Commit 199b2053 authored by Apollon Oikonomopoulos's avatar Apollon Oikonomopoulos Committed by Guido Trotter
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KVM: Add auxiliary functions to handle tap devices

Add two new functions, _OpenTap and _ProbeTapVnetHdr, to

_ProbeTapVnetHdr checks if the host kernel supports the virtio net header
(IFF_VNET_HDR) feature. This feature allows KVM to ask the the host kernel to
perform checksumming and segmentation offload of the data passing through a
guest's virtio NIC, using a special header[1] prepended to data sent through
the tap interface. A qemu-kvm version later than 0.12 is required for this
feature to work.

_OpenTap opens a new tap device suitable for use with qemu-kvm's "-net
tap,fd=n" option. It also enables vnet_hdr support if requested and safe to do

[1] struct virtio_net_hdr in include/linux/virtio_net.h
Signed-off-by: default avatarApollon Oikonomopoulos <>
Signed-off-by: default avatarGuido Trotter <>
Reviewed-by: default avatarGuido Trotter <>
parent dc022e01
......@@ -31,6 +31,8 @@ import tempfile
import time
import logging
import pwd
import struct
import fcntl
from cStringIO import StringIO
from ganeti import utils
......@@ -46,6 +48,83 @@ from ganeti import netutils
_KVM_NETWORK_SCRIPT = constants.SYSCONFDIR + "/ganeti/kvm-vif-bridge"
# TUN/TAP driver constants, taken from <linux/if_tun.h>
# They are architecture-independent and already hardcoded in qemu-kvm source,
# so we can safely include them here.
TUNSETIFF = 0x400454ca
TUNGETIFF = 0x800454d2
IFF_TAP = 0x0002
IFF_NO_PI = 0x1000
IFF_VNET_HDR = 0x4000
def _ProbeTapVnetHdr(fd):
"""Check whether to enable the IFF_VNET_HDR flag.
To do this, _all_ of the following conditions must be met:
1. TUNGETFEATURES ioctl() *must* be implemented
2. TUNGETFEATURES ioctl() result *must* contain the IFF_VNET_HDR flag
3. TUNGETIFF ioctl() *must* be implemented; reading the kernel code in
drivers/net/tun.c there is no way to test this until after the tap device
has been created using TUNSETIFF, and there is no way to change the
IFF_VNET_HDR flag after creating the interface, catch-22! However both
TUNGETIFF and TUNGETFEATURES were introduced in kernel version 2.6.27,
thus we can expect TUNGETIFF to be present if TUNGETFEATURES is.
@type fd: int
@param fd: the file descriptor of /dev/net/tun
req = struct.pack("I", 0)
res = fcntl.ioctl(fd, TUNGETFEATURES, req)
except EnvironmentError:
logging.warning("TUNGETFEATURES ioctl() not implemented")
return False
tunflags = struct.unpack("I", res)[0]
if tunflags & IFF_VNET_HDR:
return True
logging.warning("Host does not support IFF_VNET_HDR, not enabling")
return False
def _OpenTap(vnet_hdr=True):
"""Open a new tap device and return its file descriptor.
This is intended to be used by a qemu-type hypervisor together with the -net
tap,fd=<fd> command line parameter.
@type vnet_hdr: boolean
@param vnet_hdr: Enable the VNET Header
@return: (ifname, tapfd)
@rtype: tuple
tapfd ="/dev/net/tun", os.O_RDWR)
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
# The struct ifreq ioctl request (see netdevice(7))
ifr = struct.pack("16sh", "", flags)
res = fcntl.ioctl(tapfd, TUNSETIFF, ifr)
except EnvironmentError:
raise errors.HypervisorError("Failed to allocate a new TAP device")
# Get the interface name from the ioctl
ifname = struct.unpack("16sh", res)[0].strip("\x00")
return (ifname, tapfd)
def _WriteNetScript(instance, nic, index):
"""Write a script to connect a net interface to the proper bridge.
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