[Qemu-devel] [PATCHv3] tap: set IFF_ONE_QUEUE per default

[Peter Lieven]

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

v3:
 - probe if IFF_ONE_QUEUE feature is available

v2:
 - do only set the flag on linux as it breaks macvtap
 - define IFF_ONE_QUEUE in tap-linux.h

Signed-off-by: Peter Lieven <address@hidden>
---
 net/tap-linux.c |   10 ++++++----
 net/tap-linux.h |    9 +++++----
 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/net/tap-linux.c b/net/tap-linux.c
index a953189..36c09e2 100644
--- a/net/tap-linux.c
+++ b/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 "
diff --git a/net/tap-linux.h b/net/tap-linux.h
index 65087e1..5704efb 100644
--- a/net/tap-linux.h
+++ b/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

--
1.7.9.5