From mboxrd@z Thu Jan 1 00:00:00 1970 From: Feng Tang Subject: (no subject) Date: Tue, 14 May 2013 23:11:07 +0800 Message-ID: <20130514151107.GA6661__36184.5050393187$1368549639$gmane$org@feng-snb> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: David Vrabel , John Stultz Cc: Thomas Gleixner , Konrad Rzeszutek Wilk , "linux-kernel@vger.kernel.org" , "xen-devel@lists.xen.org" List-Id: xen-devel@lists.xenproject.org Bcc: Subject: Re: [PATCH 2/3] timekeeping: sync persistent clock and RTC on system time step changes Reply-To: Hi David, > From: David Vrabel > Date: 2013/5/14 > Subject: Re: [PATCH 2/3] timekeeping: sync persistent clock and RTC on > system time step changes > To: John Stultz > Cc: "xen-devel@lists.xen.org" , Konrad > Rzeszutek Wilk , Thomas Gleixner > , "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 > >> > >> 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