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Revision dda5336e docs/migration.txt

     = Migration =
     QEMU has code to load/save the state of the guest that it is running.
     This are two complementary operations.  Saving the state just does
     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 launch with the same arguments the
     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 have
     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 other machine.  I.e. being moved to
     other machine.
     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
-...
     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 lot of things).
     (notice that this depends on a lot of things).
     === Types of migration ===
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     - 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 don't cares how this file descriptor is opened.
       passed to QEMU.  QEMU doesn't care how this file descriptor is opened.
     All this four migration protocols use the same infrastructure to
     All these four migration protocols use the same infrastructure to
     save/restore state devices.  This infrastructure is shared with the
     savevm/loadvm functionality.
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     === What is the common infrastructure ===
     QEMU uses a QEMUFile abstraction to be able to do migration.  Any type
     of migration that what to use QEMU infrastructure has to create a
     of migration that wants to use QEMU infrastructure has to create a
     QEMUFile with:
     QEMUFile *qemu_fopen_ops(void *opaque,
     	 		 QEMUFilePutBufferFunc *put_buffer,
                              QEMUFilePutBufferFunc *put_buffer,
                              QEMUFileGetBufferFunc *get_buffer,
                              QEMUFileCloseFunc *close,
                              QEMUFileRateLimit *rate_limit,
                              QEMUFileSetRateLimit *set_rate_limit,
     			 QEMUFileGetRateLimit *get_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 being used for
     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,
-...
     typedef int (QEMUFileGetBufferFunc)(void *opaque, uint8_t *buf,
                                         int64_t pos, int size);
     Close a file and return an error code
     Close a file and return an error code.
     typedef int (QEMUFileCloseFunc)(void *opaque);
     Called to determine if the file has exceeded it's bandwidth allocation.  The
     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
     the old rate otherwise.
     typedef size_t (QEMUFileSetRateLimit)(void *opaque, size_t new_rate);
     typedef size_t (QEMUFileGetRateLimit)(void *opaque);
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     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 save
     the state as the newer version.  But load_state some times is able to
     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 ===
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     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 don't
     have a version_id parameter because it uses always the latest version.
     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 in sync two functions.  If we did one change
     in one of them and not on the other, we got a failed migration.
     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
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         vmstate_register(NULL, 0, &vmstate_kbd, s);
     Note: talk about how vmstate <-> qdev interact, and what the instance id's mean.
     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.
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     You can see that there are several version fields:
     - version_id: the maximum version_id supported by VMState for that device
     - 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
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     ===  Massaging functions ===
     Some times, it is not enough to be able to save the state directly
     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
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     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 some time, that bug-fix is only needed sometimes, not always.  For
     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 that only allowing
     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
     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.
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         .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_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),

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Revision dda5336e docs/migration.txt