Guest networking is stalled as virtio is stuck due to a host issue in 3.8 kernels

[Raja Sivaramakrishnan]

In Ubuntu 12.4.3 hosts with 3.8 kernels, the following kernel issue can result in guests dropping off the network.

https://bugzilla.kernel.org/show_bug.cgi?id=60620

When this happens, the tap interface corresponding to the guest shows "RX dropped" errors. Also, "tc -s disc" shows a backlog on the TX queues of the guest's eth0 interface.

This issues is fixed by the following Linux kernel commit.

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=96f8d9ecf227638c89f98ccdcdd50b569891976c

This fix is available in 3.10 to 3.14 kernels, except for 3.11. The workaround in 3.8 kernels is to turn off offloads as below.

# ethtool --offload eth0 tx off
# ethtool --offload eth0 tso off
# ethtool --offload eth0 ufo off
# ethtool --offload eth0 gso off

[Raja Sivaramakrishnan (raja-u)]

Fix committed in Linux kernel.