Re: [PATCH] vfio/migration: Send migration event before device state transition

[Peter Xu <peterx@redhat.com>]

On Wed, Jan 28, 2026 at 12:03:46PM +0100, Cédric Le Goater wrote:
> +Peter, + Fabiano

Thanks, looks benign to me from migration POV. I have one question though,
and not sure if I'm the only one wondering..

> 
> On 1/28/26 11:51, Avihai Horon wrote:
> > Currently, VFIO device migration event is sent after the device state
> > transition has been completed. However, it may be useful to additionally
> > send a "prepare" event before the state transition, to notify users that
> > it's about to happen.
> > 
> > For example, in some cases with heavy resource utilization, stopping the
> > VFIO device may take a long time. In time-sensitive scenarios, the
> > management application that consumes the event may be notified about the
> > state transition too late.

Could there be more elaborations on the problem?  For example:

(1) What would the mgmt do when receiving the notification?  What would go
    wrong if the state notification will be very late?

(2) Why would a prepare message help this situation?

(3) Doc below says, the prepare message does not imply the event will be
    guaranteed to happen.  Would it confuse the mgmt?

Thanks,

> > 
> > To overcome this issue, send an additional "prepare" migration event
> > before the device state transition.
> > 
> > Signed-off-by: Avihai Horon <avihaih@nvidia.com>
> > ---
> >   qapi/vfio.json      | 33 +++++++++++++++++++++++++++++++++
> >   hw/vfio/migration.c | 18 +++++++++++++-----
> >   2 files changed, 46 insertions(+), 5 deletions(-)
> > 
> > diff --git a/qapi/vfio.json b/qapi/vfio.json
> > index a1a9c5b673..de41211f1d 100644
> > --- a/qapi/vfio.json
> > +++ b/qapi/vfio.json
> > @@ -66,3 +66,36 @@
> >         'qom-path': 'str',
> >         'device-state': 'QapiVfioMigrationState'
> >     } }
> > +
> > +##
> > +# @VFIO_MIGRATION_PREPARE:
> > +#
> > +# This event is emitted when a VFIO device migration state is about to
> > +# be changed.  Note that even if this event is received for state X,
> > +# the VFIO device may transition to a different state if the original
> > +# state transition to X failed.
> > +#
> > +# @device-id: The device's id, if it has one.
> > +#
> > +# @qom-path: The device's QOM path.
> > +#
> > +# @device-state: The new device migration state that is about to be
> > +#     changed.
> > +#
> > +# Since: 11.0
> > +#
> > +# .. qmp-example::
> > +#
> > +#     <- { "timestamp": { "seconds": 1713771323, "microseconds": 212268 },
> > +#          "event": "VFIO_MIGRATION_PREPARE",
> > +#          "data": {
> > +#              "device-id": "vfio_dev1",
> > +#              "qom-path": "/machine/peripheral/vfio_dev1",
> > +#              "device-state": "stop" } }
> > +##
> > +{ 'event': 'VFIO_MIGRATION_PREPARE',
> > +  'data': {
> > +      'device-id': 'str',
> > +      'qom-path': 'str',
> > +      'device-state': 'QapiVfioMigrationState'
> > +  } }
> > diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
> > index b4695030c7..9f887c148f 100644
> > --- a/hw/vfio/migration.c
> > +++ b/hw/vfio/migration.c
> > @@ -90,9 +90,11 @@ mig_state_to_qapi_state(enum vfio_device_mig_state state)
> >       }
> >   }
> > -static void vfio_migration_send_event(VFIODevice *vbasedev)
> > +static void vfio_migration_send_event(VFIODevice *vbasedev,
> > +                                      enum vfio_device_mig_state state,
> > +                                      bool prep)
> >   {
> > -    VFIOMigration *migration = vbasedev->migration;
> > +    QapiVfioMigrationState qapi_state;
> >       DeviceState *dev = vbasedev->dev;
> >       g_autofree char *qom_path = NULL;
> >       Object *obj;
> > @@ -105,9 +107,13 @@ static void vfio_migration_send_event(VFIODevice *vbasedev)
> >       obj = vbasedev->ops->vfio_get_object(vbasedev);
> >       g_assert(obj);
> >       qom_path = object_get_canonical_path(obj);
> > +    qapi_state = mig_state_to_qapi_state(state);
> > -    qapi_event_send_vfio_migration(
> > -        dev->id, qom_path, mig_state_to_qapi_state(migration->device_state));
> > +    if (prep) {
> > +        qapi_event_send_vfio_migration_prepare(dev->id, qom_path, qapi_state);
> > +    } else {
> > +        qapi_event_send_vfio_migration(dev->id, qom_path, qapi_state);
> > +    }
> >   }
> >   static void vfio_migration_set_device_state(VFIODevice *vbasedev,
> > @@ -119,7 +125,7 @@ static void vfio_migration_set_device_state(VFIODevice *vbasedev,
> >                                             mig_state_to_str(state));
> >       migration->device_state = state;
> > -    vfio_migration_send_event(vbasedev);
> > +    vfio_migration_send_event(vbasedev, state, false);
> >   }
> >   int vfio_migration_set_state(VFIODevice *vbasedev,
> > @@ -146,6 +152,8 @@ int vfio_migration_set_state(VFIODevice *vbasedev,
> >           return 0;
> >       }
> > +    vfio_migration_send_event(vbasedev, new_state, true);
> > +
> >       feature->argsz = sizeof(buf);
> >       feature->flags =
> >           VFIO_DEVICE_FEATURE_SET | VFIO_DEVICE_FEATURE_MIG_DEVICE_STATE;
> 

-- 
Peter Xu