Virtual Machines

Virtual Machines

1. Tecnológico de Costa Rica, Carrera: Ingeniería en Computación, Sede: Cartago, Curso: Principios de sistemas operativos, Profesor: Esteban Arias Méndez, Estudiante: Jason David Espinoza Siles, Carné: 201223941, II semestre, 2016, Libro: Operating System Concepts 9th.pdf capítulo # 16

2. Fundamental idea – abstract hardware of a single computer into several different execution environments  Similar to layered approach  But layer creates virtual system (virtual machine, or VM) on which operation systems or applications can run  Several components  Host – underlying hardware system  Virtual machine manager (VMM) or hypervisor – creates and runs virtual machines by providing interface that is identical to the host  (Except in the case of paravirtualization)  Guest – process provided with virtual copy of the host  Usually an operating system  Single physical machine can run multiple operating systems concurrently, each in its own virtual machine

3. Benefits and Features

   • Host system protected from VMs, VMs protected from each other  I.e. A virus less likely to spread  Sharing is provided though via shared file system volume, network communication  Freeze, suspend, running VM  Then can move or copy somewhere else and resume  Snapshot of a given state, able to restore back to that state  Some VMMs allow multiple snapshots per VM  Clone by creating copy and running both original and copy  Great for OS research, better system development efficiency  Run multiple, different OSes on a single machine  Consolidation, app dev, ...

4. Nested Page Tables

   • Memory management another general challenge to VMM implementations  How can VMM keep page-table state for both guests believing they control the page tables and VMM that does control the tables?  Common method (for trap-and-emulate and binary translation) is nested page tables (NPTs)  Each guest maintains page tables to translate virtual to physical addresses  VMM maintains per guest NPTs to represent guest’s page-table state  Just as VCPU stores guest CPU state  When guest on CPU -> VMM makes that guest’s NPTs the active system page tables  Guest tries to change page table -> VMM makes equivalent change to NPTs and its own page tables  Can cause many more TLB misses -> much slower performance