Packets dropped by virtual NICs

[Michal Privoznik]

On 3/29/21 11:38 AM, Silvia Fichera wrote:
> Hi Michal,
> these are the steps:
> - start the vm with qemu
> sudo qemu-system-x86_64-spice -m 2048 -enable-kvm -smp 3 -cdrom 
> /home/machine_A/ubuntu.iso -netdev 
> tap,ifname=tap0,vhost=on,id=n1,vhostforce=on,queues=3,script=/etc/tap0_ifup.sh,downscript=/etc/tap0_ifdown.sh 
> -device virtio-net-pci,netdev=n1,mac=9E:58:00:d2:53:03,mq=on,vectors=8 
> -netdev 
> tap,ifname=tap1,vhost=on,id=n2,vhostforce=on,queues=3,script=/etc/tap1_ifup.sh,downscript=/etc/tap1_ifdown.sh 
> -device virtio-net-pci,netdev=n2,mac=7A:53:00:d1:59:04,mq=on,vectors=8  
> -device virtio-net,netdev=network2 -netdev user,id=network2 -hda 
> switch_A.img
> 
> - attach the tap to the bridges in the host machine (to have the traffic 
> coming from the outsider traffic generator injected in the vm)
> - in the VM:
>      - enable ip_forward
>      - enable promiscuous mode for the interface

This sounds fishy. Why is this needed?

>      - send the "tc qdisc" command to configure the output NIC with 2 
> traffic classes (TC) assigned to 2 different queues
>      - add the iptables mangle rules to the POSTROUTING chain to assign 
> TC1 to the traffic with dport 7777 and TC0 to the traffic with dport 8888
> I'm sending 2 UDP flows, I have no loss under 3Mbps each.
> If I capture traffic with tcpdump on the ingress and on the egress nic 
> (of the VM), I see a difference of 50% of packets
> 
> When I did a test on the host machine, to check if it has the same 
> problem, I've reached an aggregated traffic of 90Mbps with no loss.
> 
> That's why I think that there is some misconfiguration on the virtual NIC.

Well, that's fairly easy to test - create those two TAPs, do the setup 
you're doing and instad of spawning qemu, run iperf or spirent and tell 
it to use those TAPs. I think you'll find the problem elsewhere.

Anyway, this problem is not libvirt related really and I guess you can 
find more elaborate replies on a list where TC developers hang out. 
Looks like they're using latrc: https://lartc.org/#mailinglist

Michal