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Date: Tue, 17 Jan 2017 08:37:28 -0800
From: Stephen Hemminger <stephen@networkplumber.org>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: devel@linuxdriverproject.org, Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, Haiyang Zhang <haiyangz@microsoft.com>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        John Stultz <john.stultz@linaro.org>, Alex Ng <alexng@microsoft.com>,
        Olaf Hering <olaf@aepfle.de>,
        Richard Cochran <richardcochran@gmail.com>
Subject: Re: [PATCH v3 0/2] hv_util: adjust system time smoothly
Message-ID: <20170117083728.7b84feaa@xeon-e3>
In-Reply-To: <20170117152719.23618-1-vkuznets@redhat.com>
References: <20170117152719.23618-1-vkuznets@redhat.com>
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On Tue, 17 Jan 2017 16:27:17 +0100
Vitaly Kuznetsov <vkuznets@redhat.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.