3
QEMU has code to load/save the state of the guest that it is running.
4
These are two complementary operations. Saving the state just does
5
that, saves the state for each device that the guest is running.
6
Restoring a guest is just the opposite operation: we need to load the
9
For this to work, QEMU has to be launched with the same arguments the
10
two times. I.e. it can only restore the state in one guest that has
11
the same devices that the one it was saved (this last requirement can
12
be relaxed a bit, but for now we can consider that configuration has
13
to be exactly the same).
15
Once that we are able to save/restore a guest, a new functionality is
16
requested: migration. This means that QEMU is able to start in one
17
machine and being "migrated" to another machine. I.e. being moved to
20
Next was the "live migration" functionality. This is important
21
because some guests run with a lot of state (specially RAM), and it
22
can take a while to move all state from one machine to another. Live
23
migration allows the guest to continue running while the state is
24
transferred. Only while the last part of the state is transferred has
25
the guest to be stopped. Typically the time that the guest is
26
unresponsive during live migration is the low hundred of milliseconds
27
(notice that this depends on a lot of things).
29
=== Types of migration ===
31
Now that we have talked about live migration, there are several ways
34
- tcp migration: do the migration using tcp sockets
35
- unix migration: do the migration using unix sockets
36
- exec migration: do the migration using the stdin/stdout through a process.
37
- fd migration: do the migration using an file descriptor that is
38
passed to QEMU. QEMU doesn't care how this file descriptor is opened.
40
All these four migration protocols use the same infrastructure to
41
save/restore state devices. This infrastructure is shared with the
42
savevm/loadvm functionality.
44
=== State Live Migration ==
46
This is used for RAM and block devices. It is not yet ported to vmstate.
47
<Fill more information here>
49
=== What is the common infrastructure ===
51
QEMU uses a QEMUFile abstraction to be able to do migration. Any type
52
of migration that wants to use QEMU infrastructure has to create a
55
QEMUFile *qemu_fopen_ops(void *opaque,
56
QEMUFilePutBufferFunc *put_buffer,
57
QEMUFileGetBufferFunc *get_buffer,
58
QEMUFileCloseFunc *close,
59
QEMUFileRateLimit *rate_limit,
60
QEMUFileSetRateLimit *set_rate_limit,
61
QEMUFileGetRateLimit *get_rate_limit);
63
The functions have the following functionality:
65
This function writes a chunk of data to a file at the given position.
66
The pos argument can be ignored if the file is only used for
67
streaming. The handler should try to write all of the data it can.
69
typedef int (QEMUFilePutBufferFunc)(void *opaque, const uint8_t *buf,
70
int64_t pos, int size);
72
Read a chunk of data from a file at the given position. The pos argument
73
can be ignored if the file is only be used for streaming. The number of
74
bytes actually read should be returned.
76
typedef int (QEMUFileGetBufferFunc)(void *opaque, uint8_t *buf,
77
int64_t pos, int size);
79
Close a file and return an error code.
81
typedef int (QEMUFileCloseFunc)(void *opaque);
83
Called to determine if the file has exceeded its bandwidth allocation. The
84
bandwidth capping is a soft limit, not a hard limit.
86
typedef int (QEMUFileRateLimit)(void *opaque);
88
Called to change the current bandwidth allocation. This function must return
89
the new actual bandwidth. It should be new_rate if everything goes OK, and
90
the old rate otherwise.
92
typedef size_t (QEMUFileSetRateLimit)(void *opaque, size_t new_rate);
93
typedef size_t (QEMUFileGetRateLimit)(void *opaque);
95
You can use any internal state that you need using the opaque void *
96
pointer that is passed to all functions.
98
The rate limiting functions are used to limit the bandwidth used by
101
The important functions for us are put_buffer()/get_buffer() that
102
allow to write/read a buffer into the QEMUFile.
104
=== How to save the state of one device ==
106
The state of a device is saved using intermediate buffers. There are
107
some helper functions to assist this saving.
109
There is a new concept that we have to explain here: device state
110
version. When we migrate a device, we save/load the state as a series
111
of fields. Some times, due to bugs or new functionality, we need to
112
change the state to store more/different information. We use the
113
version to identify each time that we do a change. Each version is
114
associated with a series of fields saved. The save_state always saves
115
the state as the newer version. But load_state sometimes is able to
116
load state from an older version.
120
This way is going to disappear as soon as all current users are ported to VMSTATE.
122
Each device has to register two functions, one to save the state and
123
another to load the state back.
125
int register_savevm(DeviceState *dev,
129
SaveStateHandler *save_state,
130
LoadStateHandler *load_state,
133
typedef void SaveStateHandler(QEMUFile *f, void *opaque);
134
typedef int LoadStateHandler(QEMUFile *f, void *opaque, int version_id);
136
The important functions for the device state format are the save_state
137
and load_state. Notice that load_state receives a version_id
138
parameter to know what state format is receiving. save_state doesn't
139
have a version_id parameter because it always uses the latest version.
143
The legacy way of saving/loading state of the device had the problem
144
that we have to maintain two functions in sync. If we did one change
145
in one of them and not in the other, we would get a failed migration.
147
VMState changed the way that state is saved/loaded. Instead of using
148
a function to save the state and another to load it, it was changed to
149
a declarative way of what the state consisted of. Now VMState is able
150
to interpret that definition to be able to load/save the state. As
151
the state is declared only once, it can't go out of sync in the
154
An example (from hw/pckbd.c)
156
static const VMStateDescription vmstate_kbd = {
159
.minimum_version_id = 3,
160
.minimum_version_id_old = 3,
161
.fields = (VMStateField []) {
162
VMSTATE_UINT8(write_cmd, KBDState),
163
VMSTATE_UINT8(status, KBDState),
164
VMSTATE_UINT8(mode, KBDState),
165
VMSTATE_UINT8(pending, KBDState),
166
VMSTATE_END_OF_LIST()
170
We are declaring the state with name "pckbd".
171
The version_id is 3, and the fields are 4 uint8_t in a KBDState structure.
172
We registered this with:
174
vmstate_register(NULL, 0, &vmstate_kbd, s);
176
Note: talk about how vmstate <-> qdev interact, and what the instance ids mean.
178
You can search for VMSTATE_* macros for lots of types used in QEMU in
181
=== More about versions ==
183
You can see that there are several version fields:
185
- version_id: the maximum version_id supported by VMState for that device.
186
- minimum_version_id: the minimum version_id that VMState is able to understand
188
- minimum_version_id_old: For devices that were not able to port to vmstate, we can
189
assign a function that knows how to read this old state.
191
So, VMState is able to read versions from minimum_version_id to
192
version_id. And the function load_state_old() is able to load state
193
from minimum_version_id_old to minimum_version_id. This function is
194
deprecated and will be removed when no more users are left.
196
=== Massaging functions ===
198
Sometimes, it is not enough to be able to save the state directly
199
from one structure, we need to fill the correct values there. One
200
example is when we are using kvm. Before saving the cpu state, we
201
need to ask kvm to copy to QEMU the state that it is using. And the
202
opposite when we are loading the state, we need a way to tell kvm to
203
load the state for the cpu that we have just loaded from the QEMUFile.
205
The functions to do that are inside a vmstate definition, and are called:
207
- int (*pre_load)(void *opaque);
209
This function is called before we load the state of one device.
211
- int (*post_load)(void *opaque, int version_id);
213
This function is called after we load the state of one device.
215
- void (*pre_save)(void *opaque);
217
This function is called before we save the state of one device.
219
Example: You can look at hpet.c, that uses the three function to
220
massage the state that is transferred.
224
The use of version_id allows to be able to migrate from older versions
225
to newer versions of a device. But not the other way around. This
226
makes very complicated to fix bugs in stable branches. If we need to
227
add anything to the state to fix a bug, we have to disable migration
228
to older versions that don't have that bug-fix (i.e. a new field).
230
But sometimes, that bug-fix is only needed sometimes, not always. For
231
instance, if the device is in the middle of a DMA operation, it is
232
using a specific functionality, ....
234
It is impossible to create a way to make migration from any version to
235
any other version to work. But we can do better than only allowing
236
migration from older versions no newer ones. For that fields that are
237
only needed sometimes, we add the idea of subsections. A subsection
238
is "like" a device vmstate, but with a particularity, it has a Boolean
239
function that tells if that values are needed to be sent or not. If
240
this functions returns false, the subsection is not sent.
242
On the receiving side, if we found a subsection for a device that we
243
don't understand, we just fail the migration. If we understand all
244
the subsections, then we load the state with success.
246
One important note is that the post_load() function is called "after"
247
loading all subsections, because a newer subsection could change same
252
static bool ide_drive_pio_state_needed(void *opaque)
254
IDEState *s = opaque;
256
return (s->status & DRQ_STAT) != 0;
259
const VMStateDescription vmstate_ide_drive_pio_state = {
260
.name = "ide_drive/pio_state",
262
.minimum_version_id = 1,
263
.minimum_version_id_old = 1,
264
.pre_save = ide_drive_pio_pre_save,
265
.post_load = ide_drive_pio_post_load,
266
.fields = (VMStateField []) {
267
VMSTATE_INT32(req_nb_sectors, IDEState),
268
VMSTATE_VARRAY_INT32(io_buffer, IDEState, io_buffer_total_len, 1,
269
vmstate_info_uint8, uint8_t),
270
VMSTATE_INT32(cur_io_buffer_offset, IDEState),
271
VMSTATE_INT32(cur_io_buffer_len, IDEState),
272
VMSTATE_UINT8(end_transfer_fn_idx, IDEState),
273
VMSTATE_INT32(elementary_transfer_size, IDEState),
274
VMSTATE_INT32(packet_transfer_size, IDEState),
275
VMSTATE_END_OF_LIST()
279
const VMStateDescription vmstate_ide_drive = {
282
.minimum_version_id = 0,
283
.minimum_version_id_old = 0,
284
.post_load = ide_drive_post_load,
285
.fields = (VMStateField []) {
286
.... several fields ....
287
VMSTATE_END_OF_LIST()
289
.subsections = (VMStateSubsection []) {
291
.vmsd = &vmstate_ide_drive_pio_state,
292
.needed = ide_drive_pio_state_needed,
299
Here we have a subsection for the pio state. We only need to
300
save/send this state when we are in the middle of a pio operation
301
(that is what ide_drive_pio_state_needed() checks). If DRQ_STAT is
302
not enabled, the values on that fields are garbage and don't need to
added
removed
docs/migration.txt
