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| author | Cornelia Huck | 2015-09-17 18:42:57 +0200 |
|---|---|---|
| committer | Michael S. Tsirkin | 2015-10-22 13:34:48 +0200 |
| commit | 22de58fe152d68b248bc14efd7affe72286079e5 (patch) | |
| tree | 5f0b80992e174676230c5b406a381793ab04abad /docs | |
| parent | vhost: fail backend intialization early (diff) | |
| download | qemu-22de58fe152d68b248bc14efd7affe72286079e5.tar.gz qemu-22de58fe152d68b248bc14efd7affe72286079e5.tar.xz qemu-22de58fe152d68b248bc14efd7affe72286079e5.zip | |
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>
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/virtio-migration.txt | 106 |
1 files changed, 106 insertions, 0 deletions
diff --git a/docs/virtio-migration.txt b/docs/virtio-migration.txt new file mode 100644 index 0000000000..cf66458b97 --- /dev/null +++ b/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. |