[Qemu-devel] [PATCH v2 43/43] Start documenting how postcopy works.

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 docs/migration.txt | 150 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 150 insertions(+)

diff --git a/docs/migration.txt b/docs/migration.txt
index 0492a45..fec2d46 100644
--- a/docs/migration.txt
+++ b/docs/migration.txt
@@ -294,3 +294,153 @@ 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 comms; 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 fd_handler on main 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)
+
+Opening the return path generally sets the fd to be non-blocking so that a
+failed destination can't block the source; and since the non-blockingness seems
+to follow both directions it does alter the semantics of the forward path.
+
+= Postcopy =
+'Postcopy' migration is a way to deal with migrations that refuse to converge;
+it's 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 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 passed the start-time threshold; here the destination
+          checks it's 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)
+
+Normal precopy now carries on as normal, until the point that the source
+hits the start-time threshold and transitions to postcopy.  The source
+stops it's CPUs and transmits a 'discard bitmap' indicating pages that
+have been previously sent but are now dirty again and hence are out of
+date on the destination.
+
+The migration stream now contains a 'package' containing it's own chunk
+of migration stream, followed by a return to a normal stream containing
+page data.  The package (sent as CMD_PACKAGED) contains the commands to
+cycle the states on the destination, followed by all of the device
+state excluding RAM.  This lets the destination request pages from the
+source in parallel with loading device state, this is required since
+some devices (virtio) access guest memory during device initialisation.
+
+  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).
+
+The package now contains all the remaining state data and the command
+to transition to the next state.
+
+  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).
+
+Page data is sent from the source to the destination both as part
+of a linear scan (like normal migration), and received by the 'listen thread',
+When the destination tries to use a page it hasn't got, it requests
+it from the source (down the return path) and the source sends this
+page in the same stream.  When the source has transmitted all pages
+it sends a POSTCOPY_RAM_END command to transition to
+
+  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 it's
+contents are not-consistent as a local view of what's been sent since it's
+only got the masked result.
+
+=== 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)
+
-- 
1.9.3