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diff --git a/docs/migration.txt b/docs/migration.txt new file mode 100644 index 000000000..f3ddd2f1a --- /dev/null +++ b/docs/migration.txt @@ -0,0 +1,315 @@ += 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, + QEMUFileRateLimit *rate_limit, + QEMUFileSetRateLimit *set_rate_limit, + QEMUFileGetRateLimit *get_rate_limit); + +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); + +Called to determine if the file has exceeded its bandwidth allocation. The +bandwidth capping is a soft limit, not a hard limit. + +typedef int (QEMUFileRateLimit)(void *opaque); + +Called to change the current bandwidth allocation. This function must return +the new actual bandwidth. It should be new_rate if everything goes OK, and +the old rate otherwise. + +typedef size_t (QEMUFileSetRateLimit)(void *opaque, size_t new_rate); +typedef size_t (QEMUFileGetRateLimit)(void *opaque); + +You can use any internal state that you need using the opaque void * +pointer that is passed to all functions. + +The rate limiting functions are used to limit the bandwidth used by +QEMU migration. + +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/pckbd.c) + +static const VMStateDescription vmstate_kbd = { + .name = "pckbd", + .version_id = 3, + .minimum_version_id = 3, + .minimum_version_id_old = 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 +hw/hw.h. + +=== More about versions == + +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. + +So, VMState is able to read versions from minimum_version_id to +version_id. And the function load_state_old() 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. + +=== 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. + +- void (*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 no 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; +} + +const VMStateDescription vmstate_ide_drive_pio_state = { + .name = "ide_drive/pio_state", + .version_id = 1, + .minimum_version_id = 1, + .minimum_version_id_old = 1, + .pre_save = ide_drive_pio_pre_save, + .post_load = ide_drive_pio_post_load, + .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, + .minimum_version_id_old = 0, + .post_load = ide_drive_post_load, + .fields = (VMStateField []) { + .... several fields .... + VMSTATE_END_OF_LIST() + }, + .subsections = (VMStateSubsection []) { + { + .vmsd = &vmstate_ide_drive_pio_state, + .needed = ide_drive_pio_state_needed, + }, { + /* empty */ + } + } +}; + +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. |