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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

on fri, 19 feb 2010, 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). 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.
> 

Even if we decide that dom0 should sync the wallclock only once with
xen, ideally right after ntp sync's, we still need to hook into
clock_was_set and therefore do something very similar to what my patch
does (apart from the timer to keep updating the xen wallclock) because
otherwise if we just use the pvop set_wallclock it might take several
minutes before having a single wallclock update.
The reason for this is that the wallclock update policy of the kernel is
very lax and I don't think it is suitable for our use case; in
particular the classic initial time sync done with ntpdate at boot time
by most distros doesn't propagate the wallclock to the hardware and\or
xen.


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