chrony: use /dev/ptp_hyperv instead of /dev/ptp0

[jlebon]

From the last paragraph in:

https://docs.microsoft.com/en-us/azure/virtual-machines/linux/time-sync#check-for-ptp-clock-source

In Linux VMs with Accelerated Networking enabled, you may see multiple
PTP devices listed because the Mellanox mlx5 driver also creates a
/dev/ptp device. Because the initialization order can be different
each time Linux boots, the PTP device corresponding to the Azure host
might be /dev/ptp0 or it might be /dev/ptp1, which makes it difficult
to configure chronyd with the correct clock source. To solve this
problem, the most recent Linux images have a udev rule that creates
the symlink /dev/ptp_hyperv to whichever /dev/ptp entry corresponds to
the Azure host. Chrony should be configured to use this symlink
instead of /dev/ptp0 or /dev/ptp1.

See: coreos/fedora-coreos-tracker#1031

As mentioned there, we might have to backport udev rules to RHCOS as
well to make sure that symlink gets created there too.

[jlebon]

(Didn't test this; I need to dust off my Azure creds and CLI. If someone has warm access to it, would appreciate a check!)

[cgwalters]

the most recent Linux images have a udev rule that creates
the symlink /dev/ptp_hyperv to whichever /dev/ptp entry corresponds to
the Azure host

For general interest, that udev rule is part of systemd:

[root@cosa-devsh ~]# grep -r hyperv /usr/lib
...
/usr/lib/udev/rules.d/50-udev-default.rules:SUBSYSTEM=="ptp", ATTR{clock_name}=="hyperv", SYMLINK += "ptp_hyperv"
[root@cosa-devsh ~]# rpm -qf /usr/lib/udev/rules.d/50-udev-default.rules 
systemd-udev-249.4-2.fc35.x86_64
[root@cosa-devsh ~]# 

So indeed we could probably hack this by backporting to our config overlay, but we also need to be careful here because "rhcos" == "rhel8" right now but not necessarily in the future...

[dustymabe]

LGTM

[lucab]

The udev rules is already being backported to RHEL8 (8.6 likely), see https://bugzilla.redhat.com/show_bug.cgi?id=1991834.

[dustymabe]

Booted in azure on a Standard_DS1_v2 instance. Here is what I see:

[core@my-fcos-vm ~]$ ls -l /dev/ptp_hyperv 
lrwxrwxrwx. 1 root root 4 Dec  9 21:26 /dev/ptp_hyperv -> ptp0


[core@my-fcos-vm ~]$ chronyc  sources
MS Name/IP address         Stratum Poll Reach LastRx Last sample               
===============================================================================
#* PHC0                          0   3   377    10    -15us[  -24us] +/- 2661ns

Should I test on other instance types? Which ones?