ntpd looses sync on domU

[Emmanuel Dreyfus <manu%netbsd.org@localhost>]

Hello

The setup is Xen 4.1.8 (xenkernel418-20240909nb1 from pkgsrc), with 
NetBSD-10.0/amd64 dom0 and domU.

From time to time, I experience time keeping trouble on a small set
of domUs. The clock drifts a lot, loosing hours in a few hours. ntpd
is unable to cope, and the only remeidiation is to reboot the domU. 
Oddly, the dom0 and other domU running on the same dom0 have no clock
trouble.

I recently discovered that switching kern.timecounter.hardware from 
xen_system_time to clockinterrupt helped a lot. The drift remain
but is much smaller, and ntpd is able to keep an almost correct time.
It still reports being unsync, thought.

It seems also running ntpd with -G lets it sync after a while. But 
even when pll nano kernel status is reached, the numbers reported 
by ntpq -c peers show there is something wrong. 

- On the time-sick domU
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*ntp1.example.ne 145.238.80.80    2 u   29   64  377    7.812  -18.042 369.214
+ntp2.example.ne 192.0.2.20       3 u   32   64  377    7.812  +962.94 660.459

- On the dom0:
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*ntp1.example.ne  145.238.80.80   2 u 1018 1024  377    0.430   +7.106   6.703
+ntp2.example.ne  192.0.2.20      3 u  218 1024  377    1.528   +6.619   6.455

- On another domU running on the same dom0:
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*ntp1.example.ne 145.238.80.80    2 u  247  256  377    0.405   +3.995   0.931
+ntp1.example.ne 192.0.2.20       3 u  213  256  377    1.607   +2.156   1.112


Anyone already experienced that? 






-- 
Emmanuel Dreyfus
manu%netbsd.org@localhost