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    SubjectRe: [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@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.



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