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+Virtio devices and migration
+============================
+
+Copyright 2015 IBM Corp.
+
+This work is licensed under the terms of the GNU GPL, version 2 or later. See
+the COPYING file in the top-level directory.
+
+Saving and restoring the state of virtio devices is a bit of a twisty maze,
+for several reasons:
+- state is distributed between several parts:
+ - virtio core, for common fields like features, number of queues, ...
+ - virtio transport (pci, ccw, ...), for the different proxy devices and
+ transport specific state (msix vectors, indicators, ...)
+ - virtio device (net, blk, ...), for the different device types and their
+ state (mac address, request queue, ...)
+- most fields are saved via the stream interface; subsequently, subsections
+ have been added to make cross-version migration possible
+
+This file attempts to document the current procedure and point out some
+caveats.
+
+
+Save state procedure
+====================
+
+virtio core virtio transport virtio device
+----------- ---------------- -------------
+
+ save() function registered
+ via register_savevm()
+virtio_save() <----------
+ ------> save_config()
+ - save proxy device
+ - save transport-specific
+ device fields
+- save common device
+ fields
+- save common virtqueue
+ fields
+ ------> save_queue()
+ - save transport-specific
+ virtqueue fields
+ ------> save_device()
+ - save device-specific
+ fields
+- save subsections
+ - device endianness,
+ if changed from
+ default endianness
+ - 64 bit features, if
+ any high feature bit
+ is set
+ - virtio-1 virtqueue
+ fields, if VERSION_1
+ is set
+
+
+Load state procedure
+====================
+
+virtio core virtio transport virtio device
+----------- ---------------- -------------
+
+ load() function registered
+ via register_savevm()
+virtio_load() <----------
+ ------> load_config()
+ - load proxy device
+ - load transport-specific
+ device fields
+- load common device
+ fields
+- load common virtqueue
+ fields
+ ------> load_queue()
+ - load transport-specific
+ virtqueue fields
+- notify guest
+ ------> load_device()
+ - load device-specific
+ fields
+- load subsections
+ - device endianness
+ - 64 bit features
+ - virtio-1 virtqueue
+ fields
+- sanitize endianness
+- sanitize features
+- virtqueue index sanity
+ check
+ - feature-dependent setup
+
+
+Implications of this setup
+==========================
+
+Devices need to be careful in their state processing during load: The
+load_device() procedure is invoked by the core before subsections have
+been loaded. Any code that depends on information transmitted in subsections
+therefore has to be invoked in the device's load() function _after_
+virtio_load() returned (like e.g. code depending on features).
+
+Any extension of the state being migrated should be done in subsections
+added to the core for compatibility reasons. If transport or device specific
+state is added, core needs to invoke a callback from the new subsection.
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