Virtual Machines What Why How Powerpoint?

What is a Virtual Machine?

 A Piece of software that emulates hardware.

 Might emulate the I/O devices  Might emulate the CPU  Might be very high fidelity emulation  Might cheat

Different from an O.S.

 An OS has system calls.

 To write to the serial port, you call write(...)  A VM fakes hardware.

 To write to the serial port, you copy bytes and trigger interrupts.

Operating Systems and Virtual Machines  Operating Systems can run on real hardware.

 Operating Systems can run on fake hardware (virtual hardware).

 Lots of Oss can run on the same real machine.

 If the VM is good, the OS cannot tell.

 Timing issues  Performance (good or bad)

Uses of a Virtual Machine  Pretend to be a non-existent machine (prototyping)  Multitask lots of virtual machines on one physical machine (web servers).

 Emulate any machine or network  Atari  Parallels  Vmware's Teams

Examples of Virtual Machines  VMware  Has many Linux/Windows boxes running on one.

 Parallels  Has a Windows OS running under MacOS  VM  Has many applications running on a mainframe.

How to Make a Virtual Machine  Take over the CPU into supervisor mode.

 Mark a bunch of memory regions protected  Interrupt table, devices, page tables  Start running the OS code  On the real CPU in non-supervisor mode  On a fake CPU if need be  CPU is different  CPU is non-virtualizeable

What if it's the Wrong CPU  You can always fake the CPU  Read the instruction  Do what it would do  Repeat  Suffer a 5x to 20x performance hit

Reading/Writing to the Serial Port  Writing  OS writes to a device register  VM gets an interrupt, fakes the write  Reading  VM gets an interrupt.

 VM fakes an interrupt to the OS.

Serial Port (Continued)  Efficiency  Stinks  Virtualizability  Can have more serial ports than exist  Can redirect serial ports to anything else (USB?)

Virtualizing Video Framebuffer  Frame Buffer  Mark every page is r/o  When OS writes, VM gets an interrupt and writes  HUGE slowdown

Virtualizing a Modern Video Card  OS makes a command list  OS writes to a register on the video card to trigger the graphics processor unit.

 VM gets an interrupt  VM rewrites the command list if needed  VM sends the (new) commands to the video card.

Virtualizing Graphics With a Fake Device Driver  Make a device driver that does not actually drive a device, but instead communicates with the VM.

 Have the VM receive these messages and execute the instructions.

 SPEED!!!!

Virtualizing RAM  OS has a page table stored in ram it thinks it owns.

 VM has really marked that as read-only.

 When the OS makes a change, the VM gets an interrupt and reflects the change to the real page table.

RAM in a Virtual Machine  No sharing between guess OSs.

 No way for an OS to make RAM as unneeded.

 NO way to change the allocation of RAM between Vms on the fly.

CPU in a Virtual Machine  Each guest OS gets the CPU a fixed amount  If a guest OS is idle ... CPU is wasted

File Systems in a Virtual Machine  Each guest OS has it's own fake disk drive.

 Each fake disk drive is typically a file on the host.

 Yes, this is lots of disk space.

Cheating in a VM  It's called a performance enhancement :-)  Since Window's data structures are well known, and it's possible to write a windows device driver...

 Vmware can share identical RAM pages  Vmware can harvest free RAM  Vmware can harvest idle CPU  Vmware has a vmware file system

Xen  Basic Idea:  Don't emulate a real machine, that's too hard and slow.

 Edit each guest OS to communicate with the host (paravirtualization).

 Implications  better performance, easier implementation  Can no longer run unmodified Oss

Xen (again)  Xen has code inside Windows (non-published) and Linux.

 Xen (starting at 3.0) can also do straight virtualization for a few Oss (like Windows).

 Best??? of both worlds.

 Also supports live migration  Only a 300 ms delay

Performance  If you need to fake the CPU  performance is like 5-10% of real time.

 If your application is I/O intensive  performance can suffer  If you application is CPU intensive  performance is better  If you are emulating an Atari 2600(tm)  Your performance is better than real time.

Performance (again)  Dr. Dobbs says  CPU performance hit is < 10%  Ram hit is “costs ~200M”  http://www.ddj.com/hpc-high-performance computing/199000664;jsessionid=BSCMYJRQRF5 VSQSNDLRSKHSCJUNN2JVN?pgno=3

Crazy Nested VM-itude  Can you run a VM inside a VM?

 depends  Should you run a VM inside a VM  maybe not  How deep can you go  typically, either zero or infinity

Other Uses of VMs  Since any guest can be a file ...

 Ship it across country  Pause and restart  Clone thousands  Debug