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Virtual Machine Monitors

Virtual Machine Monitors. Bibliography. “Virtual Machine Monitors: Current Technology And Future Trends”, Mendel Rosenblum and Tal Garfinkel, IEEE Computer , May 2005

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Virtual Machine Monitors

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  1. Virtual Machine Monitors

  2. Bibliography • “Virtual Machine Monitors: Current Technology And Future Trends”, Mendel Rosenblum and Tal Garfinkel, IEEE Computer, May 2005 • “Xen and the Art of Virtualization”, P. Barham, R. Dragovic, K. Fraser, S. Hand, T. Harris, A Ho, R. Neugebauer, I. Pratt, A. Warfield, SOSP ’03. • The Definitive Guide to the Xen Hypervisor, David Chisnall, Prentice Hall, 2008. • “Scale and Performance in the Denali Isolation Kernel”, Andrew Whitaker, Marianne Shaw, and Steven D. Gribble, in System Design and Implementation (OSDI), Boston, MA, Dec. 2002. • Xen Homepage:http://www.cl.cam.ac.uk/research/srg/netos/xen/

  3. Outline • Overview • What is a virtual machine? • What is a virtual machine monitor (VMM)? • System or application virtual machines • History of Virtual Machines • Benefits of Virtual Machines • Issues and Implementation

  4. What is a Virtual Machine? • Several definitions and implementations. • Here, a virtual machine is an isolated environment that appears to be a whole computer, but actually only has access to a portion of the computer’s resources.

  5. A Formal Definition • “The environment in which a hosted operating system runs, providing the abstraction of a dedicated machine. A virtual machine may be identical to the underlying hardware (full virtualization) or it may differ slightly (paravirtualization).”www.linuxtopia.org/online_books/linux_virtualization/xen_3.0_user_guide/linux_virualization_xen_user_78.html

  6. What is a Virtual Machine Monitor? • A virtual machine monitor (VMM) is a thin software layer that runs directly on the bare hardware • It partitions the computer’s resources into one or more virtual machines • Each virtual machine appears to be running on the bare hardware. • End result – the appearance of multiple instances of the same computer, but all are supported on a single machine.

  7. Full Virtualization versus Paravirtualization • Full virtualization: each virtual machine runs on an exact copy of the underlying hardware. • Paravirtualization: the VMM modifies the underlying hardware somewhat • Because some aspects of the hardware can’t be virtualized • To present a simpler interface; improve performance.

  8. Application Guest OS1 VM1 Application Guest OS2 VM2 Application Guest OS3 VM3 Virtual machine layer - VMM Hardware layer

  9. Sometimes a virtual machine monitor is installed on an existing operating machine. More about this later. VM1 VM2 VMM Operating system Hardware layer

  10. VM – How They Work • When an application process makes a system call, it is received by its own OS, running (in user mode) on its private virtual machine. • When the “guest” OS tries to execute a privileged instruction, the virtual machine software traps the operation and ensures that it is executed correctly & safely • e.g., when a guest OS appears to execute an I/O system call, the “host” VM monitor is actually in charge.

  11. Virtualization versus Emulation • Virtualization presents multiple copies of the same hardware system. • Direct execution of code on the hardware • Emulation presents a model of another hardware system • Instructions are “emulated” in software – much slower than virtualization • Example: Microsoft’s VirtualPC can run on other chipsets than the x86 family; used on Mac hardware until Apple adopted Intel chips

  12. System & Process VMshttp://en.wikipedia.org/wiki/Virtual_machine • System virtual machine (hardware virtual machine) • Multiplex the underlying hardware • Each VM can run its own OS • Each VM is securely isolated from others • Process or application virtual machine • Runs inside a normal OS • Provides a platform-independent host for an application • For example, the Java Virtual Machine

  13. Virtual Machines – Examples • Denali was designed to support Internet services by providing a platform that allows a large number of servers to run on a single server machine. • Paravirtualizes x86 architecture to improve performance and scalability • “Isolation kernel”: isolates each server in a virtual machine to reduce the danger of sharing physical resources with untrusted servers.

  14. History - Why VMM’s? • Early computers were large (mainframes) and expensive • VMM approach allowed the machine to be safely multiplexed among many different applications • As an alternative to multiprogramming

  15. Virtual Machines - History • Early example: the IBM 370 • VM/370 is the virtual machine monitor • As each user logs on, a new “virtual machine” is created • CMS, a single-user, interactive OS was commonly run as the OS • Separation of powers: • Virtual machine interacts with user applications • Virtual machine monitor manages hardware resources

  16. History – 1980s & 1990s • As hardware got cheaper and operating systems became better equipped to handle multitasking, the original motivation went away. • Hardware platforms gradually eliminated hardware support for virtualization. • And then …

  17. History – late 90s to today • Massively parallel processors (MPPs) were developed during the 1990s; they were hard to program and did not support existing operating systems • Researchers at Stanford used virtualization to make MPPs look more like traditional machines • Result: VMware Inc. – supplier of VMMs for commodity hardware

  18. Rationale for VMMs Today • Today, security and encapsulation are the most important reasons for using VMMs • “…VMMs give operating systems developers another opportunity to develop functionality no longer practical in today’s complex and ossified operating systems, where innovation moves at a geologic pace.” [1]

  19. Example Virtual Machine Systems • VMware: commercial product, derived from research done at Stanford • Xen: open source, Cambridge University, widely used in research and academia • Denali: University of Washington, focuses on support for Internet services

  20. Reasons for Adopting VMM’s • Security and isolation • Ability to support several operating systems at the same time • Ability to experiment with new operating systems, or modifications of existing systems, while maintaining backward compatibility with existing operating systems.

  21. Security and Isolation • Applications running on a virtual machine are more secure than those running directly on hardware machines. • VMM controls how guest operating systems use hardware resources; what happens in one VM doesn’t affect any other VM: “…by virtualizing all hardware resources, a VMM can prevent one VM from even naming the resources of another VM, let alone modifying them.” [4]

  22. Encapsulation • The software state of a virtual machine isn’t dependent on the underlying hardware. • Rosenblum and Garfinkel [1] point out that this makes it possible to suspend and resume entire virtual machines and even move them to other platforms • For load balancing • For system maintenance • Etc.

  23. Servers • Conventionally, servers run on dedicated machines. • Protects against another server/application crashing the OS • But … wasteful of hardware resources • VMM technology makes it possible to support multiple servers, each running on its own VM, on a single hardware platform.

  24. Desirable Qualities • A good VMM • Doesn’t require applications to be modified • Doesn’t severely affect performance • Is not complex/error prone

  25. Implementation Issues • Enforce VMM control of hardware by preventing guest OS from executing privileged instructions. • Virtualize CPU • Virtualize memory

  26. CPU Virtualization • Basic technique: direct execution • The virtual machine executes on the real machine, but the VMM exercises control over privileged instructions • VMM runs in privileged (kernel) mode. • Guest OS executes all its code, privileged and unprivileged, in user mode. • If the guest OS tries to execute a privileged instruction the CPU traps to the VMM which executes the privileged operation.

  27. Protection Rings • Intel chips have 3 protection modes: • 0: equivalent to kernel mode; can execute all privileged instructions • 1: cannot execute privileged instructions but highter priority than user level • 2: where user processes run • Normally, only rings 0 and 2 are used. • Xen runs the guest OS in level 1

  28. Example: Disable Interrupts [1] • If a guest OS tries to disable interrupts, the instruction is trapped by the VMM which makes a note that interrupts are disabled for that virtual machine • If interrupts arrive for that machine, they are buffered at the VMM layer until the guest OS enables interrupts.

  29. Direct Execution Not Always Possible • Modern CPUs, esp. x86 architectures, have not been designed for virtualization. • Example: POPF (pop CPU flags from stack) • If executed in user mode, no trap - just ignore • In this case, direct execution fails – Guest OS assumes flags have been popped, but they haven’t

  30. Two Ways to Handle Non-virtualizable Instructions • Paravitualization • Modify VMM interface to use instructions that can be virtualized • Xen, Denali • Binary Translation • Monitor execution of kernel code and replace non-virtualizable instructions with other instructions • VMware

  31. Paravirtualization • Rewrite portions of the guest OS to delete this kind of instruction; replace with other instructions that are virtualizable. • Paravirtualization affects the guest OS, but not applications that run on it – the API is unchanged

  32. Binary Translation • Combines direct execution with on-the-fly binary translation (a form of emulation). • When the guest OS executes “privileged” code, the DBT (dynamic binary translator) replaces non-virtualizable instructions with equivalent code. • Paravirtualization changes the source code of a guest OS; binary translation changes the binary code as it executes.

  33. Comparison • Paravirtualization is more efficient, but requires modification to the guest OS • Paravirtualization also allows more efficient interfaces, in some cases • Binary translation is backward-compatible but has some extra overhead of run-time translation the first time an instruction is encountered. • Once translated, code is saved and used again if needed.

  34. Techniques – Hardware Support • AMD and Intel have added extensions to support virtualization. • New execution mode (-1) • Allows guest OS to run in execution ring 0 and VMM in yet a higher privileged mode • Flags to indicate if running in this mode • Will reduce the number of traps and the time to process a trap • Will support direct execution of all instructions

  35. Memory Virtualization • VMM maintains a shadow page table for each virtual machine. • When the guest OS makes an entry in its own page table, the VMM makes the same entry in the shadow table. • Shadow page table points to actual page frame • The hardware MMU uses the shadow page table when it translates virtual addresses.

  36. Paging Out the Virtual Machine • The VMM can swap one virtual machine (or parts thereof) to disk and swap in another. • Reduces the hardware requirements for a given workload • Particularly useful in environments where many servers are required, but only a few are used frequently. (Web services, for ex.)

  37. Challenges • It would make sense to let the virtual machine operating system decide which of its pages to swap out • VMware’s ESX Server uses the concept of a balloon process, running inside the guest OS, as a conduit for pages to be removed [1].

  38. Balloon Process • When the VMM wants to swap out pages from a VM it notifies the balloon process to allocate more memory to itself. • In order to get more memory for the balloon process, the guest OS must “page out” unused portions of other processes to its virtual disk. • The VMM now knows which pages the guest OS thinks it can do without.

  39. Other Virtual Memory Challenges • VMware tracks duplicate pages in different virtual machines • To avoid duplication, it only stores one copy of the actual page with pointers from the shadow page tables in sharing processes. • Copy-on-write policy • Xen focuses on total isolation of each virtual machine, which means no sharing

  40. Virtual Machines - Examples • VMware, a publicly held company, has two lines of products: • Desktop : VMware Workstation can run multiple different operating systems on a single PC. Runs in between the virtual machines and the native (host) OS. • VMware Fusion (for Mac-Intel platform) • VMware ESX Server, VMware Server run directly on hardware;

  41. Hosted versus Non-hosted VMM • Hosted has 3 advantages [1] • VMM is no harder to install than any other application • The VMM can use the host OS scheduler, pager, etc. and focus primarily on isolation • I/O support is better: the VMM can use the device drivers that are designed to work with the host OS rather than having to provide its own.

  42. Hosted versus Non-hosted VMM • Disadvantage [1] • I/O overhead is “greatly increased”: requests go from guest OS to VMM to host OS and down eventually to the device driver. • Too much for servers • More difficult to provide complete isolation, so not appropriate for servers from a security perspective.

  43. Virtual Machines - Examples • Xen is an open-source VM system for PCs • Designed to support execution of Linux, BSD Unix, Windows simultaneously on the same platform • Objective of original project: efficient hosting of up to 100 virtual machines • XenSource, Inc. provides products based on Xen and recently entered the server market in a big way.

  44. Denali • Problem addressed: hosting Internet services economically • Goal: to allow new services to hosted on third-party servers. • Requires assurances that one server won’t interfere with another. • Encapsulation of VMM model very important

  45. Isolation Kernel • “An OS structure for isolating untrusted software services” • Based on 4 principles: • Expose low-level resources rather than high-level abstractions • Prevent direct sharing by exposing only private, virtualized namespaces • Keeps one VM from “… even naming the resources of another VM, let alone modifying them”. [4]

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