Re: [Qemu-devel] [PATCH v7 01/42] Start documenting how postcopy works.

[Dr. David Alan Gilbert]

* Amit Shah (address@hidden) wrote:
> On (Tue) 16 Jun 2015 [11:26:14], Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <address@hidden>
> > 
> > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> 
> Reviewed-by: Amit Shah <address@hidden>
> 
> A few minor comments:
> 
> > ---
> >  docs/migration.txt | 167 
> > +++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 167 insertions(+)
> > 
> > diff --git a/docs/migration.txt b/docs/migration.txt
> > index f6df4be..b4b93d1 100644
> > --- a/docs/migration.txt
> > +++ b/docs/migration.txt
> > @@ -291,3 +291,170 @@ 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;
> 
> (or take too long to converge)

Added.

> 
> > +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 made to postcopy.
> > +
> > +=== Enabling postcopy ===
> > +
> > +To enable postcopy (prior to the start of migration):
> 
> How about this instead:
> 
> "To enable postcopy, issue this command ont he monitor prior to the
> start of migration:"
> 
> Otherwise, there's ambiguity that there is some way to enable this
> after a precopy migration has started.

Done.

> > +
> > +migrate_set_capability x-postcopy-ram on
> > +
> > +The migration will still start in precopy mode, however issuing:
> 
> "A future migration will then start in precopy mode.  However,
> issuing:"
> 
> ?

Ah yes, I see it's ambiguous because it doesn't say you still need
to do the normal migration stuff to start migration;

I've changed it to:

The normal commands are then used to start a migration, which is still
started in precopy mode.  Issuing:

migrate_start_postcopy

will now cause the transition from precopy to postcopy.

> > +
> > +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)
> 
> Adding:
> 
> "This gives the destination a chance to fail early if postcopy is not
> possible."
> 
> ?

I added:
 "The destination will fail early in migration at this point if the
  required OS support is not present.  "


> > +
> > +  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).
> 
> indentation went off a bit

Fixed.

Thanks,

Dave

> 
> 
>               Amit
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK