Linux KVM virtual machine and UDP transferts

I've got a setup where a windows 2008R2 VM runs under KVM on a bridged network and captures a lot of multicast UDP data (from videosurveillance cameras). Strangely, the VM network drops packets after a number of concurrent connections ( more than 128 apparently).

kernel is 2.6.35.11 amd64, kvm 0.76 (Debian Lenny).

However if I setup two different tap0 and tap1 interfaces, bridged through the same br0 to eth0 and eth1, the second interface never drops a single packet. The first interface tap0, however, drops lots of packets even if most connections go to the other one.

But if I keep using eth0 for the host, and dedicate eth1 to the VM, eth1 (the VM) will still drops tons of packets (about 20 to 25%). Apparently only the second interface behaves correctly.

Does anyone have any idea about what may be happening here? I can't make sense of it. I don't understand why the fist interface always drop packets.

Here is the kvp-ifup script :

#!/bin/sh
INTERFACE_LAN=bond0
INTERFACE_BRIDGE=br0
# set address to "DHCP" for DHCP
BRIDGE_ADDRESS=192.168.1.1
BRIDGE_BROADCAST=192.168.1.255
BRIDGE_NETMASK=255.255.255.0

echo "Configure $1 interface..."
/usr/sbin/tunctl -u 'root' -t $1
echo "Activating link for $1..."
/sbin/ip link set $1 up
sleep 1
echo "Configure $INTERFACE_BRIDGE bridge..."
/usr/sbin/brctl addbr $INTERFACE_BRIDGE
/usr/sbin/brctl addif $INTERFACE_BRIDGE $1
/usr/sbin/brctl addif $INTERFACE_BRIDGE $INTERFACE_LAN
echo "Activating promiscuous mode on $1 and $INTERFACE_LAN..."
/sbin/ifconfig $1 0.0.0.0 promisc up
/sbin/ifconfig $INTERFACE_LAN 0.0.0.0 promisc up
sleep 2
echo "IP address on $INTERFACE_BRIDGE..."

if [ "DHCP" == "$BRIDGE_ADDRESS" ]; then
        /sbin/pump -i br0
else
        # static
        /sbin/ifconfig $INTERFACE_BRIDGE $BRIDGE_ADDRESS broadcast $BRIDGE_BROADCAST netmask $BRIDGE_NETMASK
fi

echo "Done."

I tried both with a bond (different modes) and no bonding.

The physical NICS are Ethernet controller: Intel Corporation 82574L Gigabit Network
Connection (e1000e linux driver)

the VM are using the e1000 driver (better results than virtio)

iptables is perfectly empty.

As for offloading, I cannot check but it should be the defaults :

• rx-checksumming: on
• tx-checksumming: on
• scatter-gather: on
• tcp segmentation offload: on
• udp fragmentation offload: off
• generic segmentation offload: on
• large receive offload: off