Add postcopy documentation
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> Reviewed-by: Amit Shah <amit.shah@redhat.com> Reviewed-by: Juan Quintela <quintela@redhat.com> Signed-off-by: Juan Quintela <quintela@redhat.com>
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@ -291,3 +291,194 @@ save/send this state when we are in the middle of a pio operation
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(that is what ide_drive_pio_state_needed() checks). If DRQ_STAT is
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not enabled, the values on that fields are garbage and don't need to
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be sent.
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= Return path =
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In most migration scenarios there is only a single data path that runs
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from the source VM to the destination, typically along a single fd (although
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possibly with another fd or similar for some fast way of throwing pages across).
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However, some uses need two way communication; in particular the Postcopy
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destination needs to be able to request pages on demand from the source.
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For these scenarios there is a 'return path' from the destination to the source;
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qemu_file_get_return_path(QEMUFile* fwdpath) gives the QEMUFile* for the return
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path.
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Source side
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Forward path - written by migration thread
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Return path - opened by main thread, read by return-path thread
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Destination side
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Forward path - read by main thread
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Return path - opened by main thread, written by main thread AND postcopy
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thread (protected by rp_mutex)
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= Postcopy =
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'Postcopy' migration is a way to deal with migrations that refuse to converge
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(or take too long to converge) its plus side is that there is an upper bound on
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the amount of migration traffic and time it takes, the down side is that during
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the postcopy phase, a failure of *either* side or the network connection causes
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the guest to be lost.
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In postcopy the destination CPUs are started before all the memory has been
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transferred, and accesses to pages that are yet to be transferred cause
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a fault that's translated by QEMU into a request to the source QEMU.
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Postcopy can be combined with precopy (i.e. normal migration) so that if precopy
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doesn't finish in a given time the switch is made to postcopy.
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=== Enabling postcopy ===
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To enable postcopy, issue this command on the monitor prior to the
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start of migration:
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migrate_set_capability x-postcopy-ram on
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The normal commands are then used to start a migration, which is still
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started in precopy mode. Issuing:
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migrate_start_postcopy
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will now cause the transition from precopy to postcopy.
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It can be issued immediately after migration is started or any
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time later on. Issuing it after the end of a migration is harmless.
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Note: During the postcopy phase, the bandwidth limits set using
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migrate_set_speed is ignored (to avoid delaying requested pages that
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the destination is waiting for).
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=== Postcopy device transfer ===
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Loading of device data may cause the device emulation to access guest RAM
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that may trigger faults that have to be resolved by the source, as such
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the migration stream has to be able to respond with page data *during* the
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device load, and hence the device data has to be read from the stream completely
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before the device load begins to free the stream up. This is achieved by
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'packaging' the device data into a blob that's read in one go.
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Source behaviour
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Until postcopy is entered the migration stream is identical to normal
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precopy, except for the addition of a 'postcopy advise' command at
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the beginning, to tell the destination that postcopy might happen.
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When postcopy starts the source sends the page discard data and then
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forms the 'package' containing:
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Command: 'postcopy listen'
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The device state
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A series of sections, identical to the precopy streams device state stream
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containing everything except postcopiable devices (i.e. RAM)
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Command: 'postcopy run'
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The 'package' is sent as the data part of a Command: 'CMD_PACKAGED', and the
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contents are formatted in the same way as the main migration stream.
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During postcopy the source scans the list of dirty pages and sends them
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to the destination without being requested (in much the same way as precopy),
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however when a page request is received from the destination, the dirty page
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scanning restarts from the requested location. This causes requested pages
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to be sent quickly, and also causes pages directly after the requested page
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to be sent quickly in the hope that those pages are likely to be used
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by the destination soon.
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Destination behaviour
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Initially the destination looks the same as precopy, with a single thread
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reading the migration stream; the 'postcopy advise' and 'discard' commands
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are processed to change the way RAM is managed, but don't affect the stream
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processing.
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------------------------------------------------------------------------------
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1 2 3 4 5 6 7
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main -----DISCARD-CMD_PACKAGED ( LISTEN DEVICE DEVICE DEVICE RUN )
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thread | |
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| (page request)
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| \___
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v \
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listen thread: --- page -- page -- page -- page -- page --
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a b c
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------------------------------------------------------------------------------
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On receipt of CMD_PACKAGED (1)
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All the data associated with the package - the ( ... ) section in the
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diagram - is read into memory (into a QEMUSizedBuffer), and the main thread
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recurses into qemu_loadvm_state_main to process the contents of the package (2)
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which contains commands (3,6) and devices (4...)
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On receipt of 'postcopy listen' - 3 -(i.e. the 1st command in the package)
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a new thread (a) is started that takes over servicing the migration stream,
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while the main thread carries on loading the package. It loads normal
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background page data (b) but if during a device load a fault happens (5) the
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returned page (c) is loaded by the listen thread allowing the main threads
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device load to carry on.
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The last thing in the CMD_PACKAGED is a 'RUN' command (6) letting the destination
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CPUs start running.
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At the end of the CMD_PACKAGED (7) the main thread returns to normal running behaviour
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and is no longer used by migration, while the listen thread carries
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on servicing page data until the end of migration.
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=== Postcopy states ===
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Postcopy moves through a series of states (see postcopy_state) from
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ADVISE->DISCARD->LISTEN->RUNNING->END
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Advise: Set at the start of migration if postcopy is enabled, even
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if it hasn't had the start command; here the destination
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checks that its OS has the support needed for postcopy, and performs
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setup to ensure the RAM mappings are suitable for later postcopy.
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The destination will fail early in migration at this point if the
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required OS support is not present.
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(Triggered by reception of POSTCOPY_ADVISE command)
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Discard: Entered on receipt of the first 'discard' command; prior to
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the first Discard being performed, hugepages are switched off
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(using madvise) to ensure that no new huge pages are created
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during the postcopy phase, and to cause any huge pages that
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have discards on them to be broken.
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Listen: The first command in the package, POSTCOPY_LISTEN, switches
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the destination state to Listen, and starts a new thread
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(the 'listen thread') which takes over the job of receiving
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pages off the migration stream, while the main thread carries
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on processing the blob. With this thread able to process page
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reception, the destination now 'sensitises' the RAM to detect
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any access to missing pages (on Linux using the 'userfault'
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system).
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Running: POSTCOPY_RUN causes the destination to synchronise all
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state and start the CPUs and IO devices running. The main
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thread now finishes processing the migration package and
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now carries on as it would for normal precopy migration
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(although it can't do the cleanup it would do as it
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finishes a normal migration).
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End: The listen thread can now quit, and perform the cleanup of migration
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state, the migration is now complete.
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=== Source side page maps ===
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The source side keeps two bitmaps during postcopy; 'the migration bitmap'
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and 'unsent map'. The 'migration bitmap' is basically the same as in
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the precopy case, and holds a bit to indicate that page is 'dirty' -
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i.e. needs sending. During the precopy phase this is updated as the CPU
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dirties pages, however during postcopy the CPUs are stopped and nothing
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should dirty anything any more.
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The 'unsent map' is used for the transition to postcopy. It is a bitmap that
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has a bit cleared whenever a page is sent to the destination, however during
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the transition to postcopy mode it is combined with the migration bitmap
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to form a set of pages that:
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a) Have been sent but then redirtied (which must be discarded)
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b) Have not yet been sent - which also must be discarded to cause any
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transparent huge pages built during precopy to be broken.
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Note that the contents of the unsentmap are sacrificed during the calculation
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of the discard set and thus aren't valid once in postcopy. The dirtymap
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is still valid and is used to ensure that no page is sent more than once. Any
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request for a page that has already been sent is ignored. Duplicate requests
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such as this can happen as a page is sent at about the same time the
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destination accesses it.
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