots up with the following message:

  >> NetBSD/x86 PXE boot, Revision 5.1 (from NetBSD 6.0)
  >> Memory: 625/1046288 k
  Press return to boot now, any other key for boot menu
  booting netbsd - starting in 0 seconds.     
  PXE BIOS Version 2.1
  Using PCI device at bus 0 device 6 function 0
  Ethernet address 52:54:00:bb:18:46
  6091699+3349216+984352 [646536+430846]=0xc299b0
  kernel text is mapped with 4 large pages and 0 normal pages
  ksyms: checking .text
  ksyms: checking .rodata
  ksyms: checking .eh_frame
  ...

After introducing the feature, the kernel seems to hang before outputting
"kernel text is mapped ...". boot -x doesn't show nothing. Removing
virtio balloon doesn't help as well.

If disable DEBUG. the kernel boots up successfully.

Note that a kernel with or without DEBUG boots up on a physical machine
(and ESXi as saying by hikaru@) without any problems.
>How-To-Repeat:
Build a kernel with DEBUG, and boot it on a KVM.
>Fix:
n/a