Bug 733651

+++ This bug was initially created as a clone of Bug #733416 +++

Xen drops the MTU of a guest's netfront interface to 1500 after migration.

For now I'm attaching the bug to kernel-xen, but I am still not really sure whether this is the right component.

Test info for QE:

- connect two machines with a crossover cable so that eth0 is the crossover cable and eth1 is the public network

- configure Xen so that you'll be able to migrate between the machines (shared storage, relocation server, etc.)

- configure the cross link on one machine like

# Intel Corporation 82574L Gigabit Network Connection
DEVICE=eth0
HWADDR=00:21:5E:6E:F0:29
ONBOOT=yes
HOTPLUG=no
IPADDR=169.254.100.200
NETMASK=255.255.0.0
NETWORK=169.254.0.0
MTU=9000

and similarly on the other machine

- on both machines, ensure the Xen bridge connects the right devices, for example by changing the network-script line in /etc/xen/xend-config.sxp to this:

(network-script 'network-bridge netdev=eth0 bridge=xenbr0')

- on both machines, ensure the Xen bridge has its MTU set to 9000, for example by adding this (huge hack) to /etc/init.d/xend at the end of function await_daemons_up

sleep 5
ip link set mtu 9000 dev vif0.1
ip link set mtu 9000 dev peth0
ip link set mtu 9000 dev xenbr0

- install a guest in one of the machines.  ensure the guest has also a static IP configuration in the same subnet, and also has MTU=9000 in its /etc/sysconfig/network-scripts/ifcfg-eth0.  Also, ensure that the guest is using xenbr0 and not virbr0.

- run ifconfig around "xm create"

ifconfig > before
xm create test1
# wait a few seconds
ifconfig | diff before -

The diff should show that the new vif interface has MTU=9000.  Similarly, "ifconfig" in the guest should show MTU=9000.

- ensure that you can ping the guest from both hosts.

- migrate the guest from one host to the other.  before migrating, run

ifconfig > before

on the guest and *destination* host and after migration run

ifconfig | diff before -

For both the guest and the destination host, the diff should show that the virtual network interface (vifN.0 on the host, eth0 on the guest) has MTU=9000.  

- ensure that you can ping the guest from both hosts

--- Additional comment from pm-rhel on 2011-08-26 04:49:40 EDT ---

This request was evaluated by Red Hat Product Management for inclusion in a Red
Hat Enterprise Linux maintenance release.  Product Management has requested
further review of this request by Red Hat Engineering, for potential
inclusion in a Red Hat Enterprise Linux Update release for currently deployed
products.  This request is not yet committed for inclusion in an Update
release.