========
Postcopy
========

.. contents::

'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 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 interruptible 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_parameter`` is ignored (to avoid delaying requested pages that
  the destination is waiting for).

Postcopy internals
==================

State machine
-------------

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.

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.

Source side page bitmap
-----------------------

The 'migration bitmap' in postcopy is basically the same as in the precopy,
where each of the 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. Instead, dirty bits are cleared when the relevant
pages are sent during postcopy.

Postcopy features
=================

Postcopy recovery
-----------------

Comparing to precopy, postcopy is special on error handlings.  When any
error happens (in this case, mostly network errors), QEMU cannot easily
fail a migration because VM data resides in both source and destination
QEMU instances.  On the other hand, when issue happens QEMU on both sides
will go into a paused state.  It'll need a recovery phase to continue a
paused postcopy migration.

The recovery phase normally contains a few steps:

  - When network issue occurs, both QEMU will go into **POSTCOPY_PAUSED**
    migration state.

  - When the network is recovered (or a new network is provided), the admin
    can setup the new channel for migration using QMP command
    'migrate-recover' on destination node, preparing for a resume.

  - On source host, the admin can continue the interrupted postcopy
    migration using QMP command 'migrate' with resume=true flag set.
    Source QEMU will go into **POSTCOPY_RECOVER_SETUP** state trying to
    re-establish the channels.

  - When both sides of QEMU successfully reconnect using a new or fixed up
    channel, they will go into **POSTCOPY_RECOVER** state, some handshake
    procedure will be needed to properly synchronize the VM states between
    the two QEMUs to continue the postcopy migration.  For example, there
    can be pages sent right during the window when the network is
    interrupted, then the handshake will guarantee pages lost in-flight
    will be resent again.

  - After a proper handshake synchronization, QEMU will continue the
    postcopy migration on both sides and go back to **POSTCOPY_ACTIVE**
    state.  Postcopy migration will continue.

During a paused postcopy migration, the VM can logically still continue
running, and it will not be impacted from any page access to pages that
were already migrated to destination VM before the interruption happens.
However, if any of the missing pages got accessed on destination VM, the VM
thread will be halted waiting for the page to be migrated, it means it can
be halted until the recovery is complete.

The impact of accessing missing pages can be relevant to different
configurations of the guest.  For example, when with async page fault
enabled, logically the guest can proactively schedule out the threads
accessing missing pages.

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.

Postcopy preemption mode
------------------------

Postcopy preempt is a new capability introduced in 8.0 QEMU release, it
allows urgent pages (those got page fault requested from destination QEMU
explicitly) to be sent in a separate preempt channel, rather than queued in
the background migration channel.  Anyone who cares about latencies of page
faults during a postcopy migration should enable this feature.  By default,
it's not enabled.