tap: set IFF_ONE_QUEUE per default

historically the kernel queues packets two times. once
at the device and second in qdisc. this is believed to cause
interface stalls if one of these queues overruns.

setting IFF_ONE_QUEUE is the default in kernels >= 3.8. the
flag is ignored since then. see kernel commit
5d097109257c03a71845729f8db6b5770c4bbedc

Signed-off-by: Peter Lieven <pl@kamp.de>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

net/tap-linux.c

@@ -42,6 +42,7 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr,
     struct ifreq ifr;
     int fd, ret;
     int len = sizeof(struct virtio_net_hdr);
+    unsigned int features;
 
     TFR(fd = open(PATH_NET_TUN, O_RDWR));
     if (fd < 0) {
@@ -51,9 +52,12 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr,
     memset(&ifr, 0, sizeof(ifr));
     ifr.ifr_flags = IFF_TAP | IFF_NO_PI;
 
-    if (*vnet_hdr) {
-        unsigned int features;
+    if (ioctl(fd, TUNGETFEATURES, &features) == 0 &&
+        features & IFF_ONE_QUEUE) {
+        ifr.ifr_flags |= IFF_ONE_QUEUE;
+    }
 
+    if (*vnet_hdr) {
         if (ioctl(fd, TUNGETFEATURES, &features) == 0 &&
             features & IFF_VNET_HDR) {
             *vnet_hdr = 1;
@@ -78,8 +82,6 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr,
     }
 
     if (mq_required) {
-        unsigned int features;
-
         if ((ioctl(fd, TUNGETFEATURES, &features) != 0) ||
             !(features & IFF_MULTI_QUEUE)) {
             error_report("multiqueue required, but no kernel "

net/tap-linux.h

@@ -34,10 +34,11 @@
 #endif
 
 /* TUNSETIFF ifr flags */
-#define IFF_TAP		0x0002
-#define IFF_NO_PI	0x1000
-#define IFF_VNET_HDR	0x4000
-#define IFF_MULTI_QUEUE 0x0100
+#define IFF_TAP          0x0002
+#define IFF_NO_PI        0x1000
+#define IFF_ONE_QUEUE    0x2000
+#define IFF_VNET_HDR     0x4000
+#define IFF_MULTI_QUEUE  0x0100
 #define IFF_ATTACH_QUEUE 0x0200
 #define IFF_DETACH_QUEUE 0x0400