Virtualization - Dec 2022 - [RFC PATCH 4/4] virtio-net: sleep instead of busy waiting for cvq command

We used to busy waiting on the cvq command this tends to be
problematic since:

1) CPU could wait for ever on a buggy/malicous device
2) There's no wait to terminate the process that triggers the cvq
   command

So this patch switch to use sleep with a timeout (1s) instead of busy
polling for the cvq command forever. This gives the scheduler a breath
and can let the process can respond to a signal.

Signed-off-by: Jason Wang <jasowang at redhat.com>
---
 drivers/net/virtio_net.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 8225496ccb1e..69173049371f 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -405,6 +405,7 @@ static void disable_rx_mode_work(struct virtnet_info *vi)
 	vi->rx_mode_work_enabled = false;
 	spin_unlock_bh(&vi->rx_mode_lock);
 
+	virtqueue_wake_up(vi->cvq);
 	flush_work(&vi->rx_mode_work);
 }
 
@@ -1497,6 +1498,11 @@ static bool try_fill_recv(struct virtnet_info *vi, struct
receive_queue *rq,
 	return !oom;
 }
 
+static void virtnet_cvq_done(struct virtqueue *cvq)
+{
+	virtqueue_wake_up(cvq);
+}
+
 static void skb_recv_done(struct virtqueue *rvq)
 {
 	struct virtnet_info *vi = rvq->vdev->priv;
@@ -2024,12 +2030,7 @@ static bool virtnet_send_command(struct virtnet_info *vi,
u8 class, u8 cmd,
 	if (unlikely(!virtqueue_kick(vi->cvq)))
 		return vi->ctrl->status == VIRTIO_NET_OK;
 
-	/* Spin for a response, the kick causes an ioport write, trapping
-	 * into the hypervisor, so the request should be handled immediately.
-	 */
-	while (!virtqueue_get_buf(vi->cvq, &tmp) &&
-	       !virtqueue_is_broken(vi->cvq))
-		cpu_relax();
+	virtqueue_wait_for_used(vi->cvq, &tmp);
 
 	return vi->ctrl->status == VIRTIO_NET_OK;
 }
@@ -3524,7 +3525,7 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
 
 	/* Parameters for control virtqueue, if any */
 	if (vi->has_cvq) {
-		callbacks[total_vqs - 1] = NULL;
+		callbacks[total_vqs - 1] = virtnet_cvq_done;
 		names[total_vqs - 1] = "control";
 	}
 
-- 
2.25.1

Hi Jason,

Adding timeout to the cvq is a great idea IMO.
> -       /* Spin for a response, the kick causes an ioport write, trapping
> -        * into the hypervisor, so the request should be handled
immediately.
> -        */
> -       while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> -              !virtqueue_is_broken(vi->cvq))
> -               cpu_relax();
> +       virtqueue_wait_for_used(vi->cvq, &tmp);
Do you think that we should continue like nothing happened in case of a timeout?
Shouldn't we reset the device?
What happens if a device completes the control command after timeout?

Thanks

Alvaro