System Virtual Machines Introduction Key concepts Resource - - PowerPoint PPT Presentation

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System Virtual Machines

• Introduction
• Key concepts
• Resource virtualization

– processors – memory – I/O devices

• Performance issues
• Applications

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Introduction

• System virtual machine

– capable of supporting multiple system images – each guest OS manages a set of virtualized hardware resources – VMM owns the real system resources – VMM allocates resources to guest OS by partitioning or time-sharing – guest OS allocates resources to its user programs – each virtual resource may not have physical resource – see Figure 8.1

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Introduction (2)

L i n u x A p p l i c a t i o n L i n u x O S W i n d o w s A p p l i c a t i o n W i n d o w s O S O S / 2 A p p l i c a t i o n O S / 2 O S I n t e l x 8 6 H a r d w a r e V i r t u a l I n t e l x 8 6 V i r t u a l I n t e l x 8 6 V i r t u a l I n t e l x 8 6

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Key Concepts

• Outward appearance
• State management
• Resource control
• Native and hosted virtual machines
• Several concepts are simplifjed

– same guest and host ISA – uniprocessor systems

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Outward Appearance

• Create an illusion of multiple machines.
• Illusion in software

– no replication of hardware resources – switch to move devices from one VM to the

• ther
• Replication of hardware resources

– devices used directly by each user are replicated – rest of the hardware is shared

• Hosted VM

– one OS more important than the other – UI of host OS provides way to display UI of the second

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State Management

• Refers to how each individual guest state

is managed by the SVM

– each guest OS has an architected state – host resources may not be adequate

• Fixed location for all guest state in host

– VMM manages pointer and switching – indirection can be very ineffjcient (Figure 8.3a)

• Copy approach

– more effjcient (Figure 8.3b) – essential to allow direct execution of native code

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State Management (2)

P r o c e s s o r V M M M e m o r y R e g is t e r v a lu e s f o r V M 1 R e g i s t e r v a l u e s f o r V M 2 R e g i s t e r v a l u e s f o r V M 3 R e g is t e r B lo c k P o i n t e r V M M M e m o r y P r o c e s s o r P r o c e s s o r R e g is t e r s R e g is t e r v a lu e s f o r V M 1 R e g is t e r v a lu e s f o r V M 2 R e g is t e r v a lu e s f o r V M 3

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Resource Control

• Refers to how the VMM maintains overall

control of all hardware resources.

• Scheme similar to time-sharing OS

– VMM intercept accesses to privileged resources – get control on all privileged instructions – VMM handles the timer interrupt (Figure 8.4) – similar tradeofgs between fair scheduling and large switching overhead

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VMM Timesharing

• VMM timeshares resources among

guests

– similar to OS timesharing

V M M s a v e s a r c h i t e c t e d s t a t e

• f r u n n i n g V M

V M M r e s t o r e s a r c h i t e c t e d s t a t e f o r n e x t V M V M M s e t s P C t o t i m e r i n t e r r u p t h a n d l e r o f O S i n n e x t V M V M M s e t s t i m e r i n t e r v a l a n d e n a b l e s i n t e r r u p t s T i m e r i n t e r r u p t

• c c u r s

V M M d e t e r m i n e s n e x t V M t o b e a c t i v a t e d V M M A c t i v e F i r s t V M A c t i v e N e x t V M A c t i v e

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Native and Hosted VM

• Refers to the runtime privilege level of

the VMM (and OS).

– for effjciency, some part of VMM should have higher privileges

• Figure 8.5

– analogous relationship to OS and user programs – native VM system: only the VMM operates in privilege mode – hosted VM: VMM installed on top of existing OS

• some or no part of VMM in privilege mode

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Native and Hosted VM (2)

N o n - p r i v i l e g e d m o d e s P r i v i l e g e d M o d e A p p l i c a t i o n s O S T r a d i t i o n a l u n i p r o c e s s o r s y s t e m H a r d w a r e V i r t u a l M a c h i n e V M M H a r d w a r e V i r t u a l M a c h i n e H o s t O S H a r d w a r e V M M V i r t u a l M a c h i n e H o s t O S H a r d w a r e V M M N a t i v e V M s y s t e m U s e r - m o d e H o s t e d V M s y s t e m D u a l - m o d e H o s t e d V M s y s t e m

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Resource Virtualization – Processors

• T

echniques to execute guest instructions – emulation – direct native execution

• Emulation

– interpretation or binary translation – necessary if difgerent host and guest ISAs

• Direct native execution

– same host and guest ISAs required – may still require emulation of some instructions

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Conditions for ISA Virtualizability

• Conditions laid out by Popek and Goldberg.
• Assume native system VMs

– VMM runs in system mode – all other software runs in user mode

• Other assumptions

– hardware consists of processor and uniformly addressable memory – processor can operate in two modes, user and system – some subset of instructions only available in system mode – relocation register available for memory addressing

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Privileged Instructions

• Instructions that trap if the machine is

in user mode, and does not trap if the machine is in system mode.

• LPSW (Load Processor Status Word)

– PSW can change the state of the CPU – can also change the instruction address (PC)

• SPT (Set CPU Timer)

– can change the CPU interval timer

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Privileged Instructions (cont…)

• LRW (Load Real Address)

– translate a virtual address, save corresponding real address in a specifjed GPR

• POPF (Pop Stack into Flags Register)

– replace fmag register with top of memory stack – interrupt-enable fmag can only be modifjed in system mode – no trap in user mode

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Sensitive Instructions

• These specify instructions that

interact with hardware.

• Control-sensitive instructions

– can change the confjguration of system resources – e.g., physical memory assigned to a program – e.g., LPSW – change mode of operation – e.g., SPT – change CPU timer value

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Sensitive Instructions (cont…)

• Behavior-sensitive instructions

– behavior or results depend on confjguration

• f resources

– e.g., LRA – depends on mapping (state) of real memory resource – e.g., POPF – depends on mode of operation

• Innocuous instructions

– neither context-sensitive nor behavior- sensitive

• see Figure 8.6

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Instruction T ypes – Summary

P r i v i l e g e d N o n - P r i v i l e g e d S e n s i t i v e B e h a v i o r - s e n s i t i v e C o n t r o l - s e n s i t i v e I n n o c u o u s S e n s i t i v e

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Components of a VMM

I n s t r u c t i o n t r a p o c c u r s D i s p a t c h e r A l l o c a t o r I n t e r p r e t e r R o u t i n e 2 I n t e r p r e t e r R o u t i n e 1 I n t e r p r e t e r R o u t i n e n T h e s e i n s t r u c t i o n s d e s i r e t o c h a n g e m a c h i n e r e s o u r c e s , e . g . L o a d R e l o c a t i o n B o u n d s R e g i s t e r T h e s e i n s t r u c t i o n s d o n o t c h a n g e m a c h i n e r e s o u r c e s , b u t a c c e s s p r i v i l e g e d r e s o u r c e s , e . g . I N , O U T , W r i t e T L B

P r i v i l e g e d I n s t r u c t i o n P r i v i l e g e d I n s t r u c t i o n P r i v i l e g e d I n s t r u c t i o n P r i v i l e g e d I n s t r u c t i o n

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Components of a VMM (2)

• Dispatcher

– control module, called on hardware traps – decides the next module to be invoked

• Allocator

– decides allocation of system resources – invoked by dispatcher to change machine resource allocation for VMs

• Interpreter routines

– emulates trap functionality for each user VM – all traps except resource re-assignment traps

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Properties for a True VMM

• Effjciency

– All innocuous instructions must be natively executed

• Resource control

– impossible for guest s/w to directly change system resource confjgurations

• Equivalence

– guaranty of identical behavior – allowed exceptions

• performance can be slower
• available resources can be limited
• difgerences in performance due to changed

timings

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Requirement for True VMM

• For any third-generation computer, a

VMM may be constructed if the set of sensitive instructions for a computer is a subset of the set of privileged instructions

– sensitive instructions always trap in user mode – non-privileged instructions execute natively

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Handling Privileged Instruction

• Privileged instruction traps in the guest OS

G u e s t O S c o d e i n V M

( u s e r m o d e ) P r i v i l e g e d i n s t r u c t i o n ( L P S W ) … . . . … . . . N e x t i n s t r u c t i o n ( t a r g e t o f L P S W )

V M M c o d e

( p r i v i l e g e d m o d e ) L P S W R o u t i n e: C h a n g e m o d e t o p r i v i l e g e d C h e c k p r i v i l e g e l e v e l i n V M E m u l a t e i n s t r u c t i o n C o m p u t e t a r g e t R e s t o r e m o d e t o u s e r J u m p t o t a r g e t D i s p a t c h e r

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Problem Instructions

• Sensitive instructions that are not

privileged

– POPF instruction in the Intel IA-32 – IA-32 not virtualizable by Popek-Goldberg rules

• Handling problem instructions

– interpret all guest software – scan and patch problem instructions before execution – see Figure 8.11

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Patching Problem Instructions

C o n t r o l t r a n s f e r , e . g . t r a p

S c a n n e r a n d P a t c h e r V M M

C o d e P a t c h f o r d i s c o v e r e d c r i t i c a l i n s t r u c t i o n

O r i g i n a l P r o g r a m P a t c h e d P r o g r a m

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Patching Problem Instructions (2)

• VMM takes control at the head of each basic

block.

• Scan to fjnd problem instructions.
• Replace with trap to VMM.
• Place another trap at end of basic block.
• Patched basic blocks can be chained.
• Mostly similar to binary translation.

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Caching Emulation Code

• High frequency of sensitive instructions

requiring interpretation increases VMM

• verhead

– cache interpreter actions in code cache – cache block containing the problem instruction

• Each instance of the problem instruction

in the code associated with a distinct piece of cache code

– make code instance-specifjc and optimize

• Simpler management of cached code

– cached code executed in system mode

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Resource Virtualization – Memory

• Generalizes the concept of virtual memory
• Virtual memory basics

– application provided a logical view of memory – OS manages actual real memory – page tables provide logical-physical mapping – TLBs cache most common mappings

• VMM distinguishes between real memory and

physical memory – VMM does real-physical memory mapping

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VMM Memory Support

• VMM mechanism to virtualize memory

– maintain a per-VM real-map table – maps real pages to physical pages – VMM maintains a distinct swap space

• Additional indirection layer may be

ineffjcient

– virtualize architected page tables – virtualize architected TLB

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Resource Virtualization – I/O

• Virtualization of I/O is diffjcult

– large number of I/O devices – each I/O device controlled in specifjc manner – OS have to deal with similar issues

• Virtualization Scheme

– provide VMs with virtual version of a device – intercept VM requests made to virtual device – convert request, perform equivalent action

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Virtualizing Devices

• Dedicated devices

– display, keyboard, mouse etc., for each user – no device virtualization necessary – device requests could bypass the VM

• Partitioned devices

– smaller virtual disks for each user – VMM use map to translate parameters – status information from the device also needs translation and refmection in the map

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Virtualizing Devices (cont…)

• Shared devices

– network adapter, virtual network address for a VM – maintain state information for each guest VM – translate device outgoing/incoming requests

• Spooled devices

– shared at higher granularity, e.g., printer – two-level spool tables in OS and VMM – VMM schedules requests from multiple guest VMs

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Performance Degradation

• Setup

– initialize the state of the machine

• Emulation

– sensitive instructions, maybe others

• Interrupt handling

– handling interrupts generated by the guest programs

• State saving

– save/restore state of VM during switch to VMM

• Book-keeping

– special operations to maintain equivalent behavior

• Time elongation

– some instructions may require more processing time

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VM Assists

• Piece of hardware that improves performance
• f an application when running on a VM.
• May not always improve performance

– improve user program-VMM system-mode switches – improve address translation performance

• VM assists can help improve performance of

– instruction emulation – other aspects of VMM – the user-mode system running as the guest – specifjc types of guest systems

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Instruction Emulation Assists

• Emulation of privileged instructions in the

guest VM is a cause of fundamental overhead

– causes interrupt that is handled by the VMM – VMM emulation depends on whether guest VM in user or system mode when instruction called

• Assist for LPSW on system/370

– check state of guest VM – provides hardware-assisted instructions to modify physical resources

• Depends on VM system implementation
• The overhead of the trap still remains

– eliminates overhead of emulation and mode switching

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VMM Assists

• Context switch

– hardware to save/restore registers, machine state

• Decoding of privileged instructions

– helps certain critical parts of the emulation

• Virtual interval timer

– precise timer for all guest VMs to schedule jobs

• Adding to the instruction set

– to help commonly executed VMM parts

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Improve Guest VM Performance

• Requires guest system to be VMM aware

– avoid duplication of functionality – provide VMM with additional information (handshaking), e.g., DIAGNOSE instruction

• Example; non-paged mode

– disable dynamic address translation in guest VM – no translation from virtual to real address – translation from real to physical done by VMM

• Paravirtualization

– modify guest OS to work around diffjcult to virtualize ISA features – Xen for the IA-32, eliminates code detection, patching, shadow page tables

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Applications

• Implementing multiprogramming
• Multiple single-application virtual machines
• Multiple secure environments
• Mixed-OS environments
• Legacy applications
• Multiplatform application development
• Gradual migration to new system
• New system software development
• Operating system training
• Help desk support
• Operating system instrumentation
• Event monitoring
• System encapsulation and checkpointing