Virtualization - Operating System Design and Implementation - Lecture Slides, Slides of Operating Systems

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Virtualization

Dave Eckhardt

Todd Mowry

based on material from :

Mike Kasick

Roger Dannenberg

Glenn Willen

Mike Cui

Nov 25, 2013

Outline ● (^) Introduction  (^) What, why? ● (^) Basic techniques  (^) Simulation  (^) Binary translation ● (^) Kinds of instructions ● (^) Virtualization  (^) x86 Virtualization  (^) Paravirtualization ● (^) Summary

Process vs. Virtualization ● (^) The Process abstraction is a “weak, fuzzy” form of virtualization  (^) Many process resources exactly match machine resources ● (^) %eax, %ebx, …  (^) Some machine resources are not visible to processes ● (^) %cr  (^) Some process resources are “inspired by” hardware ● (^) SIGALARM  (^) Some process resources are “invented” - don't match any hardware feature ● (^) “current directory” and “umask” ● (^) Virtualization is “more like hardware” than processes  (^) What runs inside virtualization is an operating system Process : Kernel :: Kernel :?

Process vs. Virtualization ● (^) The Process abstraction is a “weak, fuzzy” form of virtualization  (^) Many process resources exactly match machine resources ● (^) %eax, %ebx, …  (^) Some machine resources are not visible to processes ● (^) %cr  (^) Some process resources are “inspired by” hardware ● (^) SIGALARM  (^) Some process resources are “invented” - don't match any hardware feature ● (^) “current directory” and “umask” ● (^) Virtualization is “more like hardware” than processes  (^) What runs inside virtualization is an operating system Process : Kernel :: Kernel : Virtual-machine monitor

Disadvantages of the Process Abstraction ● (^) Processes share the fle system  (^) Diffcult to simultaneously use different versions of: ● (^) Programs, libraries, confgurations ● (^) Single machine owner:  (^) root is the superuser  (^) Any process that attains superuser privileges controls all processes ● (^) Processes share the same kernel  (^) Kernels are huge , lots of possibly-buggy code ● (^) Processes have limited degree of protection, even from each other  (^) Linux “OOM killer” can kill one process if another uses lots of memory ● (^) Overall, processes aren't that isolated from each other...

Process Kernel Process Process Physical Machine Process/Kernel Stack

Why Use Virtualization? ● (^) Run two operating systems on the same machine!  (^) “Windows+Linux” was VMware's frst business model  (^) Hobbyists like to run ancient-history OS's ● (^) Debugging OS's is more pleasant  (^) Also: instrumenting what an OS does  (^) Monitoring a captive OS for security infestations ● (^) “Process abstraction” at the kernel layer  (^) Separate fle system  (^) Multiple machine owners  (^) Better protection than one kernel's processes (in theory) ● (^) “Small, secure” hypervisor, “small, fair” scheduler

Why Use Virtualization? ● (^) Huge impact on enterprise hosting  (^) No longer need to sell whole machines  (^) Sell machine slices ● (^) “xx GB RAM, yy cores” - smoother than “n Dell PowerEdge 2600's” ● (^) Can put competitors on the same physical hardware ● (^) Can separate instance of VM from instance of hardware  (^) Live migration of VM from machine to machine ● (^) Deal with machine failures or machine-room flooding  (^) VM replication to provide fault tolerance ● (^) “Why bother doing it at the application level?” ● (^) Can overcommit hardware  (^) Most VM's are not 100% busy all the time  (^) If one suddenly becomes 100% busy, move it to a dedicated machine for a few hours, then move it back

Disadvantages of Virtual Machines ● (^) Attempt to solve what really is an abstraction issue somewhere else  (^) Monolithic kernels  (^) Not enough partitioning of global identifers ● (^) pids, uids, etc  (^) Applications written without distribution and fault tolerance in mind ● (^) Provides some interesting mechanisms, but may not directly solve “the problem”

Disadvantages of Virtual Machines ● (^) Feasibility issues  (^) Hardware support? OS support?  (^) Admin support?  (^) Popularity of virtualization platforms argues these can be handled ● (^) Performance issues  (^) Is a 10-20% performance hit tolerable? ● (^) When an IPC becomes and RPC the cost goes up dramatically  (^) Can your NIC or disk keep up with the load of multiple virtual machines?  (^) Interdomain DoS? Thrashing? ● (^) “Nothing fails like success”  (^) VMMs are getting larger, and potentially home to security bugs

Full-System Simulation (Simics 1998) ● (^) Software simulates hardware components that make up a target machine  (^) Interpreter executes each instruction & updates the software representation of the hardware state ● (^) Approach is very accurate but very slow ● (^) Great for OS development & debugging  (^) “Break on triple fault” is better than real hardware suddenly rebooting  (^) Possible to debug a driver for a hardware device that hasn't been built yet

System Emulation (Bochs, DOSBox, QEMU) ● (^) Emulate just enough of hardware components to create an accurate “user experience” ● (^) Typically CPU & memory are emulated  (^) Buses are not  (^) Devices communicate with CPU & memory directly ● (^) Shortcuts are taken to achieve better performance  (^) Reduces overall system accuracy  (^) Code designed to run correctly on real hardware executes “pretty well”  (^) Code not designed to run correctly on real hardware exhibits wildly divergent behavior

System Emulation Techniques ● (^) Dynamic translation:  (^) Translate a block of guest instructions to host instructions just prior to execution of that block  (^) Cache translated blocks for better performance  (^) Like a Smalltalk/Java “JIT” ● (^) Dynamic recompilation & adaptive optimization:  (^) Discover which algorithm the guest code implements  (^) Substitute with an optimized version on the host  (^) Hard

Outline ● (^) Introduction  (^) What, why? ● (^) Basic techniques  (^) Simulation  (^) Binary translation ● (^) Kinds of instructions ● (^) Virtualization  (^) x86 Virtualization  (^) Paravirtualization ● (^) Summary