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Virtual Machine - Introduction
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Virtual machine systems: simulators for multiple
copies of a machine on itself.
Virtual machine (VM): the simulated machine.
Virtual machine monitor (VMM): the simulator
software.
Pure software simulator is too slow; Direct
hardware execution is necessary
Complicated when OS involved; especially with
multiple OS; build separate VMM layer
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Virtual Machine - Introduction
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Principal characteristics of the original (IBM 360)
architectures for which virtual machine systems
were developed was the dual-state hardware
organization with a privileged and non-privileged
mode.
Privileged software nucleus (kernel)
Extended machine: set of non-privileged
instructions together with the supervisory calls.
VMM runs on privileged mode, while guest OS
runs on non-privileged mode
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Survey of Virtual Machine Research
IEEE Computer, 1974
Conventional and Virtual Machine
Organizations
Flexible Virtual Machine Model
The Renaissance of Virtualization
- X86 Prospective from VMware
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1970s: Virtual machines first used
1990s:
 x86 becomes prominent server platform, not
design for virtualization
 No vertical integration in x86
 Lack of enterprise features in commodity OSs
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1999: VMWare first product to virtualize x86
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2006: AMD and Intel offer hardware support
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Virtual Machine - Performance
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Extra resources needed: main memory,
processor cycles.
Sources of overhead:
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Maintaining the status of the virtual processor.
Support of privileged instructions.
Support of paging within virtual machines.
Console functions.
A problem: resource allocation algorithms in
operating system (these are meant for
managing real resources, but the virtual
resources may not correspond precisely to
real resources).
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Virtual Machine - Applications
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Privileged software development, test and
diagnostic functions, multiple operating system
execution concurrently with production uses of
the system.
Retrofit old operating systems with new
features.
Improved testing of privileged software.
Testing of computer network software on a
single physical machine.
Debugging and performance monitoring tools.
Software reliability through isolation.
Data security.
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Basic Computer Systems
Hardware / Software Interface
3: “system” ISA:
I/O, memory mgmt, CPU
intercept & emulate
4: “user” ISA:
ALU, branch, load/store
direct execution
Basic VM Architecture
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Classic VMs are enabled by “Virtual Machine
Monitor” (VMM) software
– VMM may be in total control of the underline
physical machine
• No O/S below the VMM
– Or, VMM may share the physical machine with
a “host” operating system
• VMM runs on top of host O/S (with user-level
processes, as well as with kernel code, e.g.
modules)
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Classic VMM
VMM Integrated with Host OS
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Advantages:
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Simple to install VMM, hosted OS accommodate rich I/O devices,
take advantage hosted OS’ resource management
Disadvantages:
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Performance overhead, no performance isolation
Fundamental task of VM monitor
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Present a “replica” of the interfaces to Hardware
provided by physical machine to each VM
– Interface to processor (i.e. instruction set)
– Interface to memory (i.e. use physical memory
addresses)
– Interface to I/O devices (e.g. memory mapped
I/O, interruptions)
* Different views of physical I/O device possible;
e.g. a SCSI physical disk may be viewed as
many virtual IDE disks
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VMM – Processor Virtualization
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Analogy with operating systems:
– Most application instructions execute directly on
processor (O/S does not see them)
– Those that require access to shared resources are
handled indirectly by calling the O/S via system calls
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VMM:
– Most application instructions execute directly on
processor (VMM does not see them)
– Those that require privileged processor access are
handled indirectly – they are trapped and emulated
by the VMM
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Challenges and Techniques
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X86 architecture is not designed for
virtualization
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POPF (pop CPU flags from stack) set / clear interruptdisable flag; does not trap in user mode in guest OS
Paravirtualization
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Replace nonvirtualizable instructions with virtualized
equivalent
OS must be ported in order to run in a virtual machine
Disco (for MIPS) changes MIPS interrupt flag from a
privileged register to a special memory location
VMM - Memory Virtualization
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Aided by processor’s ability to support virtual
memory
One more level of indirection is implemented
– O/S: application loads/stores virtual address; map to
physical addresses of physical machine
– Classic VM: application loads/stores virtual address;
map to physical address of the VM; then map to
physical address of physical machine
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Page Replication and Page Sharing
in Disco
VMM - I/O Virtualization
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Hosted versus non-hosted
– Non-hosted: VMM must implement drivers for
each device Since it interfaces directly with hardware;
must go through VMM, incur overhead
– Hosted: VMM may build on top of existing device
drivers of host O/S
• E.g. VMware: low-level I/O accesses to virtual disk
are mapped into accesses to a regular file of the
host via system calls
• Host O/S drivers, in turn, maps to I/O accesses to
physical disk