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

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Date:	Tue, 14 Oct 2014 14:12:02 +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 9:26 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:
> > > I really don't see the need for this. We have NTP. If the guests
> > > want to, they may use it. Otherwise, they have a free running clock, just
> like real machines.
> > >
> > Sometimes the user can't setup NTP. For example the guest OS didn't
> > have network connection. And in some cases, they may want the guest
> time sync with host.
> > With the existing hyper-v time source, the system clock will has
> > around 1.5 second time drift per day. If the workload in the host is heavy,
> the number could be larger.
> > So this feature is really useful for some scenarios.
> 
> Any real machine without networking (and without GPS etc) will drift. That is
> just life, tough as it is. Why should we treat these guests any differently than
> real machines?
> 
> Furthermore, without networking you really don't have a compelling need
> for correct absolute time in the first place.

The host machine can be configure with NTP. And in this case, making guest time sync
with host is useful. 

> 
> Thanks,
> Richard
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