[Qemu-discuss] E1000 & virtio will not respect set MTU

[Markus Kovero]

I also forward this mail to you guys, if you can shed any light on 
this; orginally sent to kvm general ml


Hi, I've encountered interesting problem.

This happens with Linux 3.1.4, libvirt 0.9.6, qemu-kvm 0.15.1 and Linux 
3.2.1, libvirt 0.9.9, qemu-kvm 1.0.

It seems guest OS NIC does not respect MTU that has been set on 
interface when using e1000 or virtio drivers, everything works well when 
using Realtek emulation though.
Problem can be seen as degraded upload bandwidth due tcp 
retransmissions because of packets get stripped due mtu limitations.
I see this definitely as showstopper bug and it definitely needs some 
attention.

Guest OS NIC is connected to bridge that is over bond device, have 
tried to set mtu=1500 on bond devices without any help, default mtu on 
bond devices is 9000 here, because storage access, but Guest OS bridge 
is set to 1500 and Guest OS itself is on 1500 MTU link.

Problem itself can be witnessed in tcpdump when transmitting data;

(Linux, Debian Squeeze 64bit)

12:44:37.357587 IP SOURCE.48541 > DESTINATION.5001: Flags [.], seq 
770816:772264, ack 1, win 183, options [nop,nop,TS val 4294913843 ecr 
3141020848], length 1448
12:44:37.394562 IP SOURCE.48541 > DESTINATION.5001: Flags [.], seq 
772264:775160, ack 1, win 183, options [nop,nop,TS val 4294913852 ecr 
3141020857], length 2896
12:44:37.394633 IP SOURCE.48541 > DESTINATION.5001: Flags [.], seq 
775160:776608, ack 1, win 183, options [nop,nop,TS val 4294913852 ecr 
3141020857], length 1448
12:44:37.431592 IP SOURCE.48541 > DESTINATION.5001: Flags [.], seq 
776608:779504, ack 1, win 183, options [nop,nop,TS val 4294913861 ecr 
3141020866], length 2896
12:44:37.431662 IP SOURCE.48541 > DESTINATION.5001: Flags [.], seq 
779504:780952, ack 1, win 183, options [nop,nop,TS val 4294913861 ecr 
3141020866], length 1448

As we can see here, packet length varies over what it should be, when 
using realtek, packet length is strictly 1448, which fits uplink fine 
and upload degration will not occur.

This is not a pmtu problem either, same thing happens when pmtu 
discovery is disabled.
This also happens on Windows, so I'm thinking its not distribution 
related.

Thanks for any assistance

Yours
Markus Kovero