Currently, the tun/tap driver doesn't support setting it's MTU size to more than 1500 bytes: $> ip link set vnet0 mtu 9000 SIOCSIFMTU: Invalid argument In order to gain the performance improvement resulting from less packet overhead, it's quite common for customers to configure their network so that the entire path between two nodes support jumbo frames (i.e. frame size of 9000 bytes). Since tun/tap doesn't support this, it's impossible to obtain this benefit with KVM/virtio guests. The patch to implement this support is quite trivial and has been upstream since 2.6.24.
Upstream patch: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=4885a50476b95fa0f4caad179a80783508c2fe86
Created attachment 324770 [details] tun-jumbo-frame-support.patch
in kernel-2.6.18-126.el5 You can download this test kernel from http://people.redhat.com/dzickus/el5
An advisory has been issued which should help the problem described in this bug report. This report is therefore being closed with a resolution of ERRATA. For more information on therefore solution and/or where to find the updated files, please follow the link below. You may reopen this bug report if the solution does not work for you. http://rhn.redhat.com/errata/RHSA-2009-0225.html