vdpa/mlx5: Keep notifiers during suspend but ignore

Unregistering notifiers is a costly operation. Instead of removing
the notifiers during device suspend and adding them back at resume,
simply ignore the call when the device is suspended.

At resume time call queue_link_work() to make sure that the device state
is propagated in case there were changes.

For 1 vDPA device x 32 VQs (16 VQPs) attached to a large VM (256 GB RAM,
32 CPUs x 2 threads per core), the device suspend time is reduced from
~13 ms to ~2.5 ms.
Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20240816090159.1967650-9-dtatulea@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>

drivers/vdpa/mlx5/net/mlx5_vnet.c

@@ -2850,6 +2850,9 @@ static int event_handler(struct notifier_block *nb, unsigned long event, void *p
 	struct mlx5_eqe *eqe = param;
 	int ret = NOTIFY_DONE;
 
+	if (ndev->mvdev.suspended)
+		return NOTIFY_DONE;
+
 	if (event == MLX5_EVENT_TYPE_PORT_CHANGE) {
 		switch (eqe->sub_type) {
 		case MLX5_PORT_CHANGE_SUBTYPE_DOWN:
@@ -3595,7 +3598,6 @@ static int mlx5_vdpa_suspend(struct vdpa_device *vdev)
 	mlx5_vdpa_info(mvdev, "suspending device\n");
 
 	down_write(&ndev->reslock);
-	unregister_link_notifier(ndev);
 	err = suspend_vqs(ndev, 0, ndev->cur_num_vqs);
 	mlx5_vdpa_cvq_suspend(mvdev);
 	mvdev->suspended = true;
@@ -3617,7 +3619,7 @@ static int mlx5_vdpa_resume(struct vdpa_device *vdev)
 	down_write(&ndev->reslock);
 	mvdev->suspended = false;
 	err = resume_vqs(ndev, 0, ndev->cur_num_vqs);
-	register_link_notifier(ndev);
+	queue_link_work(ndev);
 	up_write(&ndev->reslock);
 
 	return err;