qemu/docs/devel/migration.rst

=========
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 (both source and
destination) 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.

Blocktime is a postcopy live migration metric, intended to show how
long the vCPU was in state of interruptable sleep due to pagefault.
That metric is calculated both for all vCPUs as overlapped value, and
separately for each vCPU. These values are calculated on destination
side.  To enable postcopy blocktime calculation, enter following
command on destination monitor:

``migrate_set_capability postcopy-blocktime on``

Postcopy blocktime can be retrieved by query-migrate qmp command.
postcopy-blocktime value of qmp command will show overlapped blocking
time for all vCPU, postcopy-vcpu-blocktime will show list of blocking
time per vCPU.

.. 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.

Postcopy with shared memory
---------------------------

Postcopy migration with shared memory needs explicit support from the other
processes that share memory and from QEMU. There are restrictions on the type of
memory that userfault can support shared.

The Linux kernel userfault support works on `/dev/shm` memory and on `hugetlbfs`
(although the kernel doesn't provide an equivalent to `madvise(MADV_DONTNEED)`
for hugetlbfs which may be a problem in some configurations).

The vhost-user code in QEMU supports clients that have Postcopy support,
and the `vhost-user-bridge` (in `tests/`) and the DPDK package have changes
to support postcopy.

The client needs to open a userfaultfd and register the areas
of memory that it maps with userfault.  The client must then pass the
userfaultfd back to QEMU together with a mapping table that allows
fault addresses in the clients address space to be converted back to
RAMBlock/offsets.  The client's userfaultfd is added to the postcopy
fault-thread and page requests are made on behalf of the client by QEMU.
QEMU performs 'wake' operations on the client's userfaultfd to allow it
to continue after a page has arrived.

.. note::
  There are two future improvements that would be nice:
    a) Some way to make QEMU ignorant of the addresses in the clients
       address space
    b) Avoiding the need for QEMU to perform ufd-wake calls after the
       pages have arrived

Retro-fitting postcopy to existing clients is possible:
  a) A mechanism is needed for the registration with userfault as above,
     and the registration needs to be coordinated with the phases of
     postcopy.  In vhost-user extra messages are added to the existing
     control channel.
  b) Any thread that can block due to guest memory accesses must be
     identified and the implication understood; for example if the
     guest memory access is made while holding a lock then all other
     threads waiting for that lock will also be blocked.