port-xen/59437: Xen time stability

[campbell+netbsd%mumble.net@localhost]

>Number:         59437
>Category:       port-xen
>Synopsis:       Xen time stability
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-xen-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu May 22 01:30:00 +0000 2025
>Originator:     Taylor R Campbell
>Release:        current
>Organization:
XenBSD Time's Up LLC
>Environment:
>Description:
NetBSD has trouble with timekeeping on Xen:

https://mail-index.netbsd.org/port-xen/2024/02/01/msg010525.html
https://mail-index.netbsd.org/port-xen/2024/06/20/msg010573.html
https://mail-index.netbsd.org/port-xen/2024/06/23/msg010599.html
https://mail-index.netbsd.org/tech-kern/2025/04/03/msg030310.html

Symptoms:

- `ntpq -np' shows offsets in the thousands (units of milliseconds, so offsets of multiple seconds)
- gettimeofday(3) returns the same result hundreds of times consecutively
- `vmstat -e | grep -w -e tsc -e systime -e hardclock -e timecounter' turns up various nonzero counts

Someone should draw up the matrix of what configurations and versions are affected like I requested at https://mail-index.netbsd.org/port-xen/2024/06/23/msg010599.html but it hasn't been drawn up yet as far as I know.
>How-To-Repeat:
unclear, but heavier load and more vCPUs may raise the probability
>Fix:
Yes, please!