--- /dev/null
+=========
+Migration
+=========
+
+QEMU has code to load/save the state of the guest that it is running.
+These are two complementary operations. Saving the state just does
+that, saves the state for each device that the guest is running.
+Restoring a guest is just the opposite operation: we need to load the
+state of each device.
+
+For this to work, QEMU has to be launched with the same arguments the
+two times. I.e. it can only restore the state in one guest that has
+the same devices that the one it was saved (this last requirement can
+be relaxed a bit, but for now we can consider that configuration has
+to be exactly the same).
+
+Once that we are able to save/restore a guest, a new functionality is
+requested: migration. This means that QEMU is able to start in one
+machine and being "migrated" to another machine. I.e. being moved to
+another machine.
+
+Next was the "live migration" functionality. This is important
+because some guests run with a lot of state (specially RAM), and it
+can take a while to move all state from one machine to another. Live
+migration allows the guest to continue running while the state is
+transferred. Only while the last part of the state is transferred has
+the guest to be stopped. Typically the time that the guest is
+unresponsive during live migration is the low hundred of milliseconds
+(notice that this depends on a lot of things).
+
+Types of migration
+==================
+
+Now that we have talked about live migration, there are several ways
+to do migration:
+
+- tcp migration: do the migration using tcp sockets
+- unix migration: do the migration using unix sockets
+- exec migration: do the migration using the stdin/stdout through a process.
+- fd migration: do the migration using an file descriptor that is
+ passed to QEMU. QEMU doesn't care how this file descriptor is opened.
+
+All these four migration protocols use the same infrastructure to
+save/restore state devices. This infrastructure is shared with the
+savevm/loadvm functionality.
+
+State Live Migration
+====================
+
+This is used for RAM and block devices. It is not yet ported to vmstate.
+<Fill more information here>
+
+Common infrastructure
+=====================
+
+The files, sockets or fd's that carry the migration stream are abstracted by
+the ``QEMUFile`` type (see `migration/qemu-file.h`). In most cases this
+is connected to a subtype of ``QIOChannel`` (see `io/`).
+
+Saving the state of one device
+==============================
+
+The state of a device is saved using intermediate buffers. There are
+some helper functions to assist this saving.
+
+There is a new concept that we have to explain here: device state
+version. When we migrate a device, we save/load the state as a series
+of fields. Some times, due to bugs or new functionality, we need to
+change the state to store more/different information. We use the
+version to identify each time that we do a change. Each version is
+associated with a series of fields saved. The `save_state` always saves
+the state as the newer version. But `load_state` sometimes is able to
+load state from an older version.
+
+Legacy way
+----------
+
+This way is going to disappear as soon as all current users are ported to VMSTATE.
+
+Each device has to register two functions, one to save the state and
+another to load the state back.
+
+.. code:: c
+
+ int register_savevm(DeviceState *dev,
+ const char *idstr,
+ int instance_id,
+ int version_id,
+ SaveStateHandler *save_state,
+ LoadStateHandler *load_state,
+ void *opaque);
+
+ typedef void SaveStateHandler(QEMUFile *f, void *opaque);
+ typedef int LoadStateHandler(QEMUFile *f, void *opaque, int version_id);
+
+The important functions for the device state format are the `save_state`
+and `load_state`. Notice that `load_state` receives a version_id
+parameter to know what state format is receiving. `save_state` doesn't
+have a version_id parameter because it always uses the latest version.
+
+VMState
+-------
+
+The legacy way of saving/loading state of the device had the problem
+that we have to maintain two functions in sync. If we did one change
+in one of them and not in the other, we would get a failed migration.
+
+VMState changed the way that state is saved/loaded. Instead of using
+a function to save the state and another to load it, it was changed to
+a declarative way of what the state consisted of. Now VMState is able
+to interpret that definition to be able to load/save the state. As
+the state is declared only once, it can't go out of sync in the
+save/load functions.
+
+An example (from hw/input/pckbd.c)
+
+.. code:: c
+
+ static const VMStateDescription vmstate_kbd = {
+ .name = "pckbd",
+ .version_id = 3,
+ .minimum_version_id = 3,
+ .fields = (VMStateField[]) {
+ VMSTATE_UINT8(write_cmd, KBDState),
+ VMSTATE_UINT8(status, KBDState),
+ VMSTATE_UINT8(mode, KBDState),
+ VMSTATE_UINT8(pending, KBDState),
+ VMSTATE_END_OF_LIST()
+ }
+ };
+
+We are declaring the state with name "pckbd".
+The `version_id` is 3, and the fields are 4 uint8_t in a KBDState structure.
+We registered this with:
+
+.. code:: c
+
+ vmstate_register(NULL, 0, &vmstate_kbd, s);
+
+Note: talk about how vmstate <-> qdev interact, and what the instance ids mean.
+
+You can search for ``VMSTATE_*`` macros for lots of types used in QEMU in
+include/hw/hw.h.
+
+More about versions
+-------------------
+
+Version numbers are intended for major incompatible changes to the
+migration of a device, and using them breaks backwards-migration
+compatibility; in general most changes can be made by adding Subsections
+(see below) or _TEST macros (see below) which won't break compatibility.
+
+You can see that there are several version fields:
+
+- `version_id`: the maximum version_id supported by VMState for that device.
+- `minimum_version_id`: the minimum version_id that VMState is able to understand
+ for that device.
+- `minimum_version_id_old`: For devices that were not able to port to vmstate, we can
+ assign a function that knows how to read this old state. This field is
+ ignored if there is no `load_state_old` handler.
+
+So, VMState is able to read versions from minimum_version_id to
+version_id. And the function ``load_state_old()`` (if present) is able to
+load state from minimum_version_id_old to minimum_version_id. This
+function is deprecated and will be removed when no more users are left.
+
+Saving state will always create a section with the 'version_id' value
+and thus can't be loaded by any older QEMU.
+
+Massaging functions
+-------------------
+
+Sometimes, it is not enough to be able to save the state directly
+from one structure, we need to fill the correct values there. One
+example is when we are using kvm. Before saving the cpu state, we
+need to ask kvm to copy to QEMU the state that it is using. And the
+opposite when we are loading the state, we need a way to tell kvm to
+load the state for the cpu that we have just loaded from the QEMUFile.
+
+The functions to do that are inside a vmstate definition, and are called:
+
+- ``int (*pre_load)(void *opaque);``
+
+ This function is called before we load the state of one device.
+
+- ``int (*post_load)(void *opaque, int version_id);``
+
+ This function is called after we load the state of one device.
+
+- ``int (*pre_save)(void *opaque);``
+
+ This function is called before we save the state of one device.
+
+Example: You can look at hpet.c, that uses the three function to
+massage the state that is transferred.
+
+If you use memory API functions that update memory layout outside
+initialization (i.e., in response to a guest action), this is a strong
+indication that you need to call these functions in a `post_load` callback.
+Examples of such memory API functions are:
+
+ - memory_region_add_subregion()
+ - memory_region_del_subregion()
+ - memory_region_set_readonly()
+ - memory_region_set_enabled()
+ - memory_region_set_address()
+ - memory_region_set_alias_offset()
+
+Subsections
+-----------
+
+The use of version_id allows to be able to migrate from older versions
+to newer versions of a device. But not the other way around. This
+makes very complicated to fix bugs in stable branches. If we need to
+add anything to the state to fix a bug, we have to disable migration
+to older versions that don't have that bug-fix (i.e. a new field).
+
+But sometimes, that bug-fix is only needed sometimes, not always. For
+instance, if the device is in the middle of a DMA operation, it is
+using a specific functionality, ....
+
+It is impossible to create a way to make migration from any version to
+any other version to work. But we can do better than only allowing
+migration from older versions to newer ones. For that fields that are
+only needed sometimes, we add the idea of subsections. A subsection
+is "like" a device vmstate, but with a particularity, it has a Boolean
+function that tells if that values are needed to be sent or not. If
+this functions returns false, the subsection is not sent.
+
+On the receiving side, if we found a subsection for a device that we
+don't understand, we just fail the migration. If we understand all
+the subsections, then we load the state with success.
+
+One important note is that the post_load() function is called "after"
+loading all subsections, because a newer subsection could change same
+value that it uses.
+
+Example:
+
+.. code:: c
+
+ static bool ide_drive_pio_state_needed(void *opaque)
+ {
+ IDEState *s = opaque;
+
+ return ((s->status & DRQ_STAT) != 0)
+ || (s->bus->error_status & BM_STATUS_PIO_RETRY);
+ }
+
+ const VMStateDescription vmstate_ide_drive_pio_state = {
+ .name = "ide_drive/pio_state",
+ .version_id = 1,
+ .minimum_version_id = 1,
+ .pre_save = ide_drive_pio_pre_save,
+ .post_load = ide_drive_pio_post_load,
+ .needed = ide_drive_pio_state_needed,
+ .fields = (VMStateField[]) {
+ VMSTATE_INT32(req_nb_sectors, IDEState),
+ VMSTATE_VARRAY_INT32(io_buffer, IDEState, io_buffer_total_len, 1,
+ vmstate_info_uint8, uint8_t),
+ VMSTATE_INT32(cur_io_buffer_offset, IDEState),
+ VMSTATE_INT32(cur_io_buffer_len, IDEState),
+ VMSTATE_UINT8(end_transfer_fn_idx, IDEState),
+ VMSTATE_INT32(elementary_transfer_size, IDEState),
+ VMSTATE_INT32(packet_transfer_size, IDEState),
+ VMSTATE_END_OF_LIST()
+ }
+ };
+
+ const VMStateDescription vmstate_ide_drive = {
+ .name = "ide_drive",
+ .version_id = 3,
+ .minimum_version_id = 0,
+ .post_load = ide_drive_post_load,
+ .fields = (VMStateField[]) {
+ .... several fields ....
+ VMSTATE_END_OF_LIST()
+ },
+ .subsections = (const VMStateDescription*[]) {
+ &vmstate_ide_drive_pio_state,
+ NULL
+ }
+ };
+
+Here we have a subsection for the pio state. We only need to
+save/send this state when we are in the middle of a pio operation
+(that is what ``ide_drive_pio_state_needed()`` checks). If DRQ_STAT is
+not enabled, the values on that fields are garbage and don't need to
+be sent.
+
+Using a condition function that checks a 'property' to determine whether
+to send a subsection allows backwards migration compatibility when
+new subsections are added.
+
+For example:
+
+ a) Add a new property using ``DEFINE_PROP_BOOL`` - e.g. support-foo and
+ default it to true.
+ b) Add an entry to the ``HW_COMPAT_`` for the previous version that sets
+ the property to false.
+ c) Add a static bool support_foo function that tests the property.
+ d) Add a subsection with a .needed set to the support_foo function
+ e) (potentially) Add a pre_load that sets up a default value for 'foo'
+ to be used if the subsection isn't loaded.
+
+Now that subsection will not be generated when using an older
+machine type and the migration stream will be accepted by older
+QEMU versions. pre-load functions can be used to initialise state
+on the newer version so that they default to suitable values
+when loading streams created by older QEMU versions that do not
+generate the subsection.
+
+In some cases subsections are added for data that had been accidentally
+omitted by earlier versions; if the missing data causes the migration
+process to succeed but the guest to behave badly then it may be better
+to send the subsection and cause the migration to explicitly fail
+with the unknown subsection error. If the bad behaviour only happens
+with certain data values, making the subsection conditional on
+the data value (rather than the machine type) allows migrations to succeed
+in most cases. In general the preference is to tie the subsection to
+the machine type, and allow reliable migrations, unless the behaviour
+from omission of the subsection is really bad.
+
+Not sending existing elements
+-----------------------------
+
+Sometimes members of the VMState are no longer needed:
+
+ - removing them will break migration compatibility
+
+ - making them version dependent and bumping the version will break backwards migration compatibility.
+
+The best way is to:
+
+ a) Add a new property/compatibility/function in the same way for subsections above.
+ b) replace the VMSTATE macro with the _TEST version of the macro, e.g.:
+
+ ``VMSTATE_UINT32(foo, barstruct)``
+
+ becomes
+
+ ``VMSTATE_UINT32_TEST(foo, barstruct, pre_version_baz)``
+
+ Sometime in the future when we no longer care about the ancient versions these can be killed off.
+
+Return path
+-----------
+
+In most migration scenarios there is only a single data path that runs
+from the source VM to the destination, typically along a single fd (although
+possibly with another fd or similar for some fast way of throwing pages across).
+
+However, some uses need two way communication; in particular the Postcopy
+destination needs to be able to request pages on demand from the source.
+
+For these scenarios there is a 'return path' from the destination to the source;
+``qemu_file_get_return_path(QEMUFile* fwdpath)`` gives the QEMUFile* for the return
+path.
+
+ Source side
+
+ Forward path - written by migration thread
+ Return path - opened by main thread, read by return-path thread
+
+ Destination side
+
+ Forward path - read by main thread
+ Return path - opened by main thread, written by main thread AND postcopy
+ thread (protected by rp_mutex)
+
+Postcopy
+========
+
+'Postcopy' migration is a way to deal with migrations that refuse to converge
+(or take too long to converge) its plus side is that there is an upper bound on
+the amount of migration traffic and time it takes, the down side is that during
+the postcopy phase, a failure of *either* side or the network connection causes
+the guest to be lost.
+
+In postcopy the destination CPUs are started before all the memory has been
+transferred, and accesses to pages that are yet to be transferred cause
+a fault that's translated by QEMU into a request to the source QEMU.
+
+Postcopy can be combined with precopy (i.e. normal migration) so that if precopy
+doesn't finish in a given time the switch is made to postcopy.
+
+Enabling postcopy
+-----------------
+
+To enable postcopy, issue this command on the monitor prior to the
+start of migration:
+
+``migrate_set_capability postcopy-ram on``
+
+The normal commands are then used to start a migration, which is still
+started in precopy mode. Issuing:
+
+``migrate_start_postcopy``
+
+will now cause the transition from precopy to postcopy.
+It can be issued immediately after migration is started or any
+time later on. Issuing it after the end of a migration is harmless.
+
+.. note::
+ During the postcopy phase, the bandwidth limits set using
+ ``migrate_set_speed`` is ignored (to avoid delaying requested pages that
+ the destination is waiting for).
+
+Postcopy device transfer
+------------------------
+
+Loading of device data may cause the device emulation to access guest RAM
+that may trigger faults that have to be resolved by the source, as such
+the migration stream has to be able to respond with page data *during* the
+device load, and hence the device data has to be read from the stream completely
+before the device load begins to free the stream up. This is achieved by
+'packaging' the device data into a blob that's read in one go.
+
+Source behaviour
+----------------
+
+Until postcopy is entered the migration stream is identical to normal
+precopy, except for the addition of a 'postcopy advise' command at
+the beginning, to tell the destination that postcopy might happen.
+When postcopy starts the source sends the page discard data and then
+forms the 'package' containing:
+
+ - Command: 'postcopy listen'
+ - The device state
+
+ A series of sections, identical to the precopy streams device state stream
+ containing everything except postcopiable devices (i.e. RAM)
+ - Command: 'postcopy run'
+
+The 'package' is sent as the data part of a Command: ``CMD_PACKAGED``, and the
+contents are formatted in the same way as the main migration stream.
+
+During postcopy the source scans the list of dirty pages and sends them
+to the destination without being requested (in much the same way as precopy),
+however when a page request is received from the destination, the dirty page
+scanning restarts from the requested location. This causes requested pages
+to be sent quickly, and also causes pages directly after the requested page
+to be sent quickly in the hope that those pages are likely to be used
+by the destination soon.
+
+Destination behaviour
+---------------------
+
+Initially the destination looks the same as precopy, with a single thread
+reading the migration stream; the 'postcopy advise' and 'discard' commands
+are processed to change the way RAM is managed, but don't affect the stream
+processing.
+
+::
+
+ ------------------------------------------------------------------------------
+ 1 2 3 4 5 6 7
+ main -----DISCARD-CMD_PACKAGED ( LISTEN DEVICE DEVICE DEVICE RUN )
+ thread | |
+ | (page request)
+ | \___
+ v \
+ listen thread: --- page -- page -- page -- page -- page --
+
+ a b c
+ ------------------------------------------------------------------------------
+
+- On receipt of ``CMD_PACKAGED`` (1)
+
+ All the data associated with the package - the ( ... ) section in the diagram -
+ is read into memory, and the main thread recurses into qemu_loadvm_state_main
+ to process the contents of the package (2) which contains commands (3,6) and
+ devices (4...)
+
+- On receipt of 'postcopy listen' - 3 -(i.e. the 1st command in the package)
+
+ a new thread (a) is started that takes over servicing the migration stream,
+ while the main thread carries on loading the package. It loads normal
+ background page data (b) but if during a device load a fault happens (5)
+ the returned page (c) is loaded by the listen thread allowing the main
+ threads device load to carry on.
+
+- The last thing in the ``CMD_PACKAGED`` is a 'RUN' command (6)
+
+ letting the destination CPUs start running. At the end of the
+ ``CMD_PACKAGED`` (7) the main thread returns to normal running behaviour and
+ is no longer used by migration, while the listen thread carries on servicing
+ page data until the end of migration.
+
+Postcopy states
+---------------
+
+Postcopy moves through a series of states (see postcopy_state) from
+ADVISE->DISCARD->LISTEN->RUNNING->END
+
+ - Advise
+
+ Set at the start of migration if postcopy is enabled, even
+ if it hasn't had the start command; here the destination
+ checks that its OS has the support needed for postcopy, and performs
+ setup to ensure the RAM mappings are suitable for later postcopy.
+ The destination will fail early in migration at this point if the
+ required OS support is not present.
+ (Triggered by reception of POSTCOPY_ADVISE command)
+
+ - Discard
+
+ Entered on receipt of the first 'discard' command; prior to
+ the first Discard being performed, hugepages are switched off
+ (using madvise) to ensure that no new huge pages are created
+ during the postcopy phase, and to cause any huge pages that
+ have discards on them to be broken.
+
+ - Listen
+
+ The first command in the package, POSTCOPY_LISTEN, switches
+ the destination state to Listen, and starts a new thread
+ (the 'listen thread') which takes over the job of receiving
+ pages off the migration stream, while the main thread carries
+ on processing the blob. With this thread able to process page
+ reception, the destination now 'sensitises' the RAM to detect
+ any access to missing pages (on Linux using the 'userfault'
+ system).
+
+ - Running
+
+ POSTCOPY_RUN causes the destination to synchronise all
+ state and start the CPUs and IO devices running. The main
+ thread now finishes processing the migration package and
+ now carries on as it would for normal precopy migration
+ (although it can't do the cleanup it would do as it
+ finishes a normal migration).
+
+ - End
+
+ The listen thread can now quit, and perform the cleanup of migration
+ state, the migration is now complete.
+
+Source side page maps
+---------------------
+
+The source side keeps two bitmaps during postcopy; 'the migration bitmap'
+and 'unsent map'. The 'migration bitmap' is basically the same as in
+the precopy case, and holds a bit to indicate that page is 'dirty' -
+i.e. needs sending. During the precopy phase this is updated as the CPU
+dirties pages, however during postcopy the CPUs are stopped and nothing
+should dirty anything any more.
+
+The 'unsent map' is used for the transition to postcopy. It is a bitmap that
+has a bit cleared whenever a page is sent to the destination, however during
+the transition to postcopy mode it is combined with the migration bitmap
+to form a set of pages that:
+
+ a) Have been sent but then redirtied (which must be discarded)
+ b) Have not yet been sent - which also must be discarded to cause any
+ transparent huge pages built during precopy to be broken.
+
+Note that the contents of the unsentmap are sacrificed during the calculation
+of the discard set and thus aren't valid once in postcopy. The dirtymap
+is still valid and is used to ensure that no page is sent more than once. Any
+request for a page that has already been sent is ignored. Duplicate requests
+such as this can happen as a page is sent at about the same time the
+destination accesses it.
+
+Postcopy with hugepages
+-----------------------
+
+Postcopy now works with hugetlbfs backed memory:
+
+ a) The linux kernel on the destination must support userfault on hugepages.
+ b) The huge-page configuration on the source and destination VMs must be
+ identical; i.e. RAMBlocks on both sides must use the same page size.
+ c) Note that ``-mem-path /dev/hugepages`` will fall back to allocating normal
+ RAM if it doesn't have enough hugepages, triggering (b) to fail.
+ Using ``-mem-prealloc`` enforces the allocation using hugepages.
+ d) Care should be taken with the size of hugepage used; postcopy with 2MB
+ hugepages works well, however 1GB hugepages are likely to be problematic
+ since it takes ~1 second to transfer a 1GB hugepage across a 10Gbps link,
+ and until the full page is transferred the destination thread is blocked.
+++ /dev/null
-= Migration =
-
-QEMU has code to load/save the state of the guest that it is running.
-These are two complementary operations. Saving the state just does
-that, saves the state for each device that the guest is running.
-Restoring a guest is just the opposite operation: we need to load the
-state of each device.
-
-For this to work, QEMU has to be launched with the same arguments the
-two times. I.e. it can only restore the state in one guest that has
-the same devices that the one it was saved (this last requirement can
-be relaxed a bit, but for now we can consider that configuration has
-to be exactly the same).
-
-Once that we are able to save/restore a guest, a new functionality is
-requested: migration. This means that QEMU is able to start in one
-machine and being "migrated" to another machine. I.e. being moved to
-another machine.
-
-Next was the "live migration" functionality. This is important
-because some guests run with a lot of state (specially RAM), and it
-can take a while to move all state from one machine to another. Live
-migration allows the guest to continue running while the state is
-transferred. Only while the last part of the state is transferred has
-the guest to be stopped. Typically the time that the guest is
-unresponsive during live migration is the low hundred of milliseconds
-(notice that this depends on a lot of things).
-
-=== Types of migration ===
-
-Now that we have talked about live migration, there are several ways
-to do migration:
-
-- tcp migration: do the migration using tcp sockets
-- unix migration: do the migration using unix sockets
-- exec migration: do the migration using the stdin/stdout through a process.
-- fd migration: do the migration using an file descriptor that is
- passed to QEMU. QEMU doesn't care how this file descriptor is opened.
-
-All these four migration protocols use the same infrastructure to
-save/restore state devices. This infrastructure is shared with the
-savevm/loadvm functionality.
-
-=== State Live Migration ===
-
-This is used for RAM and block devices. It is not yet ported to vmstate.
-<Fill more information here>
-
-=== What is the common infrastructure ===
-
-QEMU uses a QEMUFile abstraction to be able to do migration. Any type
-of migration that wants to use QEMU infrastructure has to create a
-QEMUFile with:
-
-QEMUFile *qemu_fopen_ops(void *opaque,
- QEMUFilePutBufferFunc *put_buffer,
- QEMUFileGetBufferFunc *get_buffer,
- QEMUFileCloseFunc *close);
-
-The functions have the following functionality:
-
-This function writes a chunk of data to a file at the given position.
-The pos argument can be ignored if the file is only used for
-streaming. The handler should try to write all of the data it can.
-
-typedef int (QEMUFilePutBufferFunc)(void *opaque, const uint8_t *buf,
- int64_t pos, int size);
-
-Read a chunk of data from a file at the given position. The pos argument
-can be ignored if the file is only be used for streaming. The number of
-bytes actually read should be returned.
-
-typedef int (QEMUFileGetBufferFunc)(void *opaque, uint8_t *buf,
- int64_t pos, int size);
-
-Close a file and return an error code.
-
-typedef int (QEMUFileCloseFunc)(void *opaque);
-
-You can use any internal state that you need using the opaque void *
-pointer that is passed to all functions.
-
-The important functions for us are put_buffer()/get_buffer() that
-allow to write/read a buffer into the QEMUFile.
-
-=== How to save the state of one device ===
-
-The state of a device is saved using intermediate buffers. There are
-some helper functions to assist this saving.
-
-There is a new concept that we have to explain here: device state
-version. When we migrate a device, we save/load the state as a series
-of fields. Some times, due to bugs or new functionality, we need to
-change the state to store more/different information. We use the
-version to identify each time that we do a change. Each version is
-associated with a series of fields saved. The save_state always saves
-the state as the newer version. But load_state sometimes is able to
-load state from an older version.
-
-=== Legacy way ===
-
-This way is going to disappear as soon as all current users are ported to VMSTATE.
-
-Each device has to register two functions, one to save the state and
-another to load the state back.
-
-int register_savevm(DeviceState *dev,
- const char *idstr,
- int instance_id,
- int version_id,
- SaveStateHandler *save_state,
- LoadStateHandler *load_state,
- void *opaque);
-
-typedef void SaveStateHandler(QEMUFile *f, void *opaque);
-typedef int LoadStateHandler(QEMUFile *f, void *opaque, int version_id);
-
-The important functions for the device state format are the save_state
-and load_state. Notice that load_state receives a version_id
-parameter to know what state format is receiving. save_state doesn't
-have a version_id parameter because it always uses the latest version.
-
-=== VMState ===
-
-The legacy way of saving/loading state of the device had the problem
-that we have to maintain two functions in sync. If we did one change
-in one of them and not in the other, we would get a failed migration.
-
-VMState changed the way that state is saved/loaded. Instead of using
-a function to save the state and another to load it, it was changed to
-a declarative way of what the state consisted of. Now VMState is able
-to interpret that definition to be able to load/save the state. As
-the state is declared only once, it can't go out of sync in the
-save/load functions.
-
-An example (from hw/input/pckbd.c)
-
-static const VMStateDescription vmstate_kbd = {
- .name = "pckbd",
- .version_id = 3,
- .minimum_version_id = 3,
- .fields = (VMStateField[]) {
- VMSTATE_UINT8(write_cmd, KBDState),
- VMSTATE_UINT8(status, KBDState),
- VMSTATE_UINT8(mode, KBDState),
- VMSTATE_UINT8(pending, KBDState),
- VMSTATE_END_OF_LIST()
- }
-};
-
-We are declaring the state with name "pckbd".
-The version_id is 3, and the fields are 4 uint8_t in a KBDState structure.
-We registered this with:
-
- vmstate_register(NULL, 0, &vmstate_kbd, s);
-
-Note: talk about how vmstate <-> qdev interact, and what the instance ids mean.
-
-You can search for VMSTATE_* macros for lots of types used in QEMU in
-include/hw/hw.h.
-
-=== More about versions ===
-
-Version numbers are intended for major incompatible changes to the
-migration of a device, and using them breaks backwards-migration
-compatibility; in general most changes can be made by adding Subsections
-(see below) or _TEST macros (see below) which won't break compatibility.
-
-You can see that there are several version fields:
-
-- version_id: the maximum version_id supported by VMState for that device.
-- minimum_version_id: the minimum version_id that VMState is able to understand
- for that device.
-- minimum_version_id_old: For devices that were not able to port to vmstate, we can
- assign a function that knows how to read this old state. This field is
- ignored if there is no load_state_old handler.
-
-So, VMState is able to read versions from minimum_version_id to
-version_id. And the function load_state_old() (if present) is able to
-load state from minimum_version_id_old to minimum_version_id. This
-function is deprecated and will be removed when no more users are left.
-
-Saving state will always create a section with the 'version_id' value
-and thus can't be loaded by any older QEMU.
-
-=== Massaging functions ===
-
-Sometimes, it is not enough to be able to save the state directly
-from one structure, we need to fill the correct values there. One
-example is when we are using kvm. Before saving the cpu state, we
-need to ask kvm to copy to QEMU the state that it is using. And the
-opposite when we are loading the state, we need a way to tell kvm to
-load the state for the cpu that we have just loaded from the QEMUFile.
-
-The functions to do that are inside a vmstate definition, and are called:
-
-- int (*pre_load)(void *opaque);
-
- This function is called before we load the state of one device.
-
-- int (*post_load)(void *opaque, int version_id);
-
- This function is called after we load the state of one device.
-
-- int (*pre_save)(void *opaque);
-
- This function is called before we save the state of one device.
-
-Example: You can look at hpet.c, that uses the three function to
- massage the state that is transferred.
-
-If you use memory API functions that update memory layout outside
-initialization (i.e., in response to a guest action), this is a strong
-indication that you need to call these functions in a post_load callback.
-Examples of such memory API functions are:
-
- - memory_region_add_subregion()
- - memory_region_del_subregion()
- - memory_region_set_readonly()
- - memory_region_set_enabled()
- - memory_region_set_address()
- - memory_region_set_alias_offset()
-
-=== Subsections ===
-
-The use of version_id allows to be able to migrate from older versions
-to newer versions of a device. But not the other way around. This
-makes very complicated to fix bugs in stable branches. If we need to
-add anything to the state to fix a bug, we have to disable migration
-to older versions that don't have that bug-fix (i.e. a new field).
-
-But sometimes, that bug-fix is only needed sometimes, not always. For
-instance, if the device is in the middle of a DMA operation, it is
-using a specific functionality, ....
-
-It is impossible to create a way to make migration from any version to
-any other version to work. But we can do better than only allowing
-migration from older versions to newer ones. For that fields that are
-only needed sometimes, we add the idea of subsections. A subsection
-is "like" a device vmstate, but with a particularity, it has a Boolean
-function that tells if that values are needed to be sent or not. If
-this functions returns false, the subsection is not sent.
-
-On the receiving side, if we found a subsection for a device that we
-don't understand, we just fail the migration. If we understand all
-the subsections, then we load the state with success.
-
-One important note is that the post_load() function is called "after"
-loading all subsections, because a newer subsection could change same
-value that it uses.
-
-Example:
-
-static bool ide_drive_pio_state_needed(void *opaque)
-{
- IDEState *s = opaque;
-
- return ((s->status & DRQ_STAT) != 0)
- || (s->bus->error_status & BM_STATUS_PIO_RETRY);
-}
-
-const VMStateDescription vmstate_ide_drive_pio_state = {
- .name = "ide_drive/pio_state",
- .version_id = 1,
- .minimum_version_id = 1,
- .pre_save = ide_drive_pio_pre_save,
- .post_load = ide_drive_pio_post_load,
- .needed = ide_drive_pio_state_needed,
- .fields = (VMStateField[]) {
- VMSTATE_INT32(req_nb_sectors, IDEState),
- VMSTATE_VARRAY_INT32(io_buffer, IDEState, io_buffer_total_len, 1,
- vmstate_info_uint8, uint8_t),
- VMSTATE_INT32(cur_io_buffer_offset, IDEState),
- VMSTATE_INT32(cur_io_buffer_len, IDEState),
- VMSTATE_UINT8(end_transfer_fn_idx, IDEState),
- VMSTATE_INT32(elementary_transfer_size, IDEState),
- VMSTATE_INT32(packet_transfer_size, IDEState),
- VMSTATE_END_OF_LIST()
- }
-};
-
-const VMStateDescription vmstate_ide_drive = {
- .name = "ide_drive",
- .version_id = 3,
- .minimum_version_id = 0,
- .post_load = ide_drive_post_load,
- .fields = (VMStateField[]) {
- .... several fields ....
- VMSTATE_END_OF_LIST()
- },
- .subsections = (const VMStateDescription*[]) {
- &vmstate_ide_drive_pio_state,
- NULL
- }
-};
-
-Here we have a subsection for the pio state. We only need to
-save/send this state when we are in the middle of a pio operation
-(that is what ide_drive_pio_state_needed() checks). If DRQ_STAT is
-not enabled, the values on that fields are garbage and don't need to
-be sent.
-
-Using a condition function that checks a 'property' to determine whether
-to send a subsection allows backwards migration compatibility when
-new subsections are added.
-
-For example;
- a) Add a new property using DEFINE_PROP_BOOL - e.g. support-foo and
- default it to true.
- b) Add an entry to the HW_COMPAT_ for the previous version
- that sets the property to false.
- c) Add a static bool support_foo function that tests the property.
- d) Add a subsection with a .needed set to the support_foo function
- e) (potentially) Add a pre_load that sets up a default value for 'foo'
- to be used if the subsection isn't loaded.
-
-Now that subsection will not be generated when using an older
-machine type and the migration stream will be accepted by older
-QEMU versions. pre-load functions can be used to initialise state
-on the newer version so that they default to suitable values
-when loading streams created by older QEMU versions that do not
-generate the subsection.
-
-In some cases subsections are added for data that had been accidentally
-omitted by earlier versions; if the missing data causes the migration
-process to succeed but the guest to behave badly then it may be better
-to send the subsection and cause the migration to explicitly fail
-with the unknown subsection error. If the bad behaviour only happens
-with certain data values, making the subsection conditional on
-the data value (rather than the machine type) allows migrations to succeed
-in most cases. In general the preference is to tie the subsection to
-the machine type, and allow reliable migrations, unless the behaviour
-from omission of the subsection is really bad.
-
-= Not sending existing elements =
-
-Sometimes members of the VMState are no longer needed;
- removing them will break migration compatibility
- making them version dependent and bumping the version will break backwards
- migration compatibility.
-
-The best way is to:
- a) Add a new property/compatibility/function in the same way for subsections
- above.
- b) replace the VMSTATE macro with the _TEST version of the macro, e.g.:
- VMSTATE_UINT32(foo, barstruct)
- becomes
- VMSTATE_UINT32_TEST(foo, barstruct, pre_version_baz)
-
- Sometime in the future when we no longer care about the ancient
-versions these can be killed off.
-
-= Return path =
-
-In most migration scenarios there is only a single data path that runs
-from the source VM to the destination, typically along a single fd (although
-possibly with another fd or similar for some fast way of throwing pages across).
-
-However, some uses need two way communication; in particular the Postcopy
-destination needs to be able to request pages on demand from the source.
-
-For these scenarios there is a 'return path' from the destination to the source;
-qemu_file_get_return_path(QEMUFile* fwdpath) gives the QEMUFile* for the return
-path.
-
- Source side
- Forward path - written by migration thread
- Return path - opened by main thread, read by return-path thread
-
- Destination side
- Forward path - read by main thread
- Return path - opened by main thread, written by main thread AND postcopy
- thread (protected by rp_mutex)
-
-= Postcopy =
-'Postcopy' migration is a way to deal with migrations that refuse to converge
-(or take too long to converge) its plus side is that there is an upper bound on
-the amount of migration traffic and time it takes, the down side is that during
-the postcopy phase, a failure of *either* side or the network connection causes
-the guest to be lost.
-
-In postcopy the destination CPUs are started before all the memory has been
-transferred, and accesses to pages that are yet to be transferred cause
-a fault that's translated by QEMU into a request to the source QEMU.
-
-Postcopy can be combined with precopy (i.e. normal migration) so that if precopy
-doesn't finish in a given time the switch is made to postcopy.
-
-=== Enabling postcopy ===
-
-To enable postcopy, issue this command on the monitor prior to the
-start of migration:
-
-migrate_set_capability postcopy-ram on
-
-The normal commands are then used to start a migration, which is still
-started in precopy mode. Issuing:
-
-migrate_start_postcopy
-
-will now cause the transition from precopy to postcopy.
-It can be issued immediately after migration is started or any
-time later on. Issuing it after the end of a migration is harmless.
-
-Note: During the postcopy phase, the bandwidth limits set using
-migrate_set_speed is ignored (to avoid delaying requested pages that
-the destination is waiting for).
-
-=== Postcopy device transfer ===
-
-Loading of device data may cause the device emulation to access guest RAM
-that may trigger faults that have to be resolved by the source, as such
-the migration stream has to be able to respond with page data *during* the
-device load, and hence the device data has to be read from the stream completely
-before the device load begins to free the stream up. This is achieved by
-'packaging' the device data into a blob that's read in one go.
-
-Source behaviour
-
-Until postcopy is entered the migration stream is identical to normal
-precopy, except for the addition of a 'postcopy advise' command at
-the beginning, to tell the destination that postcopy might happen.
-When postcopy starts the source sends the page discard data and then
-forms the 'package' containing:
-
- Command: 'postcopy listen'
- The device state
- A series of sections, identical to the precopy streams device state stream
- containing everything except postcopiable devices (i.e. RAM)
- Command: 'postcopy run'
-
-The 'package' is sent as the data part of a Command: 'CMD_PACKAGED', and the
-contents are formatted in the same way as the main migration stream.
-
-During postcopy the source scans the list of dirty pages and sends them
-to the destination without being requested (in much the same way as precopy),
-however when a page request is received from the destination, the dirty page
-scanning restarts from the requested location. This causes requested pages
-to be sent quickly, and also causes pages directly after the requested page
-to be sent quickly in the hope that those pages are likely to be used
-by the destination soon.
-
-Destination behaviour
-
-Initially the destination looks the same as precopy, with a single thread
-reading the migration stream; the 'postcopy advise' and 'discard' commands
-are processed to change the way RAM is managed, but don't affect the stream
-processing.
-
-------------------------------------------------------------------------------
- 1 2 3 4 5 6 7
-main -----DISCARD-CMD_PACKAGED ( LISTEN DEVICE DEVICE DEVICE RUN )
-thread | |
- | (page request)
- | \___
- v \
-listen thread: --- page -- page -- page -- page -- page --
-
- a b c
-------------------------------------------------------------------------------
-
-On receipt of CMD_PACKAGED (1)
- All the data associated with the package - the ( ... ) section in the
-diagram - is read into memory, and the main thread recurses into
-qemu_loadvm_state_main to process the contents of the package (2)
-which contains commands (3,6) and devices (4...)
-
-On receipt of 'postcopy listen' - 3 -(i.e. the 1st command in the package)
-a new thread (a) is started that takes over servicing the migration stream,
-while the main thread carries on loading the package. It loads normal
-background page data (b) but if during a device load a fault happens (5) the
-returned page (c) is loaded by the listen thread allowing the main threads
-device load to carry on.
-
-The last thing in the CMD_PACKAGED is a 'RUN' command (6) letting the destination
-CPUs start running.
-At the end of the CMD_PACKAGED (7) the main thread returns to normal running behaviour
-and is no longer used by migration, while the listen thread carries
-on servicing page data until the end of migration.
-
-=== Postcopy states ===
-
-Postcopy moves through a series of states (see postcopy_state) from
-ADVISE->DISCARD->LISTEN->RUNNING->END
-
- Advise: Set at the start of migration if postcopy is enabled, even
- if it hasn't had the start command; here the destination
- checks that its OS has the support needed for postcopy, and performs
- setup to ensure the RAM mappings are suitable for later postcopy.
- The destination will fail early in migration at this point if the
- required OS support is not present.
- (Triggered by reception of POSTCOPY_ADVISE command)
-
- Discard: Entered on receipt of the first 'discard' command; prior to
- the first Discard being performed, hugepages are switched off
- (using madvise) to ensure that no new huge pages are created
- during the postcopy phase, and to cause any huge pages that
- have discards on them to be broken.
-
- Listen: The first command in the package, POSTCOPY_LISTEN, switches
- the destination state to Listen, and starts a new thread
- (the 'listen thread') which takes over the job of receiving
- pages off the migration stream, while the main thread carries
- on processing the blob. With this thread able to process page
- reception, the destination now 'sensitises' the RAM to detect
- any access to missing pages (on Linux using the 'userfault'
- system).
-
- Running: POSTCOPY_RUN causes the destination to synchronise all
- state and start the CPUs and IO devices running. The main
- thread now finishes processing the migration package and
- now carries on as it would for normal precopy migration
- (although it can't do the cleanup it would do as it
- finishes a normal migration).
-
- End: The listen thread can now quit, and perform the cleanup of migration
- state, the migration is now complete.
-
-=== Source side page maps ===
-
-The source side keeps two bitmaps during postcopy; 'the migration bitmap'
-and 'unsent map'. The 'migration bitmap' is basically the same as in
-the precopy case, and holds a bit to indicate that page is 'dirty' -
-i.e. needs sending. During the precopy phase this is updated as the CPU
-dirties pages, however during postcopy the CPUs are stopped and nothing
-should dirty anything any more.
-
-The 'unsent map' is used for the transition to postcopy. It is a bitmap that
-has a bit cleared whenever a page is sent to the destination, however during
-the transition to postcopy mode it is combined with the migration bitmap
-to form a set of pages that:
- a) Have been sent but then redirtied (which must be discarded)
- b) Have not yet been sent - which also must be discarded to cause any
- transparent huge pages built during precopy to be broken.
-
-Note that the contents of the unsentmap are sacrificed during the calculation
-of the discard set and thus aren't valid once in postcopy. The dirtymap
-is still valid and is used to ensure that no page is sent more than once. Any
-request for a page that has already been sent is ignored. Duplicate requests
-such as this can happen as a page is sent at about the same time the
-destination accesses it.
-
-=== Postcopy with hugepages ===
-
-Postcopy now works with hugetlbfs backed memory:
- a) The linux kernel on the destination must support userfault on hugepages.
- b) The huge-page configuration on the source and destination VMs must be
- identical; i.e. RAMBlocks on both sides must use the same page size.
- c) Note that -mem-path /dev/hugepages will fall back to allocating normal
- RAM if it doesn't have enough hugepages, triggering (b) to fail.
- Using -mem-prealloc enforces the allocation using hugepages.
- d) Care should be taken with the size of hugepage used; postcopy with 2MB
- hugepages works well, however 1GB hugepages are likely to be problematic
- since it takes ~1 second to transfer a 1GB hugepage across a 10Gbps link,
- and until the full page is transferred the destination thread is blocked.
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