Comment 0 for bug 1676635

Please include the following upstream commit into lts-xenial, 16.04 HWE, Yakkity, and Zesty. This will improve the behavior of timesync on Hyper-V hosts while simultaneously using network time sync protocols like NTP.

commit 3716a49a81ba19dda7202633a68b28564ba95eb5
Author: Vitaly Kuznetsov <email address hidden>
Date: Sat Feb 4 09:57:14 2017 -0700

    hv_utils: implement Hyper-V PTP source

    With TimeSync version 4 protocol support we started updating system time
    continuously through the whole lifetime of Hyper-V guests. Every 5 seconds
    there is a time sample from the host which triggers do_settimeofday[64]().
    While the time from the host is very accurate such adjustments may cause
    issues:
    - Time is jumping forward and backward, some applications may misbehave.
    - In case an NTP server runs in parallel and uses something else for time
      sync (network, PTP,...) system time will never converge.
    - Systemd starts annoying you by printing "Time has been changed" every 5
      seconds to the system log.

    Instead of doing in-kernel time adjustments offload the work to an
    NTP client by exposing TimeSync messages as a PTP device. Users may now
    decide what they want to use as a source.

    I tested the solution with chrony, the config was:

     refclock PHC /dev/ptp0 poll 3 dpoll -2 offset 0

    The result I'm seeing is accurate enough, the time delta between the guest
    and the host is almost always within [-10us, +10us], the in-kernel solution
    was giving us comparable results.

    I also tried implementing PPS device instead of PTP by using not currently
    used Hyper-V synthetic timers (we use only one of four for clockevent) but
    with PPS source only chrony wasn't able to give me the required accuracy,
    the delta often more that 100us.

    Signed-off-by: Vitaly Kuznetsov <email address hidden>
    Signed-off-by: K. Y. Srinivasan <email address hidden>
    Signed-off-by: Greg Kroah-Hartman <email address hidden>