Re: timekeeping regression?

[Brian Buhrow <buhrow%nfbcal.org@localhost>]

	Hello.  In following this thread, I have some questions which may or may not be helpful in
pinpointing the problem.  If I remember the details of the thread correctly, Greg is running
NetBSD as a dom0 and as domu's on the same machine.  Domu's which are running FreeBSD in HVM
mode on the same machine are not exhibiting the time problem, even though both the dom0 and
domu's running NetBSD are exhibiting the behavior.  The domu's exhibiting the behavior are all
running in pv mode.

1.  In the dom0, it looks like you can tune the frequency at which NetBSD updates Xen's notion
of the time through a sysctl variable.  The variable represents the number of clock ticks on
the cpu dom0 is using for its time counter, at least, that's what it looks like to me.  In any
case, what frequency is your dom0 using for updating Xen's notion of the time?  Do you get
better, worse or no change in behavior if you change that frequency?
Is it using the same frequency as FreeBSD uses?

2.  Do you get the same trouble on domu's running in pvh mode?

3.  Do domu's with only 1 vcpu exhibit the bad behavior?  There are a lot of notes about making
sure the cpu doesn't change in the middle of time fetching operations in the xen_clock.c file.

4.  What version of xen_clock.c are you running?  It looks like some fixes were incorporated
into V1.18  to try and address this issue.

	As a data point, I'm running NetBSD-9.9977, from January 2021, as both  pv and pvh domu's
with FreeBSD-13 as the dom0, with Xen-4.16.0 and I'm not seeing these time issues on the domu's
in question.  And, that's with xen_clock.c, V1.8.  I'm also running some old NetBSD-5.2 domu's
in pv mode and they're notseeing these time issues either.


-thanks
-Brian