D-Bus VMState


The QEMU dbus-vmstate object’s aim is to migrate helpers’ data running on a QEMU D-Bus bus. (refer to the D-Bus document for some recommendations on D-Bus usage)

Upon migration, QEMU will go through the queue of org.qemu.VMState1 D-Bus name owners and query their Id. It must be unique among the helpers.

It will then save arbitrary data of each Id to be transferred in the migration stream and restored/loaded at the corresponding destination helper.

For now, the data amount to be transferred is arbitrarily limited to 1Mb. The state must be saved quickly (a fraction of a second). (D-Bus imposes a time limit on reply anyway, and migration would fail if data isn’t given quickly enough.)

dbus-vmstate object can be configured with the expected list of helpers by setting its id-list property, with a comma-separated Id list.


On object path /org/qemu/VMState1, the following org.qemu.VMState1 interface should be implemented:

<interface name="org.qemu.VMState1">
  <property name="Id" type="s" access="read"/>
  <method name="Load">
    <arg type="ay" name="data" direction="in"/>
  <method name="Save">
    <arg type="ay" name="data" direction="out"/>

“Id” property

A string that identifies the helper uniquely. (maximum 256 bytes including terminating NUL byte)


The helper ID namespace is a separate namespace. In particular, it is not related to QEMU “id” used in -object/-device objects.

Load(in u8[] bytes) method

The method called on destination with the state to restore.

The helper may be initially started in a waiting state (with an –incoming argument for example), and it may resume on success.

An error may be returned to the caller.

Save(out u8[] bytes) method

The method called on the source to get the current state to be migrated. The helper should continue to run normally.

An error may be returned to the caller.