+
+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 (prior to the start of migration):
+
+migrate_set_capability x-postcopy-ram on
+
+The migration will still start in precopy mode, however 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.
+
+=== 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.
+
+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 (into a QEMUSizedBuffer), 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->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.
+ (Triggered by reception of POSTCOPY_ADVISE command)
+
+ 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 'sent 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 'sent map' is used for the transition to postcopy. It is a bitmap that
+has a bit set whenever a page is sent to the destination, however during
+the transition to postcopy mode it is masked against the migration bitmap
+(sentmap &= migrationbitmap) to generate a bitmap recording pages that
+have been previously been sent but are now dirty again. This masked
+sentmap is sent to the destination which discards those now dirty pages
+before starting the CPUs.
+
+Note that the contents of the sentmap 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.