Radim Krcmar <rkrcmar@xxxxxxxxxx> writes: > 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. Thanks, good suggestion, as far as I see PTP_SYS_OFFSET_PRECISE support was just added to chrony: https://git.tuxfamily.org/chrony/chrony.git/commit/?id=31b6a14444a8f23147077df3c6a64518d082c35e I'll implement getcrosststamp() as well but I'll probably have to wait till K. Y.'s restructuring (https://lkml.org/lkml/2016/12/30/260) lands and do my series on top of that. We'll also be using the TSC page clocksource (when available) to not do the unneeded vmexit. > > 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. It's probably still worth it to have the 'lightweight' gettime64() implementation as going through the getcrosststamp() routine (see get_device_system_crosststamp() which we'll probably use for Hyper-V also -- it's not very simple) every time and throwing away half of the result doesn't look optimal. -- Vitaly _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
