From: Thomas Shao <huishao@microsoft.com>
To: Richard Cochran <richardcochran@gmail.com>
CC: "tglx@linutronix.de" <tglx@linutronix.de>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "devel@linuxdriverproject.org" <devel@linuxdriverproject.org>,
        "olaf@aepfle.de" <olaf@aepfle.de>,
        "apw@canonical.com" <apw@canonical.com>,
        "jasowang@redhat.com" <jasowang@redhat.com>,
        KY Srinivasan <kys@microsoft.com>
Subject: RE: [PATCH 2/2] hyperv: Implement Time Synchronization using host
 time sample
Thread-Topic: [PATCH 2/2] hyperv: Implement Time Synchronization using host
 time sample
Thread-Index: AQHP55ZRTp/VO8CmfEuTJogFjyVUzJwve/cAgAAPyNCAAAgTgIAAC/vA
Date: Tue, 14 Oct 2014 14:08:06 +0000
Message-ID: <CD2CABCB2C0A0D4682C5F8AD840141540BFEEF@HKXPRD3002MB006.064d.mgd.msft.net>
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> -----Original Message-----
> From: Richard Cochran [mailto:richardcochran@gmail.com]
> Sent: Tuesday, October 14, 2014 9:20 PM
> To: Thomas Shao
> Cc: tglx@linutronix.de; gregkh@linuxfoundation.org; linux-
> kernel@vger.kernel.org; devel@linuxdriverproject.org; olaf@aepfle.de;
> apw@canonical.com; jasowang@redhat.com; KY Srinivasan
> Subject: Re: [PATCH 2/2] hyperv: Implement Time Synchronization using host
> time sample
> 
> On Tue, Oct 14, 2014 at 01:04:35PM +0000, Thomas Shao wrote:
> > > > +		/*
> > > > +		* Use the Hyper-V time sample to adjust the guest time. The
> > > > +		* algorithm is: If the sample offsets exceeds 1 second, we
> > > > +		* directly set the clock to the server time. If the offset is
> > >
> > > So the guests will experience random time jumps in the kernel,
> > > without any rhyme or reason?
> >
> > This behavior is designed for some extreme cases. Like manually
> > setting guest time to some value. Or the host resumes from a hibernate
> > state. Normally, we should not run into this.
> 
> But when it *does* happen, the guest software will have no way of knowing
> what happened. That stinks.
> 
> Taking your example, when the guest sets its time, the time will suddenly
> jump somewhere else, and for no apparent reason.
> 
> From the guest's point of view, this is really not acceptable.
> 

If the user chooses  to sync guest time with host, that's the expected behavior, right?

> Thanks,
> Richard
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