Transcript Virtual Machine Monitors: Current Technology And Future Trends

Virtual Machine Monitors
Bibliography
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“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/
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
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.
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.
System & Process VMs
http://en.wikipedia.org/wiki/Virtual_machine
• System virtual machine (hardware virtual
machine) – See previous definitions
– 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
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
• Hybrid: shares the hardware with existing OS
– Example: VMWare Workstation
VM1
VM2
VM3
Application
Application
Application
Guest OS1
Guest OS2
Guest OS3
Virtual machine layer - VMM
Hardware layer
Traditional VMM
Hybrid
VM1
VM2
Rosenblum & Garfinkel – Fig. 2
VMM
App
I/O
App App
VMM
Operating system
Guest OS
Hardware layer
Host OS
VMM
Hardware Layer
Hosted
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
– 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.
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 provide complete isolation,
so not appropriate for servers from a
security perspective.
Hosted v Non-hosted VMM
• Conclusion:
– Hosting is a good approach for individual work
stations; reduces effort needed to get VMM
up and running.
– Hosting is not advisable for servers. Security
issues are the most important concern,
followed by added overhead for I/O.
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?
VM – How They Work (2)
• 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 performs the system call 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.
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
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.
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
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
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 …
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
• Other research groups explored different
approaches to VMs
• Result: today, virtual machines are very common
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
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 the most recent product in
this line; is a “bare-metal hypervisor”
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
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
Reasons for Adopting VMM’s
• 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
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.
– OS level security is more vulnerable than VM
security
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.
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 makes it
possible to suspend and resume entire virtual machines;
even move to other platforms
– For load balancing, system maintenance, etc.
Desirable Qualities
• A good VMM
– Doesn’t require applications to be modified
– Doesn’t severely affect performance
– Is not complex/error prone
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
CPU Virtualization
• Basic technique: direct execution
– As long as it is executing unprivileged
instructions the virtual machine (guest OS +
applications) executes hardware instructions
directly.
– 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.
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.
• Other interrupts are directed to VMs that
have not disabled them.
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.
Two Ways to Handle Nonvirtualizable 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.
Paravirtualization
• Rewrite portions of the guest OS to replace nonvirtualizable instructions with a trap 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.
Binary Translation
• Instead of modifying the OS, detect these
instructions at runtime.
• VMware’s approach: The DBT (dynamic
binary translator) controls execution of
kernel code - replaces non-virtualizable
instructions with equivalent code that can be
virtualized.
– Once translated, code is saved and used again
if needed.
Comparison
• Paravirtualization changes the source code of a
guest OS; binary translation changes the binary
code as it executes.
• 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.
Hardware-assisted Virtualization
• AMD-V and Intel VT are architecture extensions to
support virtualization.
– 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.
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.
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.
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
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.
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
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
Summary & Review (4)
• Benefits of VM technology for non-hosted 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
Appendix – Examples
Xen
Denali
Hardware Virtual Machines
Xen – Intro
• Claim: virtualization is better than multitasking 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.
VM1
Domain 0
Guest
VM2
Application
VM3
Application
Domain U
Guest OS2
Domain U
Guest OS3
Xen
Hardware layer
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
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
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 paravirtualization) operating systems. This
implementation is known as a hardware
virtual machine.
– Windows requires an HVM environment;
Linux, Solaris, and BSD systems don’t.
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.
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.
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.
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.
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]
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.
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.
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.
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.
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.
• Multi-core computing – the MPP of the
future? How useful will VMM concepts be?