On Thu, Dec 22, 2022 at 5:19 PM Eugenio Perez Martin
[off-list ref] wrote:

On Thu, Dec 22, 2022 at 7:05 AM Jason Wang [off-list ref] wrote:

quoted

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@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;

If we're using CVQ callback, what is the actual use of the timeout?

Because we can't sleep forever since locks could be held like RTNL_LOCK.

I'd say there is no right choice neither in the right timeout value
nor in the action to take.

In the next version, I tend to put BAD_RING() to prevent future requests.

Why not simply trigger the cmd and do all
the changes at command return?

I don't get this, sorry.

I suspect the reason is that it complicates the code. For example,
having the possibility of many in flight commands, races between their
completion, etc.

Actually the cvq command was serialized through RTNL_LOCK, so we don't
need to worry about this.

In the next version I can add ASSERT_RTNL().

Thanks

The virtio standard does not even cover unordered
used commands if I'm not wrong.

Is there any other fundamental reason?

Thanks!

quoted

                names[total_vqs - 1] = "control";
        }

--
2.25.1