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For a clock element as above, libvirt simply converts current system time with localtime_r(), then starts qemu with a time string that doesn't contain any timezone information. So, from qemu's point of view, the -rtc string it gets for: <clock offset='variable' basis='utc' adjustment='10800'/> is identical to the -rtc string it gets for: <clock offset='variable' basis='localtime' adjustment='0'/> (assuming the host is in a timezone that is 10800 seconds ahead of UTC, as is the case on the machine where this message is being written). Since the commandlines are identical, qemu will behave identically after this point in either case. There are two problems in the case of basis='localtime' though: Problem 1) If the guest modifies its RTC, for example to add 20 seconds, the RTC_CHANGE event from qemu will then contain offset:20 in both cases. But libvirt will have saved the original adjustment into adjustment0, and will add that value onto the offset in the event. This means that in the case of basis=;utc', it will properly emit an event with offset:10820, but in the case of basis='localtime' the event will contain offset:20, which is *not* the new offset of the RTC from UTC (as the event it documented to provide). Problem 2) If the guest is migrated to another host that is in a different timezone, or if it is migrated or saved/restored after the DST status has changed from what it was when the guest was originally started, the newly restarted guest will have a different RTC (since it will be based on the new localtime, which could have shifted by several hours). The solution to both of these problems is simple - rather than maintaining the original adjustment value along with "basis='localtime'" in the domain status, when the domain is started we convert the adjustment offset to one relative to UTC, and set the status to "basis='utc'". Thus, whatever the RTC offset was from UTC when it was initially started, that offset will be maintained when migrating across timezones and DST settings, and the RTC_CHANGE events will automatically contain the proper offset (which should by definition always be relative to UTC). This fixes a problem that was implied but not openly stated in: https://bugzilla.redhat.com/show_bug.cgi?id=964177
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>