From 199b2053a240362c5e0cf0bd7e814fad0ba67659 Mon Sep 17 00:00:00 2001
From: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Date: Mon, 17 Jan 2011 20:41:34 +0200
Subject: [PATCH] KVM: Add auxiliary functions to handle tap devices
Add two new functions, _OpenTap and _ProbeTapVnetHdr, to
hypervisors.hv_kvm.
_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
so.
[1] struct virtio_net_hdr in include/linux/virtio_net.h
Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
---
lib/hypervisor/hv_kvm.py | 79 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 79 insertions(+)
diff --git a/lib/hypervisor/hv_kvm.py b/lib/hypervisor/hv_kvm.py
index 46f41c5b4..c06545df6 100644
--- a/lib/hypervisor/hv_kvm.py
+++ b/lib/hypervisor/hv_kvm.py
@@ -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
+TUNGETFEATURES = 0x800454cf
+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)
+ try:
+ 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
+ else:
+ 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
+
+ """
+ try:
+ tapfd = os.open("/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)
+
+ try:
+ 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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