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Unformatted text preview: Are Virtual-Machine Monitors Microkernels Done Right? Gernot Heiser National ICT Australia ∗ and University of New South Wales Sydney, Australia [email protected] Volkmar Uhlig IBM T.J. Watson Research Center, Yorktown Heights, NY [email protected] Joshua LeVasseur University of Karlsruhe, Germany [email protected] Abstract A paper by Hand et al. at the recent HotOS work- shop re-examined microkernels and contrasted them to virtual-machine monitors (VMMs). It found that the two kinds of systems share architectural com- monalities but also have a number of technical dif- ferences which the paper examined. It concluded that VMMs are a special case of microkernels, “microker- nels done right”. A closer examination of that paper shows that it contains a number of statements which are poorly justified or even refuted by the literature. While we believe that it is indeed timely to reexamine the mer- its and issues of microkernels, such an examination needs to be based on facts. 1 Introduction At the HotOS workshop in June this year, Hand and coauthors presented a paper [HWF + 05] titled “Are virtual machine monitors microkernels done right?” ∗ National ICT Australia is funded by the Australian Govern- ment’s Department of Communications, Information Technol- ogy, and the Arts and the Australian Research Council through Backing Australia’s Ability and the ICT Research Centre of Ex- cellence programs. The paper compares and contrasts microkernels and virtual-machine monitors (VMMs) as platforms for systems design and implementation. While identify- ing architectural similarities, it examines the differ- ence in the approaches, and concludes that VMMs are one specific point in the microkernel design space, the “right” one. Unstated but implied is the assertion that VMMs such as Xen [BDF + 03] are the (to date) only “right” approach to building microker- nels. Taking a closer look at the main assertions made by Hand et al , we find that they are hard to justify, or even squarely at odds with the literature. While we think that reexamining the merits and failures of microkernels is a potentially valuable exercise, we strongly believe that such a discussion must be per- formed in accordance with established scientific prin- ciples, and most of all, be grounded in facts. As a contribution to an informed discussion, we examine the assertions made by Hand et al in the light of the public record. 2 Background Before addressing the specific assertions made in Hand et al ’s paper, we provide some (we hope) useful background for the discussion. 95 2.1 History Revisited Microkernels and virtual machine monitors both have a long history, dating back to the early 1970’s [BH70, Gol74]. Given that there are significant similarities, it is useful to look at a somewhat narrow definition of both....
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- Fall '11
- virtual machine, Microkernel, Microkernels, VMMs, Steven Hand, machine monitors microkernels