Constructing Services with Interposable Virtual Hardware

Virtual machine monitors (VMMs) have enjoyed a resurgence in popularity, since VMMs can help to solve difficult systems problems like migration, fault tolerance, code sandboxing, intrusion detection, and debugging. Recently, several researchers have proposed novel applications of virtual machine technology, such as Internet Suspend/Resume [25, 31] and transparent OS-level rollback and replay [13]. Unfortunately, current VMMs do not export enough functionality to budding developers of such applications, forcing them either to reverse engineer pieces of a black-box VMM, or to reimplement significant portions of a VMM. In this paper, we present the design, implementation, and evaluation of μDenali, an extensible and programmable virtual machine monitor that has the ability to run modern operating systems. μDenali allows programmers to extend the virtual architecture exposed by the VMM to a virtual machine, in effect giving systems programmers the ability to dynamically assemble a virtual machine out of either default or custombuilt virtual hardware elements. μDenali allows programmers to interpose on and modify events at the level of the virtual architecture, enabling them to easily perform tasks such as manipulating disk and network events, or capturing and migrating virtual machine state. In addition to describing and evaluating our extensible virtual machine monitor, we present an application-level API that simplifies writing extensions, and we discuss applications of virtual machines that we have built using this API.