2017-01-19 15:16+0100, Vitaly Kuznetsov: > 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 precision 1e-9 > > 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 <vkuznets@xxxxxxxxxx> > --- It is a nice coincidence that KVM is working on a PTP driver as well, https://lkml.org/lkml/2017/1/20/247, and it uses more precise/accurate method of converting the host time that Hyper-V could also use. Hyper-V provides {host_time, ref_time} tuple, but gettime64() requires that you return just host_time and a new "ref_time" is then computed to be in the middle of two guest_time reads. I recommend you use getcrosststamp PTP callback, which allows you to provide the tuple. Userspace can then use PTP_SYS_OFFSET_PRECISE ioctl. KVM patches also proposes to change PTP_SYS_OFFSET, so when gettime64 callback is not implemented, the ioctl uses getcrosststamp instead, which would avoid code duplication and improve precision/accuracy. _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
