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Date: Mon, 9 Jan 2017 09:19:50 -0800
From: Stephen Hemminger <stephen@networkplumber.org>
To: "Alex Ng (LIS)" <alexng@microsoft.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        John Stultz <john.stultz@linaro.org>,
        "devel@linuxdriverproject.org" <devel@linuxdriverproject.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 3/4] hv_util: use do_adjtimex() to update system time
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On Tue, 3 Jan 2017 19:48:29 +0000
"Alex Ng (LIS)" <alexng@microsoft.com> wrote:

> > -----Original Message-----
> > From: Vitaly Kuznetsov [mailto:vkuznets@redhat.com]
> > Sent: Tuesday, January 3, 2017 4:32 AM
> > To: Alex Ng (LIS) <alexng@microsoft.com>
> > Cc: devel@linuxdriverproject.org; linux-kernel@vger.kernel.org; KY
> > Srinivasan <kys@microsoft.com>; Haiyang Zhang <haiyangz@microsoft.com>;
> > John Stultz <john.stultz@linaro.org>; Thomas Gleixner <tglx@linutronix.de>
> > Subject: Re: [PATCH 3/4] hv_util: use do_adjtimex() to update system time
> > 
> > "Alex Ng (LIS)" <alexng@microsoft.com> writes:
> >   
> > >> -----Original Message-----
> > >> From: Vitaly Kuznetsov [mailto:vkuznets@redhat.com]
> > >> Sent: Monday, January 2, 2017 11:41 AM
> > >> To: devel@linuxdriverproject.org
> > >> Cc: linux-kernel@vger.kernel.org; KY Srinivasan <kys@microsoft.com>;
> > >> Haiyang Zhang <haiyangz@microsoft.com>; John Stultz
> > >> <john.stultz@linaro.org>; Thomas Gleixner <tglx@linutronix.de>; Alex
> > >> Ng
> > >> (LIS) <alexng@microsoft.com>
> > >> Subject: [PATCH 3/4] hv_util: use do_adjtimex() to update system time
> > >>
> > >> 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 client is run in parallel things may go south, e.g. when
> > >>   an NTP client tries to adjust tick/frequency with  
> > ADJ_TICK/ADJ_FREQUENCY  
> > >>   the Hyper-V module will not see this changes and time will oscillate and
> > >>   never converge.
> > >> - Systemd starts annoying you by printing "Time has been changed" every  
> > 5  
> > >>   seconds to the system log.  
> > >
> > > These are all good points. I am working on a patch to address point 2.
> > > It will allow new TimeSync behavior to be disabled even if the
> > > TimeSync IC is enabled from the host. This can be set to prevent
> > > TimeSync IC from interfering with NTP client.
> > >  
> > 
> > Good, this can happen in parallel to my series, right?  
> 
> Yes, that is correct.
> 
> >   
> > >>
> > >> Instead of calling do_settimeofday64() we can pretend being an NTP
> > >> client and use do_adjtimex().
> > >>
> > >> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>

An alternative would be for hyper-v util to provide a clocksource device and
let NTP manage the adjustment. The advantage of this would be HV util not fighting
with NTP, and using standard API's. The downside would be the complexity of configuring
NTP, and difficulty of writing a clock source pseudo device.