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

[David Vrabel <david.vrabel@citrix.com>]

On 12/10/12 16:02, Konrad Rzeszutek Wilk wrote:
> 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@citrix.com> 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@citrix.com>
>>>>>>
>>>>>> 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).

No,

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

If we wanted hwclock to also update the xen wallclock we would need a
xen-specific rtc driver that updated both rtc and wallclock.

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

ntpd does call adjtimex so if you are running ntpd and it is
synchronized to its clock source you do get the periodic sync of the
wallclock.

David