Re: [Qemu-devel] [PATCH] migrtion: define MigrationState/MigrationIncomingState.state as MigrationStatus

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

* Dr. David Alan Gilbert (dgilbert@redhat.com) wrote:
> * Wei Yang (richard.weiyang@gmail.com) wrote:
> > On Mon, Aug 19, 2019 at 12:26:32PM +0100, Dr. David Alan Gilbert wrote:
> > >* Wei Yang (richardw.yang@linux.intel.com) wrote:
> > >> No functional change. Add default case to fix warning.
> > >
> > >I think the problem with this is that migrate_set_state uses an
> > >atomic_cmpxchg and so we have to be careful that the type we use
> > >is compatible with that.
> > >MigrationStatus is an enum and I think compilers are allowed to
> > >choose the types of that;  so I'm not sure we're guaranteed
> > >that an enum is always OK for the atomic_cmpxchg, and if it is
> > 
> > Took a look into the definition of atomic_cmpxchg, which finally calls
> > 
> >   * __atomic_compare_exchange_n for c++11
> >   * __sync_val_compare_and_swap
> > 
> > Both of them take two pointers to compare and exchange its content.
> > 
> > Per C99 standard, http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf,
> > it mentioned:
> > 
> >   Each enumerated type shall be compatible with char, a signed integer type,
> >   or an unsigned integer type. The choice of type is implementation-defined,
> >   but shall be capable of representing the values of all the members of the
> >   enumeration.
> > 
> > Based on this, I think atomic_cmpxchg should work fine with enum.
> 
> Hmm OK, but I'd need you to test n some 32bit and other comp=ilers etc;
> make sure it works on 32bit, clang, gcc, 32bit ARM etc - because
> otherwise I'm going to have to worry about checking all those.

Actually since QEMU is defined to be C99, the test is to check with some
C99 compilers; eblake assures me that in theory C11 should be OK,
but C99 and our atomics are more of an interesting question.

Dave
> Dave
> 
> > >then we also might have to make the old_state and new_state
> > >variables match.
> > >
> > 
> > You are right.
> > >Dave
> > >
> > >> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> > >> ---
> > >>  migration/migration.c | 8 +++++++-
> > >>  migration/migration.h | 6 +++---
> > >>  2 files changed, 10 insertions(+), 4 deletions(-)
> > >> 
> > >> diff --git a/migration/migration.c b/migration/migration.c
> > >> index 2865ae3fa9..0fd2364961 100644
> > >> --- a/migration/migration.c
> > >> +++ b/migration/migration.c
> > >> @@ -946,6 +946,8 @@ static void fill_source_migration_info(MigrationInfo *info)
> > >>      case MIGRATION_STATUS_CANCELLED:
> > >>          info->has_status = true;
> > >>          break;
> > >> +    default:
> > >> +        return;
> > >>      }
> > >>      info->status = s->state;
> > >>  }
> > >> @@ -1054,6 +1056,8 @@ static void fill_destination_migration_info(MigrationInfo *info)
> > >>          info->has_status = true;
> > >>          fill_destination_postcopy_migration_info(info);
> > >>          break;
> > >> +    default:
> > >> +        return;
> > >>      }
> > >>      info->status = mis->state;
> > >>  }
> > >> @@ -1446,7 +1450,7 @@ void qmp_migrate_start_postcopy(Error **errp)
> > >>  
> > >>  /* shared migration helpers */
> > >>  
> > >> -void migrate_set_state(int *state, int old_state, int new_state)
> > >> +void migrate_set_state(MigrationStatus *state, int old_state, int new_state)
> > >>  {
> > >>      assert(new_state < MIGRATION_STATUS__MAX);
> > >>      if (atomic_cmpxchg(state, old_state, new_state) == old_state) {
> > >> @@ -1683,6 +1687,8 @@ bool migration_is_idle(void)
> > >>          return false;
> > >>      case MIGRATION_STATUS__MAX:
> > >>          g_assert_not_reached();
> > >> +    default:
> > >> +        g_assert_not_reached();
> > >>      }
> > >>  
> > >>      return false;
> > >> diff --git a/migration/migration.h b/migration/migration.h
> > >> index 5e8f09c6db..418ee00053 100644
> > >> --- a/migration/migration.h
> > >> +++ b/migration/migration.h
> > >> @@ -65,7 +65,7 @@ struct MigrationIncomingState {
> > >>  
> > >>      QEMUBH *bh;
> > >>  
> > >> -    int state;
> > >> +    MigrationStatus state;
> > >>  
> > >>      bool have_colo_incoming_thread;
> > >>      QemuThread colo_incoming_thread;
> > >> @@ -151,7 +151,7 @@ struct MigrationState
> > >>      /* params from 'migrate-set-parameters' */
> > >>      MigrationParameters parameters;
> > >>  
> > >> -    int state;
> > >> +    MigrationStatus state;
> > >>  
> > >>      /* State related to return path */
> > >>      struct {
> > >> @@ -234,7 +234,7 @@ struct MigrationState
> > >>      bool decompress_error_check;
> > >>  };
> > >>  
> > >> -void migrate_set_state(int *state, int old_state, int new_state);
> > >> +void migrate_set_state(MigrationStatus *state, int old_state, int new_state);
> > >>  
> > >>  void migration_fd_process_incoming(QEMUFile *f);
> > >>  void migration_ioc_process_incoming(QIOChannel *ioc, Error **errp);
> > >> -- 
> > >> 2.19.1
> > >> 
> > >--
> > >Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> > 
> > -- 
> > Wei Yang
> > Help you, Help me
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK