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[Qemu-devel] [PATCHv2] tap: set IFF_ONE_QUEUE per default


From: Peter Lieven
Subject: [Qemu-devel] [PATCHv2] tap: set IFF_ONE_QUEUE per default
Date: Fri, 15 Feb 2013 10:32:31 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130106 Thunderbird/17.0.2

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

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 |    4 ++++
 net/tap-linux.h |    1 +
 2 files changed, 5 insertions(+)

diff --git a/net/tap-linux.c b/net/tap-linux.c
index a953189..d49f2fd 100644
--- a/net/tap-linux.c
+++ b/net/tap-linux.c
@@ -51,6 +51,10 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr,
     memset(&ifr, 0, sizeof(ifr));
     ifr.ifr_flags = IFF_TAP | IFF_NO_PI;

+#ifdef __linux__
+    ifr.ifr_flags |= IFF_ONE_QUEUE;
+#endif
+
     if (*vnet_hdr) {
         unsigned int features;

diff --git a/net/tap-linux.h b/net/tap-linux.h
index 65087e1..13002fd 100644
--- a/net/tap-linux.h
+++ b/net/tap-linux.h
@@ -36,6 +36,7 @@
 /* TUNSETIFF ifr flags */
 #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
--
1.7.9.5





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