Re: [Xen-devel] Re: [PATCH] [PVOPS] dom0 sync xen wallclock

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2010-02-19 at 09:39 +0000, Keir Fraser wrote:
> On 19/02/2010 09:10, "Ian Campbell" <Ian.Campbell@xxxxxxxxxx> wrote:
> 
> > On Thu, 2010-02-18 at 23:43 +0000, Jeremy Fitzhardinge wrote:
> >>     * PV domUs get a rough time of day at boot either from a simple
> >>       "seconds since 1970" in the startup info put there by the domain
> >>       builder from dom0's clock (so there's no illusion that it is
> >>       anything other than a one-time start time for the domain)
> > 
> > Unfortunately there is an existing large installed base of guests which
> > use the dependent wallclock mode since it is the default in oldstyle Xen
> > kernels.
> 
> That still plays okay with dom0 setting Xen wallclock only once, or not at
> all (since Xen primes its WC values from CMOS).

I think we should try and arrange to do it once so we cope more
gracefully with wildly incorrect CMOS clocks.

>  The wallclock values would
> then never change and old kernels naturally just pick up WC at boot and
> never again. And their default mode is also compatible with them running
> ntpd -- if the kernel detects ntp sync then it changes/reduces how it syncs
> with Xen.

But these guests won't be running ntpd because they expect domain 0 to
be doing it for them, via the dependent wallclock mode.

I don't disagree that we could transition to a better solution but we
are currently not providing the host level behaviour that existing
guests expect.

Ian.



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