linux-kernel - Re: [Xen-devel] [PATCH 2/3] timekeeping: sync persistent clock and RTC on system time step changes

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Date:	Wed, 15 May 2013 09:16:05 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"David Vrabel" <david.vrabel@...rix.com>,
	"John Stultz" <john.stultz@...aro.org>
Cc:	"Thomas Gleixner" <tglx@...utronix.de>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 2/3] timekeeping: sync persistent clock
 and RTC on system time step changes

>>> On 14.05.13 at 19:15, John Stultz <john.stultz@...aro.org> 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.

So the hypervisor really doesn't care about the actual RTC getting
updated in hardware, all it needs is for Dom0 to notify it of wall
clock corrections.

Jan

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