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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 library code README
===========================
The directory provides the bulk of the libvirt codebase. Everything
except for the libvirtd daemon and client tools. The build uses a
large number of libtool convenience libraries - one for each child
directory, and then links them together for the final libvirt.so,
although some bits get linked directly to libvirtd daemon instead.
The files directly in this directory are supporting the public API
entry points & data structures.
There are two core shared modules to be aware of:
* util/ - a collection of shared APIs that can be used by any
code. This directory is always in the include path
for all things built
* conf/ - APIs for parsing / manipulating all the official XML
files used by the public API. This directory is only
in the include path for driver implementation modules
* vmx/ - VMware VMX config handling (used by esx/ and vmware/)
Then there are the hypervisor implementations:
* bhyve - bhyve - The BSD Hypervisor
* esx/ - VMware ESX and GSX support using vSphere API over SOAP
* hyperv/ - Microsoft Hyper-V support using WinRM
* lxc/ - Linux Native Containers
* openvz/ - OpenVZ containers using cli tools
* phyp/ - IBM Power Hypervisor using CLI tools over SSH
* qemu/ - QEMU / KVM using qemu CLI/monitor
* remote/ - Generic libvirt native RPC client
* test/ - A "mock" driver for testing
* uml/ - User Mode Linux
* vbox/ - Virtual Box using native API
* vmware/ - VMware Workstation and Player using the vmrun tool
* xen/ - Xen using hypercalls, XenD SEXPR & XenStore
* xenapi/ - Xen using libxenserver
Finally some secondary drivers that are shared for several HVs.
Currently these are used by LXC, OpenVZ, QEMU, UML and Xen drivers.
The ESX, Hyper-V, Power Hypervisor, Remote, Test & VirtualBox drivers all
implement the secondary drivers directly
* cpu/ - CPU feature management
* interface/ - Host network interface management
* network/ - Virtual NAT networking
* nwfilter/ - Network traffic filtering rules
* node_device/ - Host device enumeration
* secret/ - Secret management
* security/ - Mandatory access control drivers
* storage/ - Storage management drivers
Since both the hypervisor and secondary drivers can be built as
dlopen()able modules, it is *FORBIDDEN* to have build dependencies
between these directories. Drivers are only allowed to depend on
the public API, and the internal APIs in the util/ and conf/
directories