Bcc: Subject: Re: [PATCH 2/3] timekeeping: sync persistent clock and RTC on system time step changes Reply-To:
Hi David, > From: David Vrabel <david.vra...@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.stu...@linaro.org> > Cc: "xen-de...@lists.xen.org" <xen-de...@lists.xen.org>, Konrad > Rzeszutek Wilk <konrad.w...@oracle.com>, Thomas Gleixner > <t...@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.vra...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
