Operating System Support for Virtual Machines Samuel King

Operating System Support for Virtual Machines Samuel King, George Dunlap, Peter Chen Univ of Michigan Ashish Gupta

Two classifications for VM 1 VM/370 VMWare Guest tools VAX VMM Security Kernel Higher Level Interface UMLinux Sim. OS Xen Denali u-kernels JVM

Two classifications for VM Convenience Performance 2 VM/370 VMWare ESX Disco Denali Xen Type I Underlying Platform VMWare Workstation Virtual. PC Sim. OS UMLinux Type II

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 and devices Virtual I/O Devices Host Signals Virtual Interrupts mmap munmap Virtual MMU

The problem

Compiling the Linux Kernel + 510 lines to Host OS

Compiling the Linux Kernel + 510 lines to Host OS

Optimization One System calls

Lots of context switches between VMM < -- > Guest machine process

Use VMM as a Kernel module Modification to Host OS also…

Optimization Two Memory 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 • x 86 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

Optimization Three Context 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 addressspaces – Each address space different set of page tables – New system call : switch guest, whenever context switching

Multiple Page Table Sets guest proc ab guest Guest OS switchguest syscall Page Table Ptr Host operating system

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 Application Portion VM App Privileged Portion VM Driver VMM CPU Virtualization I/O Request Interrupt reasserted H/w interrupt

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

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

V M M H O S T Sending a Packet

H O S T Receiving a Packet V M M H O S T

Experimental Setup Nettest: throughput tests

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

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 MHz Optimized §VM/733 MHz Version 2. 0 Guest OS idles

Summary of three optimizations §Native §VM/350 MHz Optimized §VM/350 MHz Version 2. 0

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 ? ?