virtio-net: split the has_buffers() logic from can_receive()

We should only return zero from receive() for a condition which we'll
get notification of when it changes. Currently, we're returning zero
if the guest driver is not ready, but we won't ever flush our queue
when that status changes.

Also, don't check buffer space in can_receive(), but instead just allow
receive() to return zero when this condition occurs and have the caller
handle queueing the packet.
Signed-off-by: NMark McLoughlin <markmc@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/virtio-net.c

@@ -367,12 +367,19 @@ static void virtio_net_handle_rx(VirtIODevice *vdev, VirtQueue *vq)
     qemu_notify_event();
 }
 
-static int do_virtio_net_can_receive(VirtIONet *n, int bufsize)
+static int virtio_net_can_receive(VLANClientState *vc)
 {
+    VirtIONet *n = vc->opaque;
+
     if (!virtio_queue_ready(n->rx_vq) ||
         !(n->vdev.status & VIRTIO_CONFIG_S_DRIVER_OK))
         return 0;
 
+    return 1;
+}
+
+static int virtio_net_has_buffers(VirtIONet *n, int bufsize)
+{
     if (virtio_queue_empty(n->rx_vq) ||
         (n->mergeable_rx_bufs &&
          !virtqueue_avail_bytes(n->rx_vq, bufsize, 0))) {
@@ -384,13 +391,6 @@ static int do_virtio_net_can_receive(VirtIONet *n, int bufsize)
     return 1;
 }
 
-static int virtio_net_can_receive(VLANClientState *vc)
-{
-    VirtIONet *n = vc->opaque;
-
-    return do_virtio_net_can_receive(n, VIRTIO_NET_MAX_BUFSIZE);
-}
-
 /* dhclient uses AF_PACKET but doesn't pass auxdata to the kernel so
  * it never finds out that the packets don't have valid checksums.  This
  * causes dhclient to get upset.  Fedora's carried a patch for ages to
@@ -517,7 +517,10 @@ static ssize_t virtio_net_receive(VLANClientState *vc, const uint8_t *buf, size_
     struct virtio_net_hdr_mrg_rxbuf *mhdr = NULL;
     size_t hdr_len, offset, i;
 
-    if (!do_virtio_net_can_receive(n, size))
+    if (!virtio_net_can_receive(n->vc))
+        return -1;
+
+    if (!virtio_net_has_buffers(n, size))
         return 0;
 
     if (!receive_filter(n, buf, size))