mirror of https://gitee.com/openkylin/qemu.git
109 lines
4.0 KiB
Plaintext
109 lines
4.0 KiB
Plaintext
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 VMState wrapper on
|
|
device class
|
|
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 VMState wrapper on
|
|
device class
|
|
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.
|