D-Bus VMState — QEMU documentation

# D-Bus VMState

## Introduction

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.

## Interface

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>
      <method name="Save">
        <arg type="ay" name="data" direction="out"/>
      </method>
    </interface>

### “Id” property

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

Note

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.