vdpa: mlx5: prevent cvq work from hogging CPU

A userspace triggerable infinite loop could happen in
mlx5_cvq_kick_handler() if userspace keeps sending a huge amount of
cvq requests.

Fixing this by introducing a quota and re-queue the work if we're out
of the budget (currently the implicit budget is one) . While at it,
using a per device work struct to avoid on demand memory allocation
for cvq.

Fixes: 5262912ef3cfc ("vdpa/mlx5: Add support for control VQ and MAC setting")
Signed-off-by: Jason Wang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Michael S. Tsirkin <[e-mail]>
Acked-by: Eli Cohen <[e-mail]>

drivers/vdpa/mlx5/net/mlx5_vnet.c

···
 	u32 cur_num_vqs;
 	struct notifier_block nb;
 	struct vdpa_callback config_cb;
+	struct mlx5_vdpa_wq_ent cvq_ent;
 };
 
 static void free_resources(struct mlx5_vdpa_net *ndev);
···
 	ndev = to_mlx5_vdpa_ndev(mvdev);
 	cvq = &mvdev->cvq;
 	if (!(ndev->mvdev.actual_features & BIT_ULL(VIRTIO_NET_F_CTRL_VQ)))
-		goto out;
+		return;
 
 	if (!cvq->ready)
-		goto out;
+		return;
 
 	while (true) {
 		err = vringh_getdesc_iotlb(&cvq->vring, &cvq->riov, &cvq->wiov, &cvq->head,
···
 
 		if (vringh_need_notify_iotlb(&cvq->vring))
 			vringh_notify(&cvq->vring);
+
+		queue_work(mvdev->wq, &wqent->work);
+		break;
 	}
-out:
-	kfree(wqent);
 }
 
 static void mlx5_vdpa_kick_vq(struct vdpa_device *vdev, u16 idx)
···
 	struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
 	struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
 	struct mlx5_vdpa_virtqueue *mvq;
-	struct mlx5_vdpa_wq_ent *wqent;
 
 	if (!is_index_valid(mvdev, idx))
 		return;
···
 		if (!mvdev->wq || !mvdev->cvq.ready)
 			return;
 
-		wqent = kzalloc(sizeof(*wqent), GFP_ATOMIC);
-		if (!wqent)
-			return;
-
-		wqent->mvdev = mvdev;
-		INIT_WORK(&wqent->work, mlx5_cvq_kick_handler);
-		queue_work(mvdev->wq, &wqent->work);
+		queue_work(mvdev->wq, &ndev->cvq_ent.work);
 		return;
 	}
 
···
 	if (err)
 		goto err_mr;
 
+	ndev->cvq_ent.mvdev = mvdev;
+	INIT_WORK(&ndev->cvq_ent.work, mlx5_cvq_kick_handler);
 	mvdev->wq = create_singlethread_workqueue("mlx5_vdpa_wq");
 	if (!mvdev->wq) {
 		err = -ENOMEM;