projects / qemu.git / blame
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|---|---|---|
| 22de58fe CH |
1 | Virtio devices and migration |
| 2 | ============================ | |
| 3 | ||
| 4 | Copyright 2015 IBM Corp. | |
| 5 | ||
| 6 | This work is licensed under the terms of the GNU GPL, version 2 or later. See | |
| 7 | the COPYING file in the top-level directory. | |
| 8 | ||
| 9 | Saving and restoring the state of virtio devices is a bit of a twisty maze, | |
| 10 | for several reasons: | |
| 11 | - state is distributed between several parts: | |
| 12 | - virtio core, for common fields like features, number of queues, ... | |
| 13 | - virtio transport (pci, ccw, ...), for the different proxy devices and | |
| 14 | transport specific state (msix vectors, indicators, ...) | |
| 15 | - virtio device (net, blk, ...), for the different device types and their | |
| 16 | state (mac address, request queue, ...) | |
| 17 | - most fields are saved via the stream interface; subsequently, subsections | |
| 18 | have been added to make cross-version migration possible | |
| 19 | ||
| 20 | This file attempts to document the current procedure and point out some | |
| 21 | caveats. | |
| 22 | ||
| 23 | ||
| 24 | Save state procedure | |
| 25 | ==================== | |
| 26 | ||
| 27 | virtio core virtio transport virtio device | |
| 28 | ----------- ---------------- ------------- | |
| 29 | ||
| 30 | save() function registered | |
| 1a210f63 DDAG |
31 | via VMState wrapper on |
| 32 | device class | |
| 22de58fe CH |
33 | virtio_save() <---------- |
| 34 | ------> save_config() | |
| 35 | - save proxy device | |
| 36 | - save transport-specific | |
| 37 | device fields | |
| 38 | - save common device | |
| 39 | fields | |
| 40 | - save common virtqueue | |
| 41 | fields | |
| 42 | ------> save_queue() | |
| 43 | - save transport-specific | |
| 44 | virtqueue fields | |
| 45 | ------> save_device() | |
| 46 | - save device-specific | |
| 47 | fields | |
| 48 | - save subsections | |
| 49 | - device endianness, | |
| 50 | if changed from | |
| 51 | default endianness | |
| 52 | - 64 bit features, if | |
| 53 | any high feature bit | |
| 54 | is set | |
| 55 | - virtio-1 virtqueue | |
| 56 | fields, if VERSION_1 | |
| 57 | is set | |
| 58 | ||
| 59 | ||
| 60 | Load state procedure | |
| 61 | ==================== | |
| 62 | ||
| 63 | virtio core virtio transport virtio device | |
| 64 | ----------- ---------------- ------------- | |
| 65 | ||
| 66 | load() function registered | |
| 1a210f63 DDAG |
67 | via VMState wrapper on |
| 68 | device class | |
| 22de58fe CH |
69 | virtio_load() <---------- |
| 70 | ------> load_config() | |
| 71 | - load proxy device | |
| 72 | - load transport-specific | |
| 73 | device fields | |
| 74 | - load common device | |
| 75 | fields | |
| 76 | - load common virtqueue | |
| 77 | fields | |
| 78 | ------> load_queue() | |
| 79 | - load transport-specific | |
| 80 | virtqueue fields | |
| 81 | - notify guest | |
| 82 | ------> load_device() | |
| 83 | - load device-specific | |
| 84 | fields | |
| 85 | - load subsections | |
| 86 | - device endianness | |
| 87 | - 64 bit features | |
| 88 | - virtio-1 virtqueue | |
| 89 | fields | |
| 90 | - sanitize endianness | |
| 91 | - sanitize features | |
| 92 | - virtqueue index sanity | |
| 93 | check | |
| 94 | - feature-dependent setup | |
| 95 | ||
| 96 | ||
| 97 | Implications of this setup | |
| 98 | ========================== | |
| 99 | ||
| 100 | Devices need to be careful in their state processing during load: The | |
| 101 | load_device() procedure is invoked by the core before subsections have | |
| 102 | been loaded. Any code that depends on information transmitted in subsections | |
| 103 | therefore has to be invoked in the device's load() function _after_ | |
| 104 | virtio_load() returned (like e.g. code depending on features). | |
| 105 | ||
| 106 | Any extension of the state being migrated should be done in subsections | |
| 107 | added to the core for compatibility reasons. If transport or device specific | |
| 108 | state is added, core needs to invoke a callback from the new subsection. |