Dune allows running privileged instructions in userland using VT-x on Linux

From the paper:

This paper introduces a new approach to application use of kernel hardware features: using virtualization hardware to provide a process, rather than a machine abstraction. We have implemented this approach for Linux on 64-bit Intel CPUs in a system called Dune. Dune provides a loadable kernel module that works with unmodified Linux kernels. The module allows processes to enter "Dune mode", an irreversible transition in which, through virtualization hardware, safe and fast access to privileged hardware features is enabled, including privilege modes, virtual memory registers, page tables, and interrupt, exception, and system call vectors.

...Our evaluation shows both performance and security benefits to Dune. For instance, we built a sandbox that approaches zero overhead, modified a garbage collector to improve performance by up to 40.7%, and created a privilege separation system with 3x less context switch overhead than without Dune.