port-xen/52664: monotonic clock goes backwards

[coypu%sdf.org@localhost]

>Number:         52664
>Category:       port-xen
>Synopsis:       monotonic clock goes backwards
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-xen-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Oct 28 16:00:00 +0000 2017
>Originator:     coypu
>Release:        NetBSD 8.99.4
>Organization:
>Environment:
NetBSD lio.xen.prgmr.com 8.99.4 NetBSD 8.99.4 (XEN3_DOMU) #10: Sat Oct 28 13:28:12 UTC 2017  lio%lio.xen.prgmr.com@localhost:/home/lio/obj/sys/arch/amd64/compile/XEN3_DOMU amd64   
>Description:
dmesg:
WARNING: negative runtime; monotonic clock has gone backwards

run top:
  PID   LID USERNAME PRI STATE      TIME   WCPU    CPU NAME      COMMAND
11780     1 fly       79 wait/0     0:06  4.93%  4.93% -         pkgdata
    0    56 root     222 key_ti/2    ???  0.00%  0.00% key_timeh [system]
    0    32 root     127 xcall/3     ???  0.00%  0.00% xcall/3   [system]
    0     1 root     125 uvm/1       ???  0.00%  0.00% swapper   [system]
..

this is a domu machine with 4 vcpus.
>How-To-Repeat:

>Fix: