Virtual Machine Monitors

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. • Denali: Lightweight virtual Machines for Distributed and Networked Applications”, Andrew Whitaker, Marianne Shaw, and Steven D. Gribble, Proc. USENIX annual Technical Conference, June 2002. • Xen Homepage: http://www.cl.cam.ac.uk/research/srg/netos/xen/ • VMWare: http://www.vmware.com/products/esx/

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

4. What is it? (1) • What is virtualization? an abstraction or simulation of hardware resources • e.g., virtual memory • 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. • Similar to, but much more than, the illusion provided by a multitasking operating system.

5. What is it? (2) • A virtual machine monitor (VMM) is the software layer that supports one or more virtual machines • Each VM appears to run on bare hardware, giving the appearance of multiple instances of the same computer, but all run on a single machine. • VMM is also called a hypervisor • Guest operating system: an operating system that runs in a VM, supported by the VMM, rather than directly on the hardware.

6. System & Process VMs (1)http://en.wikipedia.org/wiki/Virtual_machine • System (hardware) virtual machine - See previous slides • Provides a complete system • Each VM can run its own OS, which in turn can run multiple applications • Process or application virtual machine; e.g., JVM • Runs inside (under the control of) a normal OS • Provides a platform-independent host for a single application at a time

7. System & Process VMs (2) • System virtual machine • One machine appears to be multiple identical machines, each running its own operating system which in turn runs user jobs which are compiled to run on the underlying hardware • Process or application virtual machine • Source code is compiled into a “machine” code that represents the instruction set of a virtual (not real) machine. • The same byte code can be “executed” by any computer that has the appropriate interpreter/virtual machine, independently of the actual underlying hardware • Examples: Java byte code + JVM, Microsoft Common Language Infrastructure + .NET framework

8. System VMMs – Three Types • Traditional: VMM is a thin software layer that runs directly on the host machine hardware • Main advantage/objective: performance • VMWare vSphere, ESXi Servers, Xen, OS370, Denali • Also called a “bare metal” VMM • Hosted: VMM runs on top of an existing OS. • Main advantage: easier to build; easier to install • Examples: User-mode Linux • Hosted/Hybrid: shares the hardware with existing OS • Example: VMWare Workstation

9. Computer System Interfaces/Traditional Model • Unprivileged machine instructions: available to any program • Privileged instructions: hardware interface for the OS/other privileged software • System calls: interface to the operating system for applications & library functions • API: An OS interface through library function calls from applications.

10. Two Ways to Virtualize Process Virtual Machine: program is compiled to intermediate code, executed by a runtime system Virtual Machine Monitor: software layer mimics the instruction set; supports an OS and its applications

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

12. Hosted/Hybrid Rosenblum & Garfinkel – Fig. 2 VM1 VM2 VMM App App App I/O VMM Operating system Guest OS Hardware layer Host OS VMM Hosted Hardware Layer

13. Hosted/Hybrid 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; (hybrid doesn’t use all host features.) • 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.

14. 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 inefficient for servers • More difficult to guarantee complete isolation, so not appropriate for servers from a security perspective.

15. Hosted v Non-hosted VMM • Conclusion: • Hosting is a good approach for individual work stations; reduces effort needed to get VMM up and running; performance isn’t a major issue. • Hosting is not advisable for servers. Security issues are the most important concern, followed by added overhead for I/O and any other host OS services that are used.

16. VM – How They Work (1) • VMM runs in kernel mode (replacing tradtional OS) • Guest OS runs in user mode • Some modern hardware has a third mode for the guest OS • For the most part, applications run normally and execute machine code directly (direct execution) • What about system calls or other attempts by user processes to execute privileged instructions?

17. VM – How They Work (2) • If the guest OS runs in user mode how can it execute privileged code? • It can’t. When it tries to execute a privileged instruction, the VMM traps the operation, and executes in place of the guest OS • e.g., when a guest OS appears to execute an I/O system call, the VMM is actually in charge of the actual I/O processing.

18. 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 could run on other chipsets than the x86 family; used on Mac hardware until Apple adopted Intel chips

19. Full Virtualization versus Paravirtualization • Full virtualization: each virtual machine runs on an exact copy of the actual hardware. • Paravirtualization: each virtual machine runs on a slightly modified copy of the actual hardware • Because some aspects of the hardware can’t be virtualized (see examples later) • To present a simpler interface; improve performance.

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

21. 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/guest OS interacts with user applications • Virtual machine monitor manages hardware resources – compare to exokernel concept

22. 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 …

23. History – late 90s Hitachi MPP • 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 • Other research groups explored different approaches to VMs • Result: today, virtual machines are very common, although the MPPs of the 90s have been mostly replaced by clusters – and in some areas MPP is now used to refer to multicore chips.

24. Example Virtual Machine Systems • VMware: commercial products, derived from research done at Stanford • Xen: open source, Cambridge University, widely used in research and academia; xen.org • Denali: University of Washington, focused on support for Internet services • Never commercialized

25. VMware • VMware, a publicly held company, founded by Stanford developers • Two lines of products: • Desktop : a range of products; advertised as a way for corporations to migrate and upgrade operating systems from a centralized IT center • VMware vSphere hypervisor is a “bare-metal hypervisor” that supports server consolidationhttp://www.vmware.com/products/vsphere/esxi-and-esx/index.html • Vmware also virtualizes datacenters, networks and cloud applications (with Vmware vSphere and vCloud suite)

26. Xen: http://xenproject.org/ • Xen: open-source VM system for x86, Itanium, ARM & others • Originated at Cambridge University Computer Lab • Now supported as an open-source product that has destktop, server, and cloud capabilities (Amazon uses it for its cloud services.) • Designed to support execution of Linux, other Unix-like systems (Solaris, BSD), Windows OS’ssimultaneously on the same platform • Objective of original project: efficient hosting of up to 100 virtual machines

27. Hyper-V • Hyper_V is Microsoft’s server virtualization software: • Each virtual machine (user program + guest OS) is encapsulated in a partition supported by the VMM • Three execution modes: Ring 0, 1, 2 • Requires special hardware to support virtualization. http://en.wikipedia.org/wiki/Hyper-V

28. Denali • Research project – U of Washington • Time frame ~ 2001-2004. • Problem addressed: hosting Internet services economically • Goal: to allow new, untrusted, services to be hosted on third-party servers. • Protection provided by VM concept lets servers safely host multiple different services. • Encapsulation lets services be swapped in and out of memory easily so multiple services can share one machine

29. Reasons for Adopting VMMs • Flexibility in choice of operating system • Encapsulation: of an operating system, (virtual) computer system, and one or more applications into a single unit • Isolation/Security: provided by encapsulation; systems compromised by internal failure or external attack are isolated and their failure doesn’t affect other VMs.

30. OS Flexibility • Support several operating systems at the same time on a single hardware platform • Ability to experiment with new operating systems, or modifications of existing systems, while maintaining backward compatibility with existing systems. • Hardware can change faster than software – now you can run an existing application and the OS that supports it on a new computer, thanks to the VMM layer.

31. Encapsulation • Conventionally, servers ran on dedicated machines. • Protects against another server/application crashing the OS • But … wasteful of hardware resources • Encapsulation means that the complete state of a given VM can be saved to one or a few files – similar to checkpointing an application. • Furthermore, the state of one VM is totally separate from the state of any other VM. This is enforced by the VMM’s resource allocation policies.

32. Isolation • Virtual machines are as separate from each other (isolated) as if they actually were separate computers. • Applications in a VM are protected from faults in other VMs, in part because of encapsulation, and because the VMM controls resource allocation and usage by the guest OS’s • Viruses, buggy applications, other problems that cause crashes or corrupt the OS they run on will not affect other VMs

33. Virtualization in Distributed Systems • Rosenblum and Garfinkel [1] point out that encapsulation supports the portability of virtual machines, which in turn means it is easy and safe to move (or replicate) servers • This supports load balancing and maintenance • Or, multiple services can safely share a single computer thanks toencapsulation & isolation. • Since many services aren’t frequently used this cana great cost saver.

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

35. Implementation Issues • Virtualize CPU • Guest OS runs as if it is executing directly on the hardware CPU, but it isn’t • Virtualize memory • Guest OS thinks it is managing memory directly, but it isn’t • Paravirtualization versus binary translation • Hardware-assisted virtualization

36. CPU Virtualization • Basic technique: direct execution • As long as it is executing unprivileged instructions the virtual machine (guest OS + applications) executes hardware instructions directly. Note that in emulation direct execution isn’t possible since applications & the OS think they are running on a different ISA. • If the guest OS tries to execute a privileged instruction the CPU traps to the VMM which executes the privileged operation. • VMM runs in privileged (kernel) mode, guest OS runs in user mode.

37. 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 only. • If interrupts arrive for the VM that disabled them, they are buffered at the VMM layer until the guest OS enables interrupts. • Other interrupts are directed to VMs that have not disabled them.

38. Direct Execution Not Always Possible • Modern CPUs, esp. x86 architectures, were not designed for virtualization. • Example: POPF (pop CPU flags from stack) • If executed in user mode, no trap – it’s just ignored by the hardware • In this case, direct execution fails – Guest OS assumes flags have been popped, but they haven’t been because the VMM isn’t notified.

39. Two Ways to Handle Non-virtualizable Instructions • Paravitualization • Xen, Denali • Binary Translation • VMware • Both use the same basic approach: catch non-virtualizable instructions and emulate them in software at the VMM level. • Difference: when they are detected

40. Paravirtualization • Rewrite portions of the guest OS to replace non-virtualizable instructions with a trap to the VMM, which executes or emulates the instruction on behalf of the guest OS • e.g., remove POPFs; substitute a call to the VMM • Paravirtualization affects the guest OS, but not applications that run on it – the API is unchanged • Paravirtualization is also used sometimes to replace inefficient operations with more efficient ones.

41. Dynamic Binary Translation • Dynamic binary translation looks at a short sequence of (binary) source code, translates it, and caches the resulting sequence. [ http://en.wikipedia.org/wiki/Binary_translation ] • Similar to JIT compilers. • During this process VMware’s DBT replaces non-virtualizable instructions with equivalent code that can be virtualized. • Compare to static binary translation, done by a compiler, which translates to binary at compile time.

42. Comparison • Paravirtualization changes the source code of a guest OS; dynamic binary translation generates modified binary code only if needed. • 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.

43. Hardware-assisted Virtualization • AMD-V and Intel VT are architecture extensions to support virtualization on AMD and Intel hardware. • New execution modes • Allows guest OS to run in a different “ring” than user programs, and VMM in yet a higher privileged mode • Flags show which mode the CPU is currently running in • Essentially, the trap and emulate mode used in paravirtualization or binary translation is now done in hardware. • Does away with need to modify guest OS; is faster than binary translation.

44. 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.

45. Challenges • Let the guest OS decide which of its pages to swap out • VMware’s ESX Server used the concept of a balloon process, running inside the guest OS [1]. • When the VMM wants to swap out pages from a given VM it notifies the balloon process to allocate more memory to itself. • 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.

46. Other Virtual Memory Challenges • To share or not to share pages across VM boundaries: • VMware tracks duplicate pages in different virtual machines & stores only 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

47. Migrating Virtual Machines • A virtual machine encapsulates an entire computing environment. • If properly implemented, the VM provides strong mobility since local resources may be part of the migrated environment • “Freeze” an environment (temporarily stop executing processes) & move entire state to another machine • e.g. In a server cluster, migrated environments support maintenance activities such as replacing a machine.

48. Migration of Virtual Machines • Example: real-time (“live”) migration of a virtualized operating system with all its running services among machines in a server cluster on a local area network. • Presented in the paper “Live Migration of Virtual Machines”, Christopher Clark, et. al. • Problems: • Migrating the memory image (page tables, in-memory pages, etc.) • Migrating bindings to local resources

49. Memory Migration in Virtual Machines • Three possible approaches • Pre-copy: push memory pages to the new machine and resend the ones that are later modified during the migration process. • Stop-and-copy: pause the current virtual machine; migrate memory, and start the new virtual machine. • Let the new virtual machine pull in new pages as needed, using demand paging • Clark et.al use a combination of pre-copy and stop-and-copy; claim downtimes of 200ms or less.

50. Looking Ahead … • How useful will virtual machine technology be for multicore processors and cloud computing???