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= Migration =
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QEMU has code to load/save the state of the guest that it is running.
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These are two complementary operations.  Saving the state just does
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that, saves the state for each device that the guest is running.
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Restoring a guest is just the opposite operation: we need to load the
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state of each device.
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For this to work, QEMU has to be launched with the same arguments the
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two times.  I.e. it can only restore the state in one guest that has
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the same devices that the one it was saved (this last requirement can
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be relaxed a bit, but for now we can consider that configuration has
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to be exactly the same).
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Once that we are able to save/restore a guest, a new functionality is
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requested: migration.  This means that QEMU is able to start in one
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machine and being "migrated" to another machine.  I.e. being moved to
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another machine.
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Next was the "live migration" functionality.  This is important
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because some guests run with a lot of state (specially RAM), and it
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can take a while to move all state from one machine to another.  Live
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migration allows the guest to continue running while the state is
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transferred.  Only while the last part of the state is transferred has
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the guest to be stopped.  Typically the time that the guest is
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unresponsive during live migration is the low hundred of milliseconds
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(notice that this depends on a lot of things).
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=== Types of migration ===
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Now that we have talked about live migration, there are several ways
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to do migration:
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- tcp migration: do the migration using tcp sockets
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- unix migration: do the migration using unix sockets
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- exec migration: do the migration using the stdin/stdout through a process.
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- fd migration: do the migration using an file descriptor that is
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  passed to QEMU.  QEMU doesn't care how this file descriptor is opened.
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All these four migration protocols use the same infrastructure to
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save/restore state devices.  This infrastructure is shared with the
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savevm/loadvm functionality.
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=== State Live Migration ==
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This is used for RAM and block devices.  It is not yet ported to vmstate.
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<Fill more information here>
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=== What is the common infrastructure ===
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QEMU uses a QEMUFile abstraction to be able to do migration.  Any type
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of migration that wants to use QEMU infrastructure has to create a
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QEMUFile with:
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QEMUFile *qemu_fopen_ops(void *opaque,
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                         QEMUFilePutBufferFunc *put_buffer,
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                         QEMUFileGetBufferFunc *get_buffer,
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                         QEMUFileCloseFunc *close,
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                         QEMUFileRateLimit *rate_limit,
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                         QEMUFileSetRateLimit *set_rate_limit,
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                         QEMUFileGetRateLimit *get_rate_limit);
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The functions have the following functionality:
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This function writes a chunk of data to a file at the given position.
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The pos argument can be ignored if the file is only used for
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streaming.  The handler should try to write all of the data it can.
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typedef int (QEMUFilePutBufferFunc)(void *opaque, const uint8_t *buf,
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                                    int64_t pos, int size);
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Read a chunk of data from a file at the given position.  The pos argument
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can be ignored if the file is only be used for streaming.  The number of
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bytes actually read should be returned.
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typedef int (QEMUFileGetBufferFunc)(void *opaque, uint8_t *buf,
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                                    int64_t pos, int size);
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Close a file and return an error code.
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typedef int (QEMUFileCloseFunc)(void *opaque);
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Called to determine if the file has exceeded its bandwidth allocation.  The
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bandwidth capping is a soft limit, not a hard limit.
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typedef int (QEMUFileRateLimit)(void *opaque);
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Called to change the current bandwidth allocation. This function must return
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the new actual bandwidth. It should be new_rate if everything goes OK, and
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the old rate otherwise.
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typedef size_t (QEMUFileSetRateLimit)(void *opaque, size_t new_rate);
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typedef size_t (QEMUFileGetRateLimit)(void *opaque);
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You can use any internal state that you need using the opaque void *
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pointer that is passed to all functions.
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The rate limiting functions are used to limit the bandwidth used by
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QEMU migration.
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The important functions for us are put_buffer()/get_buffer() that
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allow to write/read a buffer into the QEMUFile.
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=== How to save the state of one device ==
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The state of a device is saved using intermediate buffers.  There are
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some helper functions to assist this saving.
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There is a new concept that we have to explain here: device state
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version.  When we migrate a device, we save/load the state as a series
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of fields.  Some times, due to bugs or new functionality, we need to
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change the state to store more/different information.  We use the
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version to identify each time that we do a change.  Each version is
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associated with a series of fields saved.  The save_state always saves
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the state as the newer version.  But load_state sometimes is able to
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load state from an older version.
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 === Legacy way ===
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This way is going to disappear as soon as all current users are ported to VMSTATE.
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Each device has to register two functions, one to save the state and
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another to load the state back.
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int register_savevm(DeviceState *dev,
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                    const char *idstr,
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                    int instance_id,
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                    int version_id,
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                    SaveStateHandler *save_state,
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                    LoadStateHandler *load_state,
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                    void *opaque);
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typedef void SaveStateHandler(QEMUFile *f, void *opaque);
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typedef int LoadStateHandler(QEMUFile *f, void *opaque, int version_id);
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The important functions for the device state format are the save_state
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and load_state.  Notice that load_state receives a version_id
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parameter to know what state format is receiving.  save_state doesn't
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have a version_id parameter because it always uses the latest version.
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=== VMState ===
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The legacy way of saving/loading state of the device had the problem
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that we have to maintain two functions in sync.  If we did one change
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in one of them and not in the other, we would get a failed migration.
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VMState changed the way that state is saved/loaded.  Instead of using
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a function to save the state and another to load it, it was changed to
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a declarative way of what the state consisted of.  Now VMState is able
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to interpret that definition to be able to load/save the state.  As
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the state is declared only once, it can't go out of sync in the
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save/load functions.
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An example (from hw/pckbd.c)
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static const VMStateDescription vmstate_kbd = {
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    .name = "pckbd",
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    .version_id = 3,
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    .minimum_version_id = 3,
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    .minimum_version_id_old = 3,
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    .fields      = (VMStateField []) {
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        VMSTATE_UINT8(write_cmd, KBDState),
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        VMSTATE_UINT8(status, KBDState),
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        VMSTATE_UINT8(mode, KBDState),
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        VMSTATE_UINT8(pending, KBDState),
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        VMSTATE_END_OF_LIST()
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    }
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};
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We are declaring the state with name "pckbd".
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The version_id is 3, and the fields are 4 uint8_t in a KBDState structure.
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We registered this with:
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    vmstate_register(NULL, 0, &vmstate_kbd, s);
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Note: talk about how vmstate <-> qdev interact, and what the instance ids mean.
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You can search for VMSTATE_* macros for lots of types used in QEMU in
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hw/hw.h.
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=== More about versions ==
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You can see that there are several version fields:
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- version_id: the maximum version_id supported by VMState for that device.
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- minimum_version_id: the minimum version_id that VMState is able to understand
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  for that device.
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- minimum_version_id_old: For devices that were not able to port to vmstate, we can
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  assign a function that knows how to read this old state.
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So, VMState is able to read versions from minimum_version_id to
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version_id.  And the function load_state_old() is able to load state
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from minimum_version_id_old to minimum_version_id.  This function is
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deprecated and will be removed when no more users are left.
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===  Massaging functions ===
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Sometimes, it is not enough to be able to save the state directly
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from one structure, we need to fill the correct values there.  One
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example is when we are using kvm.  Before saving the cpu state, we
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need to ask kvm to copy to QEMU the state that it is using.  And the
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opposite when we are loading the state, we need a way to tell kvm to
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load the state for the cpu that we have just loaded from the QEMUFile.
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The functions to do that are inside a vmstate definition, and are called:
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- int (*pre_load)(void *opaque);
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  This function is called before we load the state of one device.
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- int (*post_load)(void *opaque, int version_id);
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  This function is called after we load the state of one device.
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- void (*pre_save)(void *opaque);
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  This function is called before we save the state of one device.
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Example: You can look at hpet.c, that uses the three function to
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         massage the state that is transferred.
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=== Subsections ===
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The use of version_id allows to be able to migrate from older versions
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to newer versions of a device.  But not the other way around.  This
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makes very complicated to fix bugs in stable branches.  If we need to
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add anything to the state to fix a bug, we have to disable migration
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to older versions that don't have that bug-fix (i.e. a new field).
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But sometimes, that bug-fix is only needed sometimes, not always.  For
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instance, if the device is in the middle of a DMA operation, it is
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using a specific functionality, ....
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It is impossible to create a way to make migration from any version to
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any other version to work.  But we can do better than only allowing
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migration from older versions no newer ones.  For that fields that are
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only needed sometimes, we add the idea of subsections.  A subsection
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is "like" a device vmstate, but with a particularity, it has a Boolean
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function that tells if that values are needed to be sent or not.  If
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this functions returns false, the subsection is not sent.
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On the receiving side, if we found a subsection for a device that we
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don't understand, we just fail the migration.  If we understand all
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the subsections, then we load the state with success.
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One important note is that the post_load() function is called "after"
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loading all subsections, because a newer subsection could change same
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value that it uses.
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Example:
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static bool ide_drive_pio_state_needed(void *opaque)
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{
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    IDEState *s = opaque;
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    return (s->status & DRQ_STAT) != 0;
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}
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const VMStateDescription vmstate_ide_drive_pio_state = {
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    .name = "ide_drive/pio_state",
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    .version_id = 1,
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    .minimum_version_id = 1,
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    .minimum_version_id_old = 1,
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    .pre_save = ide_drive_pio_pre_save,
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    .post_load = ide_drive_pio_post_load,
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    .fields      = (VMStateField []) {
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        VMSTATE_INT32(req_nb_sectors, IDEState),
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        VMSTATE_VARRAY_INT32(io_buffer, IDEState, io_buffer_total_len, 1,
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                             vmstate_info_uint8, uint8_t),
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        VMSTATE_INT32(cur_io_buffer_offset, IDEState),
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        VMSTATE_INT32(cur_io_buffer_len, IDEState),
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        VMSTATE_UINT8(end_transfer_fn_idx, IDEState),
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        VMSTATE_INT32(elementary_transfer_size, IDEState),
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        VMSTATE_INT32(packet_transfer_size, IDEState),
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        VMSTATE_END_OF_LIST()
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    }
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};
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const VMStateDescription vmstate_ide_drive = {
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    .name = "ide_drive",
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    .version_id = 3,
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    .minimum_version_id = 0,
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    .minimum_version_id_old = 0,
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    .post_load = ide_drive_post_load,
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    .fields      = (VMStateField []) {
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        .... several fields ....
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        VMSTATE_END_OF_LIST()
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    },
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    .subsections = (VMStateSubsection []) {
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        {
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            .vmsd = &vmstate_ide_drive_pio_state,
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            .needed = ide_drive_pio_state_needed,
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        }, {
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            /* empty */
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        }
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    }
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};
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Here we have a subsection for the pio state.  We only need to
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save/send this state when we are in the middle of a pio operation
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(that is what ide_drive_pio_state_needed() checks).  If DRQ_STAT is
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not enabled, the values on that fields are garbage and don't need to
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be sent.