Commit b8168679 authored by Apollon Oikonomopoulos's avatar Apollon Oikonomopoulos Committed by Hrvoje Ribicic
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KVM: set IFF_ONE_QUEUE on created tap interfaces

The IFF_ONE_QUEUE flag directs the kernel to only queue tap packets once
(as opposed to queueing them twice, once for the device, and once for the
qdisc), possibly avoiding interface stalls when one of the queues overruns.

This is the default behaviour of the kernel tun/tap driver in Linux
>= 3.8. Also, qemu >= 1.5.0 sets this flag when opening the tap device
itself (but not for tap interfaces inherited via fds), according to this

  commit d26e445c80fddcc7483b83f3115e5067fef28fe6
  Author: Peter Lieven <>
  Date:   Mon Feb 25 10:17:08 2013 +0100

      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
Signed-off-by: default avatarPeter Lieven <>
Acked-by: default avatarMichael S. Tsirkin <>
Signed-off-by: default avatarStefan Hajnoczi <>

Since we handle tap creation ourselves, we duplicate this behavior in
_OpenTap. IFF_ONE_QUEUE has been there as far back as the kernel's git
history goes (2.6.12), so it is safe to add the flag unconditionally.
Also the flag is invisible to the guest and will not break migrations of
already running instances.
Signed-off-by: default avatarApollon Oikonomopoulos <>
Reviewed-by: default avatarHrvoje Ribicic <>
parent 762a28c6
......@@ -66,6 +66,7 @@ TUNGETIFF = 0x800454d2
IFF_TAP = 0x0002
IFF_NO_PI = 0x1000
IFF_ONE_QUEUE = 0x2000
IFF_VNET_HDR = 0x4000
#: SPICE parameters which depend on L{constants.HV_KVM_SPICE_BIND}
......@@ -145,7 +146,7 @@ def _OpenTap(vnet_hdr=True):
except EnvironmentError:
raise errors.HypervisorError("Failed to open /dev/net/tun")
flags = IFF_TAP | IFF_NO_PI
if vnet_hdr and _ProbeTapVnetHdr(tapfd):
flags |= IFF_VNET_HDR
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