Re: [PATCH v4 2/2] hv_utils: implement Hyper-V PTP source

[Radim Krcmar <rkrcmar@redhat.com>]

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@redhat.com>
> ---

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.