On 12/05/2011 06:55 PM, Stefan Hajnoczi wrote:

On Mon, Dec 5, 2011 at 8:59 AM, Jason Wang[off-list ref]  wrote:

quoted

+static int virtnet_set_fd(struct net_device *dev, u32 pfn)
+{
+       struct virtnet_info *vi = netdev_priv(dev);
+       struct virtio_device *vdev = vi->vdev;
+
+       if (virtio_has_feature(vdev, VIRTIO_NET_F_HOST_FD)) {
+               vdev->config->set(vdev,
+                                 offsetof(struct virtio_net_config_fd, addr),
+&pfn, sizeof(u32));

Please use the virtio model (i.e. virtqueues) instead of shared
memory.  Mapping a page breaks the virtio abstraction.

Using control virtqueue is more suitable but there's are also some problems:

One problem is the interface,  if we use control virtqueue, we need a 
interface between the backend and tap/macvtap to change the flow 
mapping. But qemu and vhost_net only know about the file descriptor, 
more informations or interfaces need to be exposed in order to let 
ethtool or ioctl work.

Another problem is the delay introduced by ctrl vq, as the ctrl vq would 
be used in the critical path in guest and it use busy wait to get the 
response, the delay is not neglectable.

Stefan
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