I have seen on the Internet that a few people experience problems using AMD-V with VirtualBox on Ubuntu, but I do not know what is causing mine. When I have it enabled, the whole computer will freeze up after a while. Could be a few seconds or a few minutes. However, when I tried using the same VM in Windows with the feature enabled, I had no problems whatsoever. Currently using VirtualBox 3.1.6 off the website.