linux-kernel - Re: [PATCH v3 0/2] hv_util: adjust system time smoothly

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Date:   Tue, 17 Jan 2017 08:37:28 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     devel@...uxdriverproject.org, Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        John Stultz <john.stultz@...aro.org>,
        Alex Ng <alexng@...rosoft.com>, Olaf Hering <olaf@...fle.de>,
        Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH v3 0/2] hv_util: adjust system time smoothly

On Tue, 17 Jan 2017 16:27:17 +0100
Vitaly Kuznetsov <vkuznets@...hat.com> wrote:

> 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.
> 
> Changes since v2:
> - Implement Hyper-V PTP device instead of doint in-kernel time sync.
> 
> Changes since "[PATCH RFC] hv_utils: implement Hyper-V PTP source":
> - Richard Cochran: implement .adjfreq, .adjtime, .settime64 returning
>   -EOPNOTSUPP.
> - Olaf Hering: change IS_ERR->IS_ERR_OR_NULL as CONFIG_PTP_1588_CLOCK
>   can be disabled.
> - Thomas Gleixner: formatting fixes, comments added.
> 
> Vitaly Kuznetsov (2):
>   hv_util: switch to using timespec64
>   hv_utils: implement Hyper-V PTP source
> 
>  drivers/hv/hv_util.c | 142 +++++++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 116 insertions(+), 26 deletions(-)
> 

It would be good to update Documentation files to describe any configuration needed.