Re: [Xen-devel] [PATCH 3/3] xen/privcmd: add IOCTL_PRIVCMD_SYNC_WALLCLOCK to sync Xen's wallclock

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Oct 12, 2012 at 03:59:25PM +0100, David Vrabel wrote:
> On 12/10/12 15:29, Konrad Rzeszutek Wilk wrote:
> > On Fri, Oct 12, 2012 at 10:02 AM, David Vrabel <david.vrabel@xxxxxxxxxx> 
> > wrote:
> >> On 12/10/12 14:41, Konrad Rzeszutek Wilk wrote:
> >>> On Fri, Oct 12, 2012 at 01:57:14PM +0100, David Vrabel wrote:
> >>>> From: David Vrabel <david.vrabel@xxxxxxxxxx>
> >>>>
> >>>> Add a new ioctl to synchronize Xen's wallclock with the current system
> >>>> time.
> >>>>
> >>>> This may be used by the tools to ensure that newly created domains see
> >>>> the correct wallclock time if NTP is not used in dom0 or if domains
> >>>> are started before NTP has synchronized.
> >>>
> >>> So... how does this work with NTPD? As in does ntpd _not_ update the
> >>> hwclock enough?
> >>
> >> Once NTPD is synchronized then the kernel updates the wallclock (and the
> >> RTC with patch #1) every 11 mins.  I assume this is often enough given
> >> how NTP adjusts the system time.
> >>
> >> You only really need the tools to sync wallclock if system time was
> >> stepped at start of day.  e.g., init scripts could do something like:
> >>
> >> ntpdate pool.ntp.org
> >> hwclock --systohc
> >> xen-wallclock --systowc
> > 
> > I think I am missing something. The hwclock should end up in the
> > xen_set_wallclock call. And from there on, the ntpd would update the
> > wallclock if it got skewed enough? Or is the system time not calling
> > the wall-clock enough? If that is the case, would just adding this in
> > the crontab be enough:
> 
> hwclock talks to /dev/rtc which writes to the CMOS directly and does not
> call update_persistent_clock() (or xen_set_wallclock()).

/me scratches his head.

I recall that the update_persistent_clock() was being called.. with
hwclock -w (which is the same as --systohc).

That seems like a bug in the generic code - I would think that
hwclock would update the wallclock, not just the RTC.

Oh, it is whoever calls 'adjtimex' syscall ends up calling in
update_persistent_clock(). So .. ntpdate or ntpd don't call that?

Somebody does.. I hope?

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