UML and the Intel VT extensions
Intel has added virtualization extensions (VT) to the x86 architecture. It adds a new set of rings, guest rings 0 through 3, to the traditional rings, which are now called the host rings.
User-mode Linux (UML) is in the process of being enhanced to make use of these extensions for greater performance. It will run in guest ring 0, gaining the ability to directly receive software interrupts. This will allow it to handle process system calls without needing assistance from the host kernel, which will let UML handle system calls at hardware speed.
In spite of running in a ring 0, UML will appear to remain in userspace, making system calls to the host kernel and receiving signals from it. So, it will retain its current manageability, while getting a performance boost from its use of the hardware.