| Popek And Goldberg Virtualization Requirements |
Website Links For Goldberg |
Information AboutPopek And Goldberg Virtualization Requirements |
| CATEGORIES ABOUT POPEK AND GOLDBERG VIRTUALIZATION REQUIREMENTS | |
| computer architecture | |
| virtualization software | |
|
INTRODUCTION System Virtual Machines are virtual machines capable of virtualizing a full set of hardware resources, including a processor (or processors), memory and storage resources and peripheral devices. A Virtual Machine Monitor (VMM) is the piece of software that provides the abstraction of a virtual machine. There are three properties of interest when analyzing the environment created by a VMM: ; Equivalence : A program running under the VMM should exhibit a behavior essentially identical to that demonstrated when running on an equivalent machine directly. ; Resource control : The VMM must be in complete control of the virtualized resources. ; Efficiency : A statistically dominant fraction of machine instructions must be executed without VMM intervention. In Popek and Goldberg terminology, a VMM must present all 3 properties. In today's terminology, VMM are typically assumed to satisfy the equivalence and resource control properties. So, in a sense, Popek and Goldberg's VMMs are today's efficient VMM. The problem addressed by Popek and Goldberg then is determining what are the characteristics that the Instruction Set Architecture (ISA) of the original machine must possess in order to allow the creation of VMMs with the above mentioned properties. Their analysis derives such characteristics for a model of "third generation architectures" (e.g., IBM 360, Honeywell 6000, DEC PDP-10), but is general enough to be naturally extended to current machines. The model includes a processor that can operate either in system or user mode and a linear, uniformly addressable memory. It is assumed that a subset of the instruction set is available only when in system mode and that memory is addressed relative to a relocation register. I/O and interrupts are not modeled. VIRTUALIZATION REQUIREMENTS To derive their virtualization requirements, Popek and Goldberg introduce a classification of instructions of an ISA into 3 different groups: ; Privileged instructions : Those that trap if the processor is in user mode and do not trap if it is in system mode. ; Control sensitive instructions : Those that attempt to change the configuration of resources in the system. ; Behavior sensitive instructions : Those whose behavior or result depends on the configuration of resources (the content of the relocation register or the processor's mode). The main result of Popek and Goldberg's analysis can then be expressed as follows. Theorem 1. For any conventional third generation computer, a VMM may be constructed if the set of sensitive instructions for that computer is a subset of the set of privileged instructions. Intuitively, the theorem states that to build a VMM it is sufficient that all instructions that could affect the correct functioning of the VMM (sensitive instructions) always trap and pass control to the VMM. This guarantees the resource control property. Non privileged instructions must instead be executed natively (efficiency). The holding of the equivalence property also follows. A related problem is that of deriving the ISA requirements for recursive virtualization, that is, the conditions under which a VMM that can run on a copy of itself can be built. Popek and Goldberg presents the following (sufficient) conditions. Theorem 2. A conventional third generation computer is recursively virtualizable if # it is virtualizable and # a VMM without any timing dependencies can be constructed for it. HANDLING CRITICAL INSTRUCTIONS The conditions for ISA virtualization expressed in Theorem 1 are sufficient but not necessary. Indeed, VMMs for non virtualizable ISA (in the Popek and Goldberg's sense) have routinely been built, at the expense of the efficiency property. The virtualization of such architectures requires correct handling of ''critical instructions'', i.e., sensitive but unprivileged instructions. One approach, known as ''patching'' adopts techniques commonly used in , that requires guest operating systems to be modified (''ported'') before running in the virtual environment. INSTRUCTION SETS In this section some relevant architectures and how they relate to the virtualization requirements are presented. PDP-10 The PDP-10 architecture has a few instructions which are sensitive (alter or query the processor's mode) but not privileged1 |
|
|