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
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:
VMWare:

3. Outline Overview History of Virtual Machines
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 on a 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

7. System & Process VMs (2) System virtual machine
One machine appears to be multiple identical machines, each running its own operating system
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 byte code can be “executed” by any computer that has the appropriate interpreter.
Many different machines can execute the same byte code.
Examples: Java byte code + JVM, Microsoft Common Language Infrastructure + .NET framework

8. System Virtual Machines
Traditional: VMM is a thin software layer that runs directly on the host machine hardware
Main advantage/objective: performance
VMWare ESX, 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. Virtual machine layer - VMM
Application
Guest OS1
VM1
Application
Guest OS2
VM2
VM3
Application
Guest OS3
Virtual machine layer - VMM
Hardware layer
Traditional VMM

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

11. 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.

12. 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.

13. 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.

14. 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?

15. 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.

16. 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

17. 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.

18. 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

19. 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

20. 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 …

21. 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.

22. 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

23. 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 ESXi Server is a line of “bare-metal hypervisors”
Expanding into datacenter and cloud applications (with Vmware vSphere and vCloud suite)

24. Xen 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 simultaneously on the same platform
Objective of original project: efficient hosting of up to 100 virtual machines

25. Hyper-V Hyper_V is Microsoft’s server virtualization software:
“Using Windows Server 2012 with Hyper-V, you can take advantage of new hardware technology, while still utilizing the servers you already have. This way you can virtualize today, and be ready for the future.”
Claims to handle server migration more easily than Vmware.

26. Denali Research project – U of Washington
Time frame ~
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

27. Reasons for Adopting VMMs
Flexibility in choice of operating system
Encapsulation: A VM collects together an operating system, a complete (virtual) computer system, and one or more applications into a single unit that can be treated like any other software application.
Can be saved to a file, for example
Security and isolation: provided by encapsulation

28. 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.

29. Encapsulation Conventionally, servers ran 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
Rosenblum and Garfinkel [1] point out that this also makes it possible to suspend and resume entire virtual machines; even move them to other platforms (migration)
For load balancing, system maintenance, etc.

30. Security and Isolation
Applications running on a VM are more secure than those running directly on hardware machines.
An application that crashes or corrupts the OS it runs on will have not affect other VMs
VMM controls how guest operating systems use hardware resources; what happens in one VM doesn’t affect any other VM.
OS level security is more vulnerable than VM security

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

32. 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

33. 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.

34. 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.

35. 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 – 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.

36. 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

37. Paravirtualization
Rewrite portions of the guest OS to replace non-virtualizable instructions with a trap to the VMM, which emulates the instruction on behalf of the guest OS
e.g., remove POPFs; substitute something else
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.

38. Dynamic Binary Translation
Dynamic binary translation looks at a short sequence of source code, translates it, and caches the resulting sequence. [ ]
Similar to JIT compilers.
VMware’s DBT controls execution of kernel code - 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.

39. 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.

40. 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 execution ring 0 and VMM in yet a higher privileged mode
Flags to indicate if running in this mode
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.

41. OS-level Virtualization
A server virtualization technology that works at the OS layer. The physical server and a single instance of the OS is virtualized into multiple isolated partitions, where each partition replicates a real server.
Result: multiple servers, running the same OS, running on top of a single OS; e.g. multiple virtual Linux servers running on a Linux server

42. OS-Level Virtualization
This kind of VM is sometimes called a VE (Virtual Execution Environment), Virtual Private System (VPS), or container.
Advantage: quicker to create OS-level VM than one directly on top of a VMM.
Disadvantage: can only support one operating system per physical server.
Example: OpenVZ for Linux

43. 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.

44. Challenges Let the guest OS decide which of its pages to swap out
VMware’s ESX Server uses the concept of a balloon process, running inside the guest OS [1].
When the VMM wants to swap out pages from a 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.

45. 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

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

47. Summary & Review (1) A virtual machine is a copy of a real machine
Applications don’t know if they are running on real or virtual hardware, other than having fewer resources.
A virtual machine is isolated: if several VMs execute on the same hardware they do not interact with each other directly or indirectly.
The performance of a virtual machine should be about the same as that of the actual hardware.
So most instructions should be directly executed by the hardware as opposed to being emulated.

48. Summary and Review (2)
Process virtual machines (JVM) virtualize at a higher level, do not necessarily even correspond to real machines.
System virtual machines virtualize at the level of the hardware-software interface
Variations of classic system virtual machine:
Hosted (run on another operating system
Emulation (provides virtual hardware and OS, as in Virtual PC) – not really a virtual machine

49. Summary & Review (3)
Virtual Machine Monitor (hypervisor) runs on a bare machine, implements one or more virtual machines.
The VMM allocates resources and controls resource sharing among all VMs
Operation:
Each VM runs a guest OS
VMM runs in kernel mode
Guest OS and applications run in user mode
Privileged instructions trap to the VMM
Hypercalls (the VMM equivalent of system calls) may be used by a guest OS to request service from the VMM

50. Summary & Review (4)
Benefits of VM technology for non-hosted (traditional, or native) VMs
Isolation and security
Multiple servers on a single machine
Encapsulation of an entire environment: OS and application for the purpose of
Migration
Checkpointing
Supporting system maintenance
Running several OS’s concurrently
Older versions, experimental systems, Linux & Windows, …
For hosted VMs, the major advantage is the ability to run two or more OS’s at once

51. Reading for Next Class
7. “The Multikernel: A new OS architecture for scalable multicore systems”, Andrew Baumann, et al., SOSP ’09 October 11-14, 2009.
8. “Factored Operating Systems (fos): The Case for a Scalable Operating System for Multicores”, David Wentzlaff and Anant Agarwal, ACM SIGOPS Operating System Review, Special Issue, April 2009
9. “Resource Management in the Tessellation Manycore OS”, Juan A. Colmenares, et. al., HOTPAR 10, 2nd USENIX Workshop on Hot Topics in Parallelism, June 14-15, 2010.

52. Xen Denali Hardware Virtual Machines
Appendix – Examples
Xen
Denali
Hardware Virtual Machines

53. Xen – Intro
Claim: virtualization is better than multi-tasking as a way to share hardware.
CPU requests, memory demand, disk accesses, other resource needs of one process impact the performance of other processes
Xen solution: multiplex resources at the OS level instead of the process level.

54. Xen implementation of VMM
Domain 0 guest has privileged access to the Xen hypervisor and can be used by the system administrator to manage the system.
Separation of powers
Xen only has to worry about multiplexing hardware to multiple guests
Domain 0
Guest
Application
Domain U
Guest OS2
Application
Domain U
Guest OS3
VM1
VM2
VM3
Xen
Hardware layer
Xen implementation of VMM

55. Xen Design Principles
Virtualize all architecture features that are required by standard binary interfaces.
To support existing applications without modification
Support multi-application guest operating systems
Use paravirtualization to get improved performance and resource isolation

56. Xen HVM (Hardware Virtual Machine)
Some versions of Xen are designed to run on Intel VT and AMD-V chips with special virtualizing hardware.
Able to run un-modified (no para-virtualization) operating systems. This implementation is known as a hardware virtual machine.
Windows requires an HVM environment; Linux, Solaris, and BSD systems don’t.

57. Xen Memory Management
Unlike VMWare and Denali, Xen expects the guest OS’s to manage their own hardware page tables.
To support this, each VM receives a fixed allocation of page frames which it can use as it wishes.
New page tables must be registered with Xen and updates must be validated by Xen.
Make the page table write protected.

58. Xen CPU Management
Xen is designed for the X86 architecture which supports 4 rings, or privilege levels.
Traditional OS’s execute in ring 0 (most privileged) and applications in ring 3 (least)
Xen executes in ring 0 (only level that can execute privileged instructions)
Guest OS runs in ring 1, which isolates it from applications.
Note: since this paper was written there have been some modifications to X86 to better support virtualization.

59. Xen CPU Management
Privileged instructions must be validated (is it OK?) and executed by Xen
Exceptions (page faults, system calls, other traps to OS) are handled as much as possible by the guest OS.
Exception handlers are registered & validated with Xen
System calls stop at the guest OS; Xen is involved only if the OS executes a privileged instruction.

60. Denali Isolation Kernel
Authors define Denali as a small-kernel operating system with similarities to microkernels and exokernels
Once thought to be inefficient, modern hardware has improved performance of this kernel architecture
They expected Denali to support multiple (up to 10,000) untrusted applications that are virtually independent.

61. Isolation Kernel Design Principles
Expose low-level resources rather than high-level abstractions for greater security
Avoid “layer-below” attacks
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]

62. Isolation Kernel Design Principles
Design for scalability
Be able to support a work load that has a few popular services and many that are accessed infrequently.
Modify the virtualized architecture for simplicity, scale and performance.
Paravirtualization for reasons other than necessity.
They do not believe isolation depends on providing an exact copy of hardware so they provide a hardware version that is modified to be more efficient and secure.

63. Zipf’s Law
Given a table that ranks something on the basis of its frequency of occurrence, Zipf’s law states that the most frequent item occurs about twice as often as the next most frequent item, which in turn occurs twice as often as the next item, and so on.
Zipf made this observation about words in a natural language. Here, we’re talking about accesses to various web services.

64. Statistically Multiplexing Services
Studies showed that the popularity of most network services (server requests, document searches, etc) followed a Zipfian distribution.
Implications:
Most requests go to a small number of services
Most services aren’t popular, but the total number of requests for unpopular services is non-trivial
With isolation it can be safe and efficient to run hundreds or even thousands of services concurrently on a single platform.

65. Proof-of-concept Denali is the virtualized architecture
Yakima: a VMM which was designed to run in ring 0 on x86 hardware.
Ilwaco: a simple prototype guest OS which provides a full set of abstractions to its applications while hiding the Denali architecture
Reasonable performance in tests
1.4 μsec to 9 μsec context switch time, depending on number of VMs
End-to-end run times of network apps were “comparable” to those of a traditional operating system.

66. Conclusion The Denali research project terminated in the mid-2000’s.
The Denali research group was right in supposing that virtual machine technology would be most useful today to enable efficient use of server hardware.