dbus-vmstate.rst (2127B)
1============= 2D-Bus VMState 3============= 4 5Introduction 6============ 7 8The QEMU dbus-vmstate object's aim is to migrate helpers' data running 9on a QEMU D-Bus bus. (refer to the :doc:`dbus` document for 10some recommendations on D-Bus usage) 11 12Upon migration, QEMU will go through the queue of 13``org.qemu.VMState1`` D-Bus name owners and query their ``Id``. It 14must be unique among the helpers. 15 16It will then save arbitrary data of each Id to be transferred in the 17migration stream and restored/loaded at the corresponding destination 18helper. 19 20For now, the data amount to be transferred is arbitrarily limited to 211Mb. The state must be saved quickly (a fraction of a second). (D-Bus 22imposes a time limit on reply anyway, and migration would fail if data 23isn't given quickly enough.) 24 25dbus-vmstate object can be configured with the expected list of 26helpers by setting its ``id-list`` property, with a comma-separated 27``Id`` list. 28 29Interface 30========= 31 32On object path ``/org/qemu/VMState1``, the following 33``org.qemu.VMState1`` interface should be implemented: 34 35.. code:: xml 36 37 <interface name="org.qemu.VMState1"> 38 <property name="Id" type="s" access="read"/> 39 <method name="Load"> 40 <arg type="ay" name="data" direction="in"/> 41 </method> 42 <method name="Save"> 43 <arg type="ay" name="data" direction="out"/> 44 </method> 45 </interface> 46 47"Id" property 48------------- 49 50A string that identifies the helper uniquely. (maximum 256 bytes 51including terminating NUL byte) 52 53.. note:: 54 55 The helper ID namespace is a separate namespace. In particular, it is not 56 related to QEMU "id" used in -object/-device objects. 57 58Load(in u8[] bytes) method 59-------------------------- 60 61The method called on destination with the state to restore. 62 63The helper may be initially started in a waiting state (with 64an --incoming argument for example), and it may resume on success. 65 66An error may be returned to the caller. 67 68Save(out u8[] bytes) method 69--------------------------- 70 71The method called on the source to get the current state to be 72migrated. The helper should continue to run normally. 73 74An error may be returned to the caller.