virtio: add some migration doc

Try to cover the basics of virtio migration.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>

docs/virtio-migration.txt

@@ -0,0 +1,106 @@
+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.