Implement virtual PTP hardware clock (PHC) for use in virtual machines

[g-v-egidy]

I'm running many virtual machines with qemu/kvm. In the past I had issues with jitter when using NTP on the VMs and there was high load on the host. Qemu/kvm provides a virtual PTP hardware clock (PHC) feature that allows VMs to get a precise time from the host.

I now manage the time on the host with chrony and then use PHC to forward the time info to chrony running on my virtual machines. This works much better than NTP on the VMs in my experience.

This is what I have in my /etc/chrony.conf:

[...]
refclock PHC /dev/ptp0 poll 4 stratum 3 prefer

I know that ntpd-rs currently doesn't support all features of chrony yet. But this feature would be important for me before I could migrate my virtual machines to ntpd-rs.

[davidv1992]

Thank you for the feature request, we will consider implementing this once we have started general reference clock support. One question, are these hardware clocks expected to carry TAI or UTC?

[Artoria2e5]

They are expected to carry the system time of the host, which typically is UTC: if the host is synced with an external time source at all, it probably does so via NTP, which runs UTC. In any case, you don't want to do any extra operations on the time provided, so UTC it is.