The bug description said this was a regression from 13.04 to 14.04, but comment #9 says 12.04 is affected.
Pierre, could you show us your kernel and qemu versions? dpkg -l | egrep "(linux|qemu)" -- You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. https://bugs.launchpad.net/bugs/1297487 Title: MTU not honored in virtio vnet Status in QEMU: New Status in “qemu-kvm” package in Ubuntu: Confirmed Bug description: I am observing a potential regression/different behavior between rel. 14.04 (dev branch) and release 13.04. My hardware is: Cisco UCS blade B200-M3 and the network adapter card: Cisco UCS VIC 1240. lsb_release -rd Description: Ubuntu Trusty Tahr (development branch) Release: 14.04 uname -a Linux konan2 3.13.0-19-generic #40-Ubuntu SMP Mon Mar 24 02:36:06 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux The problem: After starting a kvm with virtio interfaces I am passing HTTP traffic via an external network traffic simulator. The tool is sending a TCP packet of 3481B, because the tool MTU is set to 1400B, it splits the packets into 3 TCP segments. When the 3 segments are received at the host eth1 interface, the host (ubuntu 14.04) reassembles the TCP packets into a larger packet (GRO), then passes the packet up on vnet1. At this point, because vnet1 MTU is 1500B, it is supposed to re-segment the packet and pass the 3 segments up to the VM. But it passes the big 3481B packet instead. This behavior did not happen when I tried the same scenario in release 13.04 I can disable this behavior by disabling TSO (TCP segment offloading in the vnet), but I did not have to do this in rel. 13.04 and I feel the MTU is not honored as it should be with rel. 14.04. ip link show | grep eth1 3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master br1 state UP mode DEFAULT group default qlen 1000 ip link show | grep vnet1 16: vnet1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br1 state UNKNOWN mode DEFAULT group default qlen 500 I am attaching two tcpdump/pcap traces that show a TCP transaction passing on vnet1 when TSO is on and when TSO is off. Please see: - vnet1_tso_on.pcap - vnet1_tso_off.pcap in attachment. I noticed there was a driver upgrade in rel. 14.04: in 14.04: ethtool -i eth1 driver: enic version: 2.1.1.50 firmware-version: 2.1(3a) bus-info: 0000:07:00.0 supports-statistics: yes supports-test: no supports-eeprom-access: no supports-register-dump: no supports-priv-flags: no ethtool -i vnet1 driver: tun version: 1.6 firmware-version: bus-info: tap supports-statistics: no supports-test: no supports-eeprom-access: no supports-register-dump: no supports-priv-flags: no ethtool -k vnet1 Features for vnet1: rx-checksumming: off [fixed] tx-checksumming: on tx-checksum-ipv4: off [fixed] tx-checksum-ip-generic: on tx-checksum-ipv6: off [fixed] tx-checksum-fcoe-crc: off [fixed] tx-checksum-sctp: off [fixed] scatter-gather: on tx-scatter-gather: on tx-scatter-gather-fraglist: on tcp-segmentation-offload: off tx-tcp-segmentation: off tx-tcp-ecn-segmentation: off tx-tcp6-segmentation: off udp-fragmentation-offload: on generic-segmentation-offload: on generic-receive-offload: on large-receive-offload: off [fixed] rx-vlan-offload: off [fixed] tx-vlan-offload: on ntuple-filters: off [fixed] receive-hashing: off [fixed] highdma: off [fixed] rx-vlan-filter: off [fixed] vlan-challenged: off [fixed] tx-lockless: off [fixed] netns-local: off [fixed] tx-gso-robust: off [fixed] tx-fcoe-segmentation: off [fixed] tx-gre-segmentation: off [fixed] tx-ipip-segmentation: off [fixed] tx-sit-segmentation: off [fixed] tx-udp_tnl-segmentation: off [fixed] tx-mpls-segmentation: off [fixed] fcoe-mtu: off [fixed] tx-nocache-copy: on loopback: off [fixed] rx-fcs: off [fixed] rx-all: off [fixed] tx-vlan-stag-hw-insert: on rx-vlan-stag-hw-parse: off [fixed] rx-vlan-stag-filter: off [fixed] l2-fwd-offload: off [fixed] in 13.04 : ethtool -i eth1 driver: enic version: 2.1.1.39 firmware-version: 2.1(3a) bus-info: 0000:07:00.0 supports-statistics: yes supports-test: no supports-eeprom-access: no supports-register-dump: no supports-priv-flags: no ethtool -i vnet1 driver: tun version: 1.6 firmware-version: bus-info: tap supports-statistics: no supports-test: no supports-eeprom-access: no supports-register-dump: no supports-priv-flags: no ethtool -k vnet1 Features for vnet1: rx-checksumming: off [fixed] tx-checksumming: on tx-checksum-ipv4: off [fixed] tx-checksum-ip-generic: on tx-checksum-ipv6: off [fixed] tx-checksum-fcoe-crc: off [fixed] tx-checksum-sctp: off [fixed] scatter-gather: on tx-scatter-gather: on tx-scatter-gather-fraglist: on tcp-segmentation-offload: on tx-tcp-segmentation: on tx-tcp-ecn-segmentation: on tx-tcp6-segmentation: on udp-fragmentation-offload: on generic-segmentation-offload: on generic-receive-offload: on large-receive-offload: off [fixed] rx-vlan-offload: off [fixed] tx-vlan-offload: off [fixed] ntuple-filters: off [fixed] receive-hashing: off [fixed] highdma: off [fixed] rx-vlan-filter: off [fixed] vlan-challenged: off [fixed] tx-lockless: off [fixed] netns-local: off [fixed] tx-gso-robust: off [fixed] tx-fcoe-segmentation: off [fixed] fcoe-mtu: off [fixed] To manage notifications about this bug go to: https://bugs.launchpad.net/qemu/+bug/1297487/+subscriptions
