path: root/docs/migration.txt

= 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 ===

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.

===  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 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.