2015-09-17 18:42:57 +02:00
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Virtio devices and migration
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============================
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Copyright 2015 IBM Corp.
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This work is licensed under the terms of the GNU GPL, version 2 or later. See
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the COPYING file in the top-level directory.
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Saving and restoring the state of virtio devices is a bit of a twisty maze,
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for several reasons:
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- state is distributed between several parts:
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- virtio core, for common fields like features, number of queues, ...
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- virtio transport (pci, ccw, ...), for the different proxy devices and
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transport specific state (msix vectors, indicators, ...)
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- virtio device (net, blk, ...), for the different device types and their
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state (mac address, request queue, ...)
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- most fields are saved via the stream interface; subsequently, subsections
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have been added to make cross-version migration possible
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This file attempts to document the current procedure and point out some
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caveats.
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Save state procedure
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====================
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virtio core virtio transport virtio device
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----------- ---------------- -------------
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save() function registered
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2016-07-14 19:22:56 +02:00
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via VMState wrapper on
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device class
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2015-09-17 18:42:57 +02:00
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virtio_save() <----------
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------> save_config()
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- save proxy device
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- save transport-specific
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device fields
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- save common device
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fields
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- save common virtqueue
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fields
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------> save_queue()
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- save transport-specific
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virtqueue fields
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------> save_device()
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- save device-specific
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fields
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- save subsections
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- device endianness,
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if changed from
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default endianness
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- 64 bit features, if
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any high feature bit
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is set
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- virtio-1 virtqueue
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fields, if VERSION_1
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is set
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Load state procedure
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====================
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virtio core virtio transport virtio device
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----------- ---------------- -------------
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load() function registered
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2016-07-14 19:22:56 +02:00
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via VMState wrapper on
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device class
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2015-09-17 18:42:57 +02:00
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virtio_load() <----------
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------> load_config()
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- load proxy device
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- load transport-specific
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device fields
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- load common device
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fields
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- load common virtqueue
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fields
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------> load_queue()
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- load transport-specific
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virtqueue fields
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- notify guest
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------> load_device()
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- load device-specific
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fields
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- load subsections
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- device endianness
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- 64 bit features
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- virtio-1 virtqueue
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fields
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- sanitize endianness
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- sanitize features
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- virtqueue index sanity
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check
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- feature-dependent setup
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Implications of this setup
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==========================
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Devices need to be careful in their state processing during load: The
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load_device() procedure is invoked by the core before subsections have
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been loaded. Any code that depends on information transmitted in subsections
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therefore has to be invoked in the device's load() function _after_
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virtio_load() returned (like e.g. code depending on features).
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Any extension of the state being migrated should be done in subsections
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added to the core for compatibility reasons. If transport or device specific
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state is added, core needs to invoke a callback from the new subsection.
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