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Re: [libvirt] [PATCH] Xen: Fix <clock> handling

Hello Daniel,

On Tuesday 17 January 2012 22:24:11 Daniel P. Berrange wrote:
> On Tue, Jan 17, 2012 at 05:02:14PM +0100, Philipp Hahn wrote:
> > For PV, Xen implements the <clock offset='utc'> and <clock
> > offset='localtime'> behaviour, while for HV it implements <clock
> > offset='variable'>.
> > This difference is important for domUs, which switch daylight saving on
> > there own, since the state is kept inside the VM and must be in sync
> > with the RTC provided by Xen.
> What versions of Xen is that true for ?  I didn't think that old
> versions behaved this way for HVM. The libvirt Xen driver is still
> intended to support the RHEL5 vintage Xen code (which is based on
> 3.0.3)

To me it looks like CentOS5 (since I don't have RHEL5) has the Xen-3.0.3 
user-land XenD (rpm -q xen → xen-3.0.3-132.el5) but the Xen-3.1.2 hypervisor 
(xm info → 3.1.2-.2-274.el5). Relevant for this change seems only the version 
of XenD, which returns xendConfigVersion=2.

This is a version without "managed domain" support, which was added in the 
3.1.x branch <http://xenbits.xen.org/hg/xen-3.1-testing.hg/rev/7e431ea834a8>.
The generation bump to xendConfig_version=3 happend in 3.0.4 for "Live Cycle" 
support <http://xenbits.xen.org/hg/xen-3.0.4-testing.hg/rev/1c51c580dc05>. 
The bump to 4 was in the 3.0.5 → 3.1.0 transition 

So for XenD version _ its _:
- < 3.1: "utc" or "localtime" (because there is no XenD keeping track of the 
modified rtc_timeoffset)
- >= 3.1: managed HV domains: "variable" (XenD keeps track)
- >= 3.1: managed PV domains: "utc" or "localtime" (they don't have a RTC but 
can access a PV-clock, which gets initialized to either "utc" or "localtime". 
As far as I know the offset can't be changed by the domU and is not visible 
in any tool, only the "is_utc/localtime" flag is visible)
- >= 3.1: "xm" domains: same as with "managed domains". The "variable" offset 
is tracked by XenD and survives a "reboot", but as with other transient 
domains, the information is lost after "destroy".

Patch will (hopefully) follow soon.

Philipp Hahn           Open Source Software Engineer      hahn univention de
Univention GmbH        Linux for Your Business        fon: +49 421 22 232- 0
Mary-Somerville-Str.1  D-28359 Bremen                 fax: +49 421 22 232-99

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