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[Feng Tang]

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Subject: Re: [PATCH 2/3] timekeeping: sync persistent clock and RTC on system time step changes
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Hi David,

> From: David Vrabel <david.vrabel@citrix.com>
> Date: 2013/5/14
> Subject: Re: [PATCH 2/3] timekeeping: sync persistent clock and RTC on
> system time step changes
> To: John Stultz <john.stultz@linaro.org>
> Cc: "xen-devel@lists.xen.org" <xen-devel@lists.xen.org>, Konrad
> Rzeszutek Wilk <konrad.wilk@oracle.com>, Thomas Gleixner
> <tglx@linutronix.de>, "linux-kernel@vger.kernel.org"
> <linux-kernel@vger.kernel.org>
> 
> 
> On 14/05/13 01:40, John Stultz wrote:
> > On 05/13/2013 10:56 AM, David Vrabel wrote:
> >> From: David Vrabel <david.vrabel@citrix.com>
> >>
> >> The persistent clock or the RTC is only synchronized with system time
> >> every 11 minutes if NTP is running.  This gives a window where the
> >> persistent clock may be incorrect after a step change in the time
> >> (such as on first boot).
> >>
> >> This particularly affects Xen guests as until an update to the control
> >> domain's persistent clock, new guests will start with the incorrect
> >> system time.
> >>
> >> When there is a step change in the system time, call
> >> update_persistent_clock or rtc_set_ntp_time() to synchronize the
> >> persistent clock or RTC to the new system time.
> >
> > I'm sorry, this isn't quite making sense to me. Could you further
> > describe the exact problematic behavior you're seeing here, and why its
> > a problem?
> 
> The Xen wallclock is used as the persistent clock for Xen guests.  This
> is initialized (by Xen) with the CMOS RTC at the start of day.  If the
> RTC is incorrect then guests will see an incorrect wallclock time until
> dom0 has corrected it.
> 
> Currently dom0 only updates the Xen wallclock with the 11 min periodic
> work when NTP is synced.  This leaves a window where newly started
> guests will see an incorrect wallclock time.  This can cause guests to
> fail to start correctly if the wallclock is now behind what it was when
> the guest last started. (e.g., fsck of its disk fails as its last mount
> time appears to be far into the future).

Is it possible for Xen to resync its wallclock time from the RTC device
whenever it gets a request of starting a new guest? as usually a new
guest OS needs to get the time base from Xen's wallclock anyway.

Thanks,
Feng