Supervisor-Mode Virtualization for x86 in VDebug

Machine virtualization techniques offer many ways to improve both debugging and performance analysis facilities available to kernel developers. A minimal hardware interposition, exposing as much as possible of the underlying hardware device model, would enable high-level debugging of almost all parts of an operating system. Existing emulators either lack a debugging interface, impose excessive performance penalties, or obscure details of machine-specific hardware, all of which impede their value as debugging platforms. Because it is a paragon of complexity, techniques for emulating the protection state of today's most popular processor – the Pentium – have not been widely published. This paper presents the design of supervisor-mode virtualization in the VDebug kernel debugger system, which uses dynamic translation and dynamic shadowing to provide translucent CPU emulation. By running directly on the bare machine, VDebug is able to achieve an unusually low translation overhead and is able to exploit the hardware protection mechanisms to provide interposition and protection. We focus here on our design for supervisor-mode emulation. We also identify some of the fundamental challenges posed by dynamic translation of supervisor-mode code, and propose new ways of overcoming them. While the work described is not yet running fully, it is far enough along to have confidence in the design presented here, and several of the techniques used have not previously been published.