Re: [Qemu-devel] [PATCH v7 22/42] migrate_start_postcopy: Command to trigger transition to postcopy

[Dr. David Alan Gilbert]

* Juan Quintela (address@hidden) wrote:
> "Dr. David Alan Gilbert (git)" <address@hidden> wrote:
> > From: "Dr. David Alan Gilbert" <address@hidden>
> >
> > Once postcopy is enabled (with migrate_set_capability), the migration
> > will still start on precopy mode.  To cause a transition into postcopy
> > the:
> >
> >   migrate_start_postcopy
> >
> > command must be issued.  Postcopy will start sometime after this
> > (when it's next checked in the migration loop).
> >
> > Issuing the command before migration has started will error,
> > and issuing after it has finished is ignored.
> >
> > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> > Reviewed-by: Eric Blake <address@hidden>
> 
> > diff --git a/include/migration/migration.h b/include/migration/migration.h
> > index a5951ac..e973490 100644
> > --- a/include/migration/migration.h
> > +++ b/include/migration/migration.h
> > @@ -111,6 +111,9 @@ struct MigrationState
> >      int64_t xbzrle_cache_size;
> >      int64_t setup_time;
> >      int64_t dirty_sync_count;
> > +
> > +    /* Flag set once the migration has been asked to enter postcopy */
> > +    bool start_postcopy;
> >  };
> >  
> >  void process_incoming_migration(QEMUFile *f);
> > diff --git a/migration/migration.c b/migration/migration.c
> > index e77b8b4..6fc47f9 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -465,6 +465,28 @@ void qmp_migrate_set_parameters(bool 
> > has_compress_level,
> >      }
> >  }
> >  
> > +void qmp_migrate_start_postcopy(Error **errp)
> > +{
> > +    MigrationState *s = migrate_get_current();
> > +
> > +    if (!migrate_postcopy_ram()) {
> > +        error_setg(errp, "Enable postcopy with migration_set_capability 
> > before"
> > +                         " the start of migration");
> > +        return;
> > +    }
> > +
> > +    if (s->state == MIGRATION_STATUS_NONE) {
> 
> I would claim that this check should be:
> 
>     if (s->state != MIGRATION_STATUS_ACTIVE) {
> ??
> 
> FAILED, COMPLETED, CANCELL* don't make sense, right?

What I'm trying to catch here is people doing:
     migrate_start_postcopy
     migrate tcp:pppp:whereever

  which wont work, because migrate_init reinitialises
the flag that start previously set.

However, I also don't want to create a race, since what you do is
typically:
     migrate  tcp:pppp:whereever
   <wait some time, get bored>
     migrate_start_postcopy

if you're unlucky, and the migration finishes just
at the same time you do the migrate_start_postcopy, do you
want migrate_start_postcopy to fail?  My guess was it
was best for it not to fail, in this case.

Dave

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