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

From: Vitaly Kuznetsov
Date: Mon Jan 30 2017 - 04:30:59 EST

KY Srinivasan <kys@xxxxxxxxxxxxx> writes:

>> -----Original Message-----
>> From: Vitaly Kuznetsov [mailto:vkuznets@xxxxxxxxxx]
>> Sent: Thursday, January 19, 2017 6:17 AM
>> To: devel@xxxxxxxxxxxxxxxxxxxxxx
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx;
>> Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>; KY Srinivasan
>> <kys@xxxxxxxxxxxxx>; John Stultz <john.stultz@xxxxxxxxxx>; Alex Ng (LIS)
>> <alexng@xxxxxxxxxxxxx>; Stephen Hemminger
>> <stephen@xxxxxxxxxxxxxxxxxx>; Olaf Hering <olaf@xxxxxxxxx>; Richard
>> Cochran <richardcochran@xxxxxxxxx>; Radim Krcmar
>> <rkrcmar@xxxxxxxxxx>
>> Subject: [PATCH v4 2/2] 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 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>
> Vitaly,
> This patch does not apply. Please rebase and send.

Sure, I'm currently testing v5 which includes the .getcrosststamp()
callback support. Will be sending it out shortly.