tap: vhost busy polling support

This patch add the capability of basic vhost net busy polling which is
supported by recent kernel. User could configure the maximum number of
us that could be spent on busy polling through a new property of tap
"poll-us".

Cc: Greg Kurz <groug@kaod.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

net/tap.c

@@ -663,6 +663,11 @@ static void net_init_tap_one(const NetdevTapOptions *tap, NetClientState *peer,
 
         options.backend_type = VHOST_BACKEND_TYPE_KERNEL;
         options.net_backend = &s->nc;
+        if (tap->has_poll_us) {
+            options.busyloop_timeout = tap->poll_us;
+        } else {
+            options.busyloop_timeout = 0;
+        }
 
         if (vhostfdname) {
             vhostfd = monitor_fd_param(cur_mon, vhostfdname, &err);
@@ -687,7 +692,7 @@ static void net_init_tap_one(const NetdevTapOptions *tap, NetClientState *peer,
             return;
         }
     } else if (vhostfdname) {
-        error_setg(errp, "vhostfd= is not valid without vhost");
+        error_setg(errp, "vhostfd(s)= is not valid without vhost");
     }
 }
 

net/vhost-user.c

@@ -91,6 +91,7 @@ static int vhost_user_start(int queues, NetClientState *ncs[])
 
         options.net_backend = ncs[i];
         options.opaque      = s->chr;
+        options.busyloop_timeout = 0;
         s->vhost_net = vhost_net_init(&options);
         if (!s->vhost_net) {
             error_report("failed to init vhost_net for queue %d", i);