Operating System Support for Virtual Machines

Samuel King, George Dunlap, Peter Chen

Univ of Michigan

Ashish Gupta

Two classifications for VM

Higher Level Interface

VM/370VMWare

DenaliUMLinuxSimOSXen

VMWare Guest toolsVAX VMM Security Kernel

u-kernels JVM

1

Two classifications for VM

Underlying Platform

VM/370VMWare ESXDiscoDenaliXen

VMWare WorkstationVirtualPC

SimOSUMLinux

2

Type IIType I

Convenience

Performance

UMLinux• Higher level interface slightly different

• Guest OS needs to be modified– Simple device drivers added– Emulation of certain instructions (iret and in/out)– Kernel Re-linked to different address

• 17,000 lines of change

• ptrace virtualization– Intercepts guest system calls– Tracks transitions

Advantage of Type II VM

Guest Machine Process

Virtual CPU

Host files anddevices

Virtual I/O Devices

Host SignalsVirtual

Interrupts

mmapmunmap

Virtual MMU

The problem

Compiling the Linux Kernel

+ 510 lines to Host OS

Compiling the Linux Kernel

+ 510 lines to Host OS

Optimization OneSystem calls

Lots of context switches betweenVMM < -- > Guest machine process

Use VMM as a Kernel module

Modification to

Host OS also…

?

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POV-Ray SPECweb 99 Kernel Build

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VMware Workstation 3.1

Original UMLinux

UMLinux + VMM in host

Optimization TwoMemory protection

Frequent switching between Guest Kernel and Guest application

Guest Kernel to Guest User

Guest User to Guest Kernel

Through mmap, munmap and mprotect

Very expensive…

Host Linux Memory Management

• x86 paging provides built-in protection to memory pages

• Linux uses page tables for translation and protection• Segments used only to switch between privilege

levels• Uses supervisor bit to disallow ring 3 to access

certain pages

The idea: segments bound features are relatively unused

Solution:

Change Segment bounds for each mode

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POV-Ray SPECweb 99 Kernel Build

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Original UMLinux

UMLinux + VMM in host

UMLinux + VMM in host + seg.bounds prot.

Optimization ThreeContext Switching

• The problem with context switching:– Have to remap user process’s virtual memory to

the “virtual” physical memory– Generates large number of mmaps costly

• The solution:– Allow one process to maintain multiple address-

spaces– Each address space different set of page tables– New system call : switch guest, whenever context

switching

Multiple Page Table Sets

Page Table Ptr

Host operating system

Guest OS

guest proc aguest proc b

switchguest syscall

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POV-Ray SPECweb 99 Kernel Build

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VMware Workstation 3.1

Original UMLinux

UMLinux + VMM in host

UMLinux + VMM in host + seg.bounds prot.

Fully optimized UMLinux

Conclusion• Type II VMM CAN be as fast as type I

by modifying the Host OS

• Is the title of paper justified ?

Virtualizing I/O Devices on VMware Workstation’s

Hosted VMM

Jeremy Sugerman, Ganesh Venkitachalam and Beng-Hong Lim

VMware, Inc.

Introduction• VM Definition from IBM:

– a “virtual machine” is a fully protected and isolated copy of the underlying physical machine’s hardware.

• The choice for hosted architecture– Relies upon host OS for device support

• Primary Advantage– Copes with diversity of hardware– Compatible with pre-existing PC software– Near native performance for CPU intensive

workloads

The major tradeoff• I/O performance degradation

• I/O emulation done in host world– Switching between the host world and the VMM

world

How I/O works

VM App VMMVM

Driver

ApplicationPortion

PrivilegedPortion

I/O RequestI/O Virtualization

CPU Virtualization

H/w interruptInterrupt reasserted

I/O Virtualization• VMM intercepts all I/O operations

– Usually privileged IN , OUT operations

• Emulated either in VMM on in VMApp

• Host OS drivers understand the semantics of port I/O, VMM doesn’t

• Physical Hardware I/O must be handled in Host OS

• Lot of Overhead from world switching– Which devices get affected ?– CPU gets saturated before I/O…

The Goal of this paper

I/O CPU I/O CPU

The Network Card

• Virtual NIC appears as a full fledged PCI Ethernet Controller, with its own MAC address

• Connection implemented by a VMNet driver loaded in the Host OS

• Virtual NIC : a combination of code in the VMM and VMApp– Virtual I/O Ports and Virtual IRQs

HOST

VMM

Sending a Packet

VMM

HOST

HOST

Receiving a Packet

Experimental Setup

Nettest: throughput tests

Time profilingExtra work:

• Switching worlds for every I/O instruction: most expensive

• I/O interrupt for every packet sent and received:– VMM, host and guest interrupt handlers are run !

• Packet trans: two device drivers• Packet copy on transmit

Optimization One• Primary aim: Reduce world switches

• Idea: Only a third of the I/O instructions trigger packet trans.– Emulate the rest in VMM

• The Lance NIC address I/O has memory semantics– I/O MOV !– Strips away several layers of virtualization

Optimization Two• Very high interrupt rate for data trans.

• When does a world switch occur:– A packet is to be transmitted– A real interrupt occurs e.g. timer interrupt

• The Idea: Piggyback the packet interrupts on the real interrupts– Queue the packets in a ring buffer– Transmit all buffered packets on next switch

• Works well for I/O intensive workloads

Packet Transmit

Real Interrupt

Optimization Three• Reduce host system calls for packet sends

and receives

• Idea: Instead of select, use a shared bit-vector, to indicate packet availability

• Eliminates costly select() ?

Summary of three optimizations

Native

VM/733 MHzVersion 2.0

VM/733 MHzOptimized

Guest OS idles

Summary of three optimizations

Native

VM/350 MHzVersion 2.0

VM/350 MHzOptimized

Most effective Optimization ?• Emulating IN and OUT to Lance I/O ports

directly in VMM

• Why ?– Eliminates lots of world switches– I/O changed to MOV instruction

Further avenues for Optimization ?• Modify the Guest OS

– Substitute expensive-to-virtualize instructions e.g. MMU instructions . Example ??

– Import some OS functionality into VMM– Tradeoff: can use off-the-shelf Oses

• An idealized virtual NIC (Example ??)– Only one I/O for packet transmit instead of 12 !– Cost: custom device drivers for every OS– VMWare Server version

Further avenues for Optimization ?• Modify the Host OS: Example ??

– Change the Linux networking stack• Poor buffer management

– Cost: requires co-operation from OS Vendors

• Direct Control of Hardware: VMWare ESX– Fundamental limitations of Hosted Architecture– Idea: Let VMM drive I/O directly, no switching– Cost ??