linux-kernel - Re: [PATCH v2 1/2] keep guest wallclock in sync with host clock

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Date:	Tue, 8 Sep 2009 17:15:12 -0300
From:	Glauber Costa <glommer@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com
Subject: Re: [PATCH v2 1/2] keep guest wallclock in sync with host clock

On Tue, Sep 08, 2009 at 05:00:04PM -0300, Marcelo Tosatti wrote:
> On Tue, Sep 08, 2009 at 04:37:52PM -0300, Glauber Costa wrote:
> > On Tue, Sep 08, 2009 at 03:41:59PM -0300, Marcelo Tosatti wrote:
> > > On Wed, Sep 02, 2009 at 10:34:57AM -0400, Glauber Costa wrote:
> > > > KVM clock is great to avoid drifting in guest VMs running ontop of kvm.
> > > > However, the current mechanism will not propagate changes in wallclock value
> > > > upwards. This effectively means that in a large pool of VMs that need accurate timing,
> > > > all of them has to run NTP, instead of just the host doing it.
> > > > 
> > > > Since the host updates information in the shared memory area upon msr writes,
> > > > this patch introduces a worker that writes to that msr, and calls do_settimeofday
> > > > at fixed intervals, with second resolution. A interval of 0 determines that we
> > > > are not interested in this behaviour. A later patch will make this optional at
> > > > runtime
> > > > 
> > > > Signed-off-by: Glauber Costa <glommer@...hat.com>
> > > 
> > > As mentioned before, ntp already does this (and its not that heavy is
> > > it?).
> > > 
> > > For example, if ntp running on the host, it avoids stepping the clock
> > > backwards by slow adjustment, while the periodic frequency adjustment on
> > > the guest bypasses that.
> > 
> > Simple question: How do I run ntp in guests without network?
> 
> You don't.
For those guests, the mechanism I am proposing comes handy.

Furthermore, it is not only optional, but disabled by default. And then even if you
have a network, but a genuine reason not to use ntp in your VMs, you can use it too.

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