Download VirtualMachines

CSC 660: Advanced OS
Virtual Machines
CSC 660: Advanced Operating Systems
Slide #1
Topics
1.
2.
3.
4.
5.
6.
7.
8.
What is a VM?
Process vs System VMs
Virtualizing the Processor
Virtualizing Memory
Virtualizing I/O
VM Performance Issues
Intel VT-x Technology
Paravirtualization
CSC 660: Advanced Operating Systems
Slide #2
What is a VM?
A virtualized system that
–
–
–
–
Provides a consistent ABI to guest programs.
Runs on a host system (software + hardware.)
Controls resources available to guest programs.
May provide different resources than hardware
• Different Type (ex: JVML in Java VM)
• Different Quantity (ex: more/fewer CPUs, disks, etc.)
– May be of two major types
• Process: provides VM to a single process.
• System: emulates an entire machine w/ guest OS.
CSC 660: Advanced Operating Systems
Slide #3
System Models
Non-virtual Machine
CSC 660: Advanced Operating Systems
Virtual Machine
Slide #4
Why use Virtual Machines?
Portability
Run software on a different OS.
Run software on a different CPU.
Aggregation
Modern machines are fast and underused.
Put multiple servers in VMs on one real machine.
Development
Complex software environments.
Processor testing and simulation.
Debugging
Can analyze every aspect of hardware behavior.
Security
VMs provide greater isolation of software than regular OS.
CSC 660: Advanced Operating Systems
Slide #5
Types of VMs
CSC 660: Advanced Operating Systems
Slide #6
Process VMs
• Multitasking
– Each process in a multitasking OS.
– VM = System call interface + ISA + VirtMem
• Emulators
– Allow a process to run on a different OS/ISA.
– Types:
• Interpreter
• Dynamic binary translator
• High Level Language VMs
– ex: Pascal, JVM, CLR
CSC 660: Advanced Operating Systems
Slide #7
HLL VMs
HLL Program
HLL Program
Compiler Front End
Compiler
Intermediate Code
Compiler Back End
Byte Code
Dist
Object Code
Loader
VM Loader
Virtual Memory Image
Dist
Memory Image
CSC 660: Advanced Operating Systems
VM
Host Instructions
Slide #8
System VMs
• Virtual Machine Monitor (VMM)
– Provides illusion of multiple isolated machines.
– Manages allocation of and access to hardware
resources for multiple guest OSes.
– Layer between hardware and guest OS.
• VMM tasks
– State management
– Resource control
CSC 660: Advanced Operating Systems
Slide #9
System VMs
Guest Apps
Guest Apps
Guest OS
Guest OS
VMM
VMM
Host OS
Hardware
Hardware
Applications
OS
Hardware
a. Traditional OS
b. Native VMM
CSC 660: Advanced Operating Systems
c. User-mode Hosted VMM
Slide #10
VMM Modes
• Requirement: guest OS may not change hardware
resources to impact other VMs or the VMM.
• Guest OS runs in user mode.
• VMM runs in supervisor mode.
– Tracks virtual mode of VM.
– User programs run in virtual user mode.
– OS runs in virtual supervisor mode.
• Exceptions & interrupts invoke VMM.
– VMM can handle directly
– or produce a virtual exception for guest OS.
CSC 660: Advanced Operating Systems
Slide #11
System VM Execution
1.
2.
3.
4.
5.
6.
7.
Timer Interrupt in running VM.
Context switch to VMM.
VMM saves state of running VM.
VMM determines next VM to execute.
VMM sets timer interrupt.
VMM restores state of next VM.
VMM sets PC to timer interrupt handler of
next VM.
8. Next VM active.
CSC 660: Advanced Operating Systems
Slide #12
IBM VM/370
Mainframe VMM OS.
–
–
–
–
First VM environment in System/360 1965.
Control program was a native VMM.
Each user had VM running single-user CMS.
Principles still used in z/VM on IBM zSeries.
CSC 660: Advanced Operating Systems
Slide #13
VMWare
• x86-based VM.
• VMWare Workstation
– Hosted VMM runs on Linux or Windows.
– Any x86 OS can be used as guest OS.
• VMWare ESX Server
– Native VMM runs directly on x86 hardware.
– VMotion allows VM migration.
CSC 660: Advanced Operating Systems
Slide #14
Virtualizing the Processor
• Emulation
– VMM examines each guest instruction and
emulates on virtualized resources the effects that
instruction would have had on real resources.
– Only method when host/guest ISA differs.
– May be necessary with identical host/guest ISA.
• Direct Native Execution
– Host ISA must be identical to guest ISA.
– Most instructions executed directly on CPU.
– Some sensitive instructions must be emulated.
CSC 660: Advanced Operating Systems
Slide #15
Privileged Instructions
• Instrs that trap if executed in user mode.
– I/O
– Memory Management
• Can only be executed in supervisor mode.
CSC 660: Advanced Operating Systems
Slide #16
Handling Privileged Instructions
Instruction Trap
Dispatcher
Instructions to
change machine
resources.
Interpreter
Routine #1
Allocator
Interpreter
Routine #N
CSC 660: Advanced Operating Systems
Slide #17
Handling Privileged Instructions
1.
2.
3.
4.
5.
6.
7.
Instruction Trap invokes VMM Dispatcher.
Dispatcher calls Instruction Routine.
Changes mode to supervisor.
Emulates instruction.
Computes return target.
Restores mode to user.
Jumps to target.
CSC 660: Advanced Operating Systems
Slide #18
Sensitive Instructions
Some instructions cannot be executed
directly on the CPU because they would
interfere with the VMM or other VMs.
Control-sensitive Instructions
Attempt to modify resource configuration.
Behavior-sensitive Instructions
Behavior depends on resource configuration,
including user/kernel mode setting.
CSC 660: Advanced Operating Systems
Slide #19
Non-Privileged Sensitive
Instructions
Example: POPF
Pops flag registers from stack.
Includes interrupt-enable flag.
User mode, POPF modifies all but interrupt flag.
Kernel mode, POPF modifies all flags.
CSC 660: Advanced Operating Systems
Slide #20
Efficient VMs
• If all sensitive instructions are privileged, the
processor is considered to be virtualizable.
– All sensitive instructions produce a trap.
• If non-privileged sensitive instructions exist,
then the VMM must examine all instructions
before execution to trap these instructions.
– Emulation
– Dynamic Binary Translation (Patching)
CSC 660: Advanced Operating Systems
Slide #21
Memory Virtualization
• Virtual Memory in a VM
– Each guest OS maintains own set of page tables.
– Guest OS translates virtual memory locations to real
memory locations (“physical memory” of VM.)
– Guest OS has swap space on virtual disk.
• VMM
– Translates real memory to physical memory using MMU.
– Doubles number of page faults.
– VMM has swap space on physical disk.
CSC 660: Advanced Operating Systems
Slide #22
Shadow Page Tables
• VMM maintains shadow page tables
– Map virtual->physical instead of real->physical.
– VMM validates guest page table updates.
– Replicates guest changes in shadow page table.
• Virtualize page table pointer register.
– VMM manages real page table pointer.
– Updates page table ptr when switching VMs.
CSC 660: Advanced Operating Systems
Slide #23
Shadow Page Tables
guest reads
Guest Page Table
Guest OS
guest writes
Accessed &
dirty bits
Updates
Shadow Page Table
VMM
MMU
CSC 660: Advanced Operating Systems
Hardware
Slide #24
I/O Virtualization
• VMM must intercept all guest I/O ops.
– PC: privileged IN and OUT instructions.
– I/O operation may consist of many INs/OUTs.
• Problem: huge array of diverse hardware
– Native VMM needs driver for each device.
– Hosted VMM uses host drivers w/ perf penalty.
CSC 660: Advanced Operating Systems
Slide #25
Virtualizing Devices
• Dedicated Devices
– VM has sole control of device.
• Partitioned Devices
– VM has dedicated slice of device, treats as full.
– VMM translates virtual full dev parameters to parameters
for underlying physical device.
• Shared Devices
– VMM can multiplex devices.
– Each VM may have own virtual device state.
• Nonexistent Devices
– Virtual software devices with no physical counterpart.
CSC 660: Advanced Operating Systems
Slide #26
Virtualizing a Network Card
CSC 660: Advanced Operating Systems
Slide #27
VM Performance
Why is VM slower than physical hardware?
Emulation: Sensitive instructions must be emulated.
Interrupt Handling: VMM must handle interrupts, even if
eventually passed to guest.
Context Switches: VMM must save VM state when
controlled transferred to VMM.
Bookkeeping: VMM has to do work to simulate behavior
of real machine, such as keeping track of time for VMs.
Memory: Memory accesses may require access to both
shadow and local page tables.
CSC 660: Advanced Operating Systems
Slide #28
VT-x Technology
• New CPU modes: VMX root/non-root
– VMM runs in VMX root.
– Guest VMs run in VMX non-root.
– Each mode has rings 0..3.
• Virtual Machine Control Structure (VMCS)
– Guest Area, Host Area.
• Transitions
– VM Entry: root to non-root transition.
• Load processor state from VMCS guest area.
– VM Exit: non-root to root transition.
• Save state to VMCS guest area, load state from host area.
CSC 660: Advanced Operating Systems
Slide #29
VT-x Technology
• Instructions
– Some sensitive instructions operate on non-root
VMX state; others produce a VM exit.
– VMM controls which instructions VM exit.
• Interrupts
– External interrupts cause VM exits.
– VMM controls which exceptions VM exit.
CSC 660: Advanced Operating Systems
Slide #30
Paravirtualization: Xen
• Provide VM abstraction similar to hardware.
– Modifies guest OS to use Xen/x86 architecture.
• Memory
– Guest has read access to hardware page tables.
– Updates batched and validated by Xen VMM.
• CPU
– Guest OS installs direct system call handler.
– Sensitive instructions replaced with Xen calls.
• I/O
– Event mechanism replaces hardware interrupts.
CSC 660: Advanced Operating Systems
Slide #31
Xen 1.2 Architecture
CSC 660: Advanced Operating Systems
Slide #32
• VMM resides in
top 64MB.
• Protected by
segmentation, not
page tbl for perf.
4GB
3GB
Xen
S
Kernel
S
User
U
ring 3
ring 1
ring 0
Xen VMM
0GB
CSC 660: Advanced Operating Systems
Slide #33
Xen System Performance
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
L
X
V
U
SPEC INT2000 (score)
L
X
V
U
Linux build time (s)
L
X
V
U
OSDB-OLTP (tup/s)
L
X
V
U
SPEC WEB99 (score)
Benchmark suite running on Linux (L), Xen (X), VMware Workstation (V), and UML (U)
CSC 660: Advanced Operating Systems
Slide #34
References
1.
2.
3.
4.
5.
6.
7.
8.
Paul Barham et. al., “Xen and the Art of Virtualization,” 19th ACM
Symposium on Operating Systems Principles, Oct. 19-22 2003.
Ian Pratt, “Xen 3.0 and the Art of Virtualization,” Ottawa Linux Symposium
2005.
John Scott Robin and Cynthia E. Irvine, “Analysis of the Intel Pentium’s
Ability to Support a Secure Virtual Machine Monitor,” Proceedings of the 9th
USENIX Security Symposium, Aug 14-17 2000.
Mendel Rosenblum and Tal Garfinkel, “Virtual Machine Monitors: Current
Technology and Future Trends,” IEEE Computer, May 2005.
James E. Smith and Ravi Nair, Virtual Machines, Elsevier, 2005.
Abraham Silberschatz, Peter Baer Galvin, and Greg Gagne, Operating
System Concepts, 6th edition, Wiley, 2003.
Jeremy Sugerman, et. al., “Virtualizing I/O Devices on VMware
Workstation’s Hosted Virtual Machine Monitor,” Proceedings of the 2001
USENIX Annual Technical Conference, 2001.
Rich Uhlig et. al., “Intel Virtualization Technology,” IEEE Computer, May
2005.
CSC 660: Advanced Operating Systems
Slide #35