Re: [Xen-devel] [PATCH 2/3] timekeeping: sync persistent clock andRTC on system time step changes
From: John Stultz
Date: Wed May 15 2013 - 14:10:49 EST
On 05/15/2013 01:16 AM, Jan Beulich wrote:
On 14.05.13 at 19:15, John Stultz <john.stultz@xxxxxxxxxx> wrote:
On 05/14/2013 02:47 AM, David Vrabel wrote:
On 14/05/13 01:40, John Stultz wrote:
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.
Start of the day? I assume you mean on dom0 bootup? Or is it done
pre-dom0 bootup by Xen itself?
It is, indeed - Xen reads the CMOS clock (or consults EFI) once
when it starts up, but leaves those alone as soon as it launched
Dom0.
If the
RTC is incorrect then guests will see an incorrect wallclock time until
dom0 has corrected it.
Sorry, just a bit more clarifying context here: So there is a 1:1
relationship between xen_wall_clock and the RTC for all domN guests? And
even if dom0 has set its system time properly, domN guests will
initialize (in effect) from the hardware RTC and not from dom0's system
time?
No, (PV) DomU-s will get their time from the software clock that
Xen maintains, which Dom0 is helping keep in good shape when
NTP-synced.
Ok, so really, as soon as the Dom0 time is set by NTP, all guests will
see the right time? That makes more sense, and means the window for
these sorts of issues is reasonably quite small.
David: So I'm less inclined to merge this individual change, but if you
still feel strongly about it, let me know and we can circle around on it
after you've addressed the specific issues I pointed out earlier.
thanks
-john
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