From: "Dr. David Alan Gilbert" <dgilb...@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilb...@redhat.com>
---
docs/migration.txt | 188 +++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 188 insertions(+)
diff --git a/docs/migration.txt b/docs/migration.txt
index 0492a45..7f0fdc4 100644
--- a/docs/migration.txt
+++ b/docs/migration.txt
@@ -294,3 +294,191 @@ 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.
+
+= 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;
+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 automatically made to precopy.
+
+=== 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 postcopy,
+except for the addition of a 'postcopy advise' command at the beginning to
+let the destination know that postcopy might happen. When postcopy starts
+the source sends the page discard data and then forms the 'package' containing:
+
+ Command: 'postcopy ram 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 ram 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 ram 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_ram_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_RAM_ADVISE command)
+
+ Listen: The first command in the package, POSTCOPY_RAM_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_RAM_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 once in postcopy mode, the sent map is still updated; however,
+its contents are not necessarily consistent with the pages already sent
+due to the masking with the migration bitmap.
+
+=== Destination side page maps ===
+
+(Needs to be changed so we can update both easily - at the moment updates are
done
+ with a lock)
+The destination keeps a 'requested map' and a 'received map'.
+Both maps are initially 0, as pages are received the bits are set in
'received map'.
+Incoming requests from the kernel cause the bit to be set in the 'requested
map'.
+When a page is received that is marked as 'requested' the kernel is notified.
+If the kernel requests a page that has already been 'received' the kernel is
notified
+without re-requesting.
+
+This leads to three valid page states:
+page states:
+ missing (!rc,!rq) - page not yet received or requested
+ received (rc,!rq) - Page received
+ requested (!rc,rq) - page requested but not yet received
+
+state transitions:
+ received -> missing (only during setup/discard)
+
+ missing -> received (normal incoming page)
+ requested -> received (incoming page previously requested)
+ missing -> requested (userfault request)
+
