Re: [Qemu-devel] [PATCH v4 30/32] migration: delay the postcopy-active state switch

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

* Peter Xu (peterx@redhat.com) wrote:
> Switch the state until we try to start the VM on destination side.  The
> problem is that without doing this we may have a very small window that
> we'll be in such a state:
> 
> - dst VM is in postcopy-active state,
> - main thread is handling MIG_CMD_PACKAGED message, which loads all the
>   device states,
> - ram load thread is reading memory data from source.
> 
> Then if we failed at this point when reading the migration stream we'll
> also switch to postcopy-paused state, but that is not what we want.  If
> device states failed to load, we should fail the migration directly
> instead of pause.
> 
> Postponing the state switch to the point when we have already loaded the
> devices' states and been ready to start running destination VM.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>

If it's the only way, then this is OK; but I'd prefer you use a separate
flag somewhere to let you know this, because this means that
POSTCOPY_ACTIVE on the destination happens at a different point that it
does on the source (and changing it on the source I think will break
lots of things).
Can't you use the PostcopyState value and check if it's in
POSTCOPY_INCOMING_RUNNING?

Dave

> ---
>  migration/savevm.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/migration/savevm.c b/migration/savevm.c
> index bc87b0e5b1..3bc792e320 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -1584,8 +1584,6 @@ static void *postcopy_ram_listen_thread(void *opaque)
>      QEMUFile *f = mis->from_src_file;
>      int load_res;
>  
> -    migrate_set_state(&mis->state, MIGRATION_STATUS_ACTIVE,
> -                                   MIGRATION_STATUS_POSTCOPY_ACTIVE);
>      qemu_sem_post(&mis->listen_thread_sem);
>      trace_postcopy_ram_listen_thread_start();
>  
> @@ -1748,6 +1746,14 @@ static int loadvm_postcopy_handle_run(MigrationIncomingState *mis)
>          return -1;
>      }
>  
> +    /*
> +     * Declare that we are in postcopy now.  We should already have
> +     * all the device states loaded ready when reach here, and also
> +     * the ram load thread running.
> +     */
> +    migrate_set_state(&mis->state, MIGRATION_STATUS_ACTIVE,
> +                                   MIGRATION_STATUS_POSTCOPY_ACTIVE);
> +
>      data = g_new(HandleRunBhData, 1);
>      data->bh = qemu_bh_new(loadvm_postcopy_handle_run_bh, data);
>      qemu_bh_schedule(data->bh);
> -- 
> 2.13.6
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK