linux-kernel - RE: [PATCH 2/2] hyperv: Implement Time Synchronization using host time sample

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Date:	Tue, 14 Oct 2014 14:08:06 +0000
From:	Thomas Shao <huishao@...rosoft.com>
To:	Richard Cochran <richardcochran@...il.com>
CC:	"tglx@...utronix.de" <tglx@...utronix.de>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>,
	KY Srinivasan <kys@...rosoft.com>
Subject: RE: [PATCH 2/2] hyperv: Implement Time Synchronization using host
 time sample


> -----Original Message-----
> From: Richard Cochran [mailto:richardcochran@...il.com]
> Sent: Tuesday, October 14, 2014 9:20 PM
> To: Thomas Shao
> Cc: tglx@...utronix.de; gregkh@...uxfoundation.org; linux-
> kernel@...r.kernel.org; devel@...uxdriverproject.org; olaf@...fle.de;
> apw@...onical.com; jasowang@...hat.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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