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

Glauber Costa
2009-09-08T20:15:15Z

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@redhat.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.