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

[Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>]

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:

*/11 * * * * hwclock --systohc

?

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