net: Update netdev peer on link change

When a link change occurs on a backend (like tap), we currently do
not propage such change to the nic.  As a result, when someone turns
off a link on a tap device, for instance, then a guest doesn't see
that change and continues to try to send traffic or run DHCP even
though the lower-layer is disconnected.  This is OK when the network
is set up as a HUB since the the guest may be connected to other HUB
ports too, but when it's set up as a netdev, it makes thinkgs worse.

The patch addresses this by setting the peers link down only when the
peer is not a HUBPORT device.  With this patch, in the following config
  -netdev tap,id=net0 -device e1000,mac=XXXXX,netdev=net0
when net0 link is turned off, the guest e1000 shows lower-layer link
down. This allows guests to boot much faster in such configurations.
With windows guest, it also allows the network to recover properly
since windows will not configure the link-local IPv4 address, and
when the link is turned on, the proper address address is configured.
Signed-off-by: NVlad Yasevich <vyasevic@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

net/net.c

@@ -1071,15 +1071,23 @@ void qmp_set_link(const char *name, bool up, Error **errp)
         nc->info->link_status_changed(nc);
     }
 
-    /* Notify peer. Don't update peer link status: this makes it possible to
-     * disconnect from host network without notifying the guest.
-     * FIXME: is disconnected link status change operation useful?
-     *
-     * Current behaviour is compatible with qemu vlans where there could be
-     * multiple clients that can still communicate with each other in
-     * disconnected mode. For now maintain this compatibility. */
-    if (nc->peer && nc->peer->info->link_status_changed) {
-        nc->peer->info->link_status_changed(nc->peer);
+    if (nc->peer) {
+        /* Change peer link only if the peer is NIC and then notify peer.
+         * If the peer is a HUBPORT or a backend, we do not change the
+         * link status.
+         *
+         * This behavior is compatible with qemu vlans where there could be
+         * multiple clients that can still communicate with each other in
+         * disconnected mode. For now maintain this compatibility.
+         */
+        if (nc->peer->info->type == NET_CLIENT_OPTIONS_KIND_NIC) {
+            for (i = 0; i < queues; i++) {
+                ncs[i]->peer->link_down = !up;
+            }
+        }
+        if (nc->peer->info->link_status_changed) {
+            nc->peer->info->link_status_changed(nc->peer);
+        }
     }
 }