Yesterday I updated a lot of computers (hosts as well as virtual machines) running Ubuntu 12.04 LTS (Precise Pangolin) to the backported version of the 3.13 kernel. This kernel is provided by the linux-image-generic-lts-trusty package which is installed (together with the linux-headers-generic-lts-trusty package) when installing linux-generic-lts-trusty. By installing the backported kernel (before the update all Ubuntu 12.04 LTS systems where running on the 3.5 kernel provided by linux-generic-lts-quantal) I wanted to increase the uniformity between the Ubuntu 12.04 LTS and Ubuntu 14.04 LTS systems.
After installing the new kernel and rebooting the machines, funny network problems started to happen. For some virtual machines, IPv6 communication between virtual machines running on the same VM host became very unreliable. For other virtual machines, I experienced occassional huge delays (up to several seconds) for IPv4 packets.
After testing around for a few hours (at the same time I had upgraded a virtual-machine host to Ubuntu 14.04 LTS and first suspected this upgrade, specifically the new version of OpenVSwitch), I found out that these network problems were indeed caused by the new kernel in the virtual machines. If one of two virtual machines running on the same host had the new kernel running, the problems with IPv6 appeared. If both were running the old kernel version, the problems disappeared. The other problem with the massively delayed IPv4 packets was a bit harder to reproduce. Funnily, it already became much better when I downgraded just one of the virtual machines on the host.
At the current stage (linux-image-generic-lts-quantal-3.13.0-30), there seems to be a massive problem with the IP stack of the kernel. For some reasons, this problems only seem to be triggered if the kernel is running in a (Linux KVM) virtual machine. For now, I downgraded all virtual machines back to the old kernel version.
I have to do some more tests to find out whether these problems are caused by the newer kernel in general or whether they are specifically caused by the backported version. At the moment I only have one virtual machine with Ubuntu 14.04 LTS, so I will have to setup some test VMs to carry out more tests.
Until then, I can only recommend to stay away from the backported 3.13 kernel, at least for virtual machines.