[Qemu-devel] [Bug 1297487] Re: MTU not honored in virtio vnet

[Serge Hallyn]

** Also affects: qemu
   Importance: Undecided
       Status: New

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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]

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