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

From: Thomas Shao
Date: Tue Oct 14 2014 - 10:13:11 EST

> -----Original Message-----
> From: Richard Cochran [mailto:richardcochran@xxxxxxxxx]
> Sent: Tuesday, October 14, 2014 9:26 PM
> To: Thomas Shao
> Cc: tglx@xxxxxxxxxxxxx; gregkh@xxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx;
> apw@xxxxxxxxxxxxx; jasowang@xxxxxxxxxx; 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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