Operating System Support for Virtual Machines

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Operating System Support for Virtual Machines

• Samuel King, George Dunlap, Peter Chen
• Univ of Michigan

Ashish Gupta
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Overview

• Motivation
• Classification of VMs
• Advantage of Type II VMs
• About UMLinux exploiting Linux caps
• How UMLinux works ?
• The three bottlenecks, their solutions
• Performance results
• Conclusions Modifying host OS helps !

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Two classifications for VM
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Higher Level Interface
VMWare Guest toolsVAX VMM Security Kernel
VM/370VMWare
Denali
UMLinuxSimOSXen
u-kernels
JVM
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Two classifications for VM
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Underlying Platform
VM/370VMWare ESXDiscoDenaliXen
VMWare WorkstationVirtualPC
SimOSUMLinux
Type II
Type I
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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

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Advantage of Type II VM
Virtual CPU
Guest Machine Process
Virtual I/O Devices
Host files anddevices
Virtual Interrupts
Host Signals
Virtual MMU
mmapmunmap
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The problem
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Compiling the Linux Kernel
510 lines to Host OS
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Compiling the Linux Kernel
510 lines to Host OS
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Optimization OneSystem calls
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Lots of context switches between VMM lt -- gt Guest
machine process
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Use VMM as a Kernel module Modification to
Host OS also
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?
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Optimization TwoMemory protection
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Frequent switching between Guest Kernel and Guest
application
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Guest Kernel to Guest User
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Guest User to Guest Kernel
Through mmap, munmap and mprotect Very expensive
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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
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Solution Change Segment bounds for each mode
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Optimization ThreeContext Switching
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• The problem with context switching
• Have to remap user processs 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

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Multiple Page Table Sets
guest proc a
Guest OS
Page Table Ptr
Host operating system
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Conclusion

• Type II VMM CAN be as fast as type Iby modifying
  the Host OS
• Is the title of paper justified ?

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Virtualizing I/O Devices on VMware
WorkstationsHosted VMM

• Jeremy Sugerman, Ganesh Venkitachalam and
  Beng-Hong Lim
• VMware, Inc.

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Introduction

• VM Definition from IBM
• a virtual machine is a fully protected and
  isolated copy of the underlying physical
  machines 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

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The major tradeoff

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

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How I/O works
VM Driver
ApplicationPortion
PrivilegedPortion
VM App
VMM
CPU Virtualization
I/O Request
H/w interrupt
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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 doesnt
• 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

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The Goal of this paper
I/O
CPU
I/O
CPU
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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

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VMM
H O S T
Sending a Packet
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H O S T
Receiving a Packet
V M M
H O S T
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Experimental Setup
Nettest throughput tests
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Time profiling

• Extra 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

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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

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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

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Packet Transmit
Real Interrupt
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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()

?
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Summary of three optimizations

• Native

• VM/733 MHz
• Optimized

• VM/733 MHz
• Version 2.0

Guest OS idles
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Summary of three optimizations

• Native

• VM/350 MHz
• Optimized

• VM/350 MHz
• Version 2.0

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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

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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

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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 ??