commit d26e445c80fddcc7483b83f3115e5067fef28fe6
author Peter Lieven <pl@dlhnet.de> Mon Feb 25 10:17:08 2013 +0100
committer Stefan Hajnoczi <stefanha@redhat.com> Wed Feb 27 16:10:47 2013 +0100
tree 38ddab0d25178b7b2bb792b6bc80078f43f39178
parent ce675a7579fea498397c5d2da3c5367671e9f02a

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

diff --git a/net/tap-linux.c b/net/tap-linux.c
index a9531892..36c09e2 100644
--- a/net/tap-linux.c
+++ b/net/tap-linux.c
@@ -42,6 +42,7 @@
     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 @@
     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 @@
     }
 
     if (mq_required) {
-        unsigned int features;
-
         if ((ioctl(fd, TUNGETFEATURES, &features) != 0) ||
             !(features & IFF_MULTI_QUEUE)) {
             error_report("multiqueue required, but no kernel "