Re: [Qemu-devel] [PATCH v3 03/47] Start documenting how postcopy works.

[Dr. David Alan Gilbert]

* Hongyang Yang (address@hidden) wrote:
> 
> 
> ??? 08/28/2014 11:03 PM, Dr. David Alan Gilbert (git) ??????:
> >From: "Dr. David Alan Gilbert" <address@hidden>
> >
> >Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> >---


> >+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.
> 
> I think you mean "automatically made to postcopy" here?

Thanks!

> >+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
> 
> A comma here?

Yes, thanks.

Dave

> 
> >+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)
> >+
> >
> 
> -- 
> Thanks,
> Yang.
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK