1. CPS110: Wrapping up memory Landon Cox March 6, 2008

2. Traditional OS structure Host Machine Operating System App

3. OS abstractions Hardware OS Applications What are the interfaces and the resources? What is being virtualized? Instructions CPU Virtual addrs Physical mem Syst calls I/O devices Last month of class

4. Courser abstraction: virtual machine  We’ve already seen a kind of virtual machine  OS gives processes virtual memory  Each process runs on a virtualized CPU  Virtual machine  An execution environment  May or may not correspond to physical reality

5. Virtual machine options  How to implement a virtual machine? 1.Interpreted virtual machines  Translate every VM instruction  Kind of like on-the-fly compilation  VM instruction  HW instruction(s) 2.Direct execution  Execute instructions directly  Emulate the hard ones

6. Interpreted virtual machines  Implement the machine in software  Must translate emulated to physical  Java: byte codes  x86, PPC, ARM, etc  Software fetches/executes instructions Program (foo.class) Byte code Interpreter (java) x86 What does this picture look like? Dynamic virtual memory translator

7. Java virtual machine  What is the interface?  Java byte-code instructions  What is the abstraction?  Stack-machine architecture  What are the resources?  CPU, physical memory, disk, network  The Java programming language  High-level language compiled into byte code  Library of services (kind of like a kernel)  Like C++/STL, C#

8. Direct execution  What is the interface?  Hardware ISA (e.g. x86 instructions)  What is the abstraction?  Physical machine (e.g. x86 processor)  What are the resources?  CPU, physical memory, disk, network Program (XP kernel) x86 Monitor (VMware) x86

9. Different techniques  Emulation  Bochs, QEMU  Full virtualization  VMware  Paravirtualization  Xen  Dynamic recompilation  Virtual PC

10. Virtual machines are hot VMware IPO: $19.1 billion Xen sale: $500 million

11. Views of the CPU  How is a process’s view of the CPU different than the OS’s?  Kernel mode  Access to physical memory  Manipulation of page tables  Other “privileged instructions”  Turn off interrupts  Traps  Keep these in mind when thinking about virtual machines

12. Virtual machine structure Host Machine Virtual Machine Monitor (Hypervisor) Guest OS Guest App Guest App Guest OS Guest App

13. Why are hypervisors useful?  Code reuse  Can run old operating systems + apps on new hardware  Original purpose of VMs by IBM in the 60s  Encapsulation  Can put entire state of an “application” in one thing  Move it, restore it, copy it, etc  Isolation, security  All interactions with hardware are mediated  Hypervisor can keep one VM from affecting another  Hypervisor cannot be corrupted by guest operating systems

14. Encapsulation  Say I want to suspend/restore an application  I decide to write the process mem + PCB to disk  I reboot my kernel and restart the process  Will this work?  No, application state is spread out in many places  Application might involve multiple processes  Applications have state in the kernel (lost on reboot)  (e.g. open files, locks, process ids, driver states, etc)

15. Encapsulation  Virtual machines capture all of this state  Can suspend/restore an application  On same machine between boots  On different machines  Very useful in server farms  We’ll talk more about this with Xen

16. Security  Can user processes corrupt the kernel?  Can overwrite logs  Overwrite kernel file  Can boot a new kernel  Exploit a bug in the system call interface  Ok, so I’ll use a hypervisor. Is my data any less vulnerable?  All the state in the guest is still vulnerable (file systems, etc)  So what’s the point?  Hypervisors can observe the guest OS  Security services in hypervisor are safe, makes detection easier

17. Security  Hypervisors buggy too, why trust them more than kernels?  Narrower interface to malicious code (no system calls)  No way for kernel to call into hypervisor  Smaller, (hopefully) less complex codebase  Should be fewer bugs  Anything wrong with this argument?  Hypervisors are still complex  May be able to take over hypervisor via non-syscall interfaces  E.g. what if hypervisor is running IP-accessible services?  Paravirtualization (in Xen) may compromise this

18. VMware architecture Host Machine Host OS VM AppTarget OS Target App Target App Virtual Machine Monitor VM Driver Host App VMM World Host World

19. SimOS (proto-VMware) arch. Host Machine Host OS SimOS Target OS Target App Target App Host App Host App

20. SimOS memory SimOS Target OS Host Machine Host OS Target App Mem File SimDisk File SimDisk SimOS VMemory Target App SimOS code, data TargOS code, data TargApp code, data Virtual MMU

21. SimOS page fault SimOS Target OS Host Machine Host OS Target App Mem File SimDisk File SimDisk SimOS VMemory Target App Unmapped addr SimOS Fault handler TargOS Fault handler Virtual MMU What if I want to suspend and migrate the target OS?

22. Full vs interpreted  Why would I use VMware instead of Java?  Support for legacy applications  Do not force users to use a particular language  Do not force users to use a particular OS  Why would I use Java instead of VMware?  Lighter weight  Nice properties of type-safe language  Can prove safety at compile time

23. Full vs interpreted  What about protection?  What does Java use for protection? VMware?  Java relies on language features (cannot express unsafe computation)  VMware relies on the hardware to enforce protection (like an OS)  What are the trade-offs? Which protection model is better?  Java gives you stronger (i.e. provable) safety guarantees  Hardware protection doesn’t constrain programming expressiveness  What about sharing (kind of the opposite of protection)?  Sharing among components in Java is easy  (call a function, compiler makes sure it is safe)  Sharing between address spaces is more work, has higher overhead  (use sockets, have to context switch, flush TLB, etc)

24. Singularity (could try both)

25. Virtual machine challenges  Privilege modes  Memory management  Protection  Performance  Many more for every architecture…

26. Course administration  Multi-process test cases  Autograder will test your pager with > 1 process  But don’t submit any to the autograder  How to write multi-process test cases  Use vm_yield  Can quickly open processes in different windows  Can use sleep (unsigned int seconds)  Could use fork

27. Course administration  Extra office hours after spring break  Will announce over Blackboard  Other questions?

28. Sharing machines among users  PlanetLab (752 nodes at 361 sites)  Platform for distributed applications  Research testbed  Why is this more useful than a cluster?  See real Internet problems  Latency, failures, etc  Service fault tolerance

29. Sharing machines among users X Consolidate under-utilized servers to reduce CapEx and OpEx Avoid downtime with relocation Dynamically re-balance workload to guarantee application SLAs X Enforce security policy X What about the enterprise?

30. Sharing machines among users  When?  PlanetLab (testbeds, distributed services)  Data centers (three-tier web applications)  Scientific computing (protein folding, etc)  What should the interface be?

31. Shared infrastructure interfaces  Unmodified OS  Each app gets a login username  Access resources through system calls  Users can see other users’ files, processes  Drawbacks of this approach?  Administration, configuration headaches  (e.g. which libraries are installed?)  No performance isolation  (one process can dominate CPU, buffer cache, bandwidth)

32. Shared infrastructure interfaces  Unmodified OS  Retrofit resource accounting into OS (V-Servers)  Access resources through system calls  Virtualize some resources  (e.g. each app has own process table, file system)  Drawbacks of this approach?  How do you know that you’ve virtualized everything you need to  Especially hard for software resources  (e.g. what about entries in the file descriptor table?)  (e.g. who gets charged on a page fault?)

33. Shared infrastructure interfaces  Unmodified OS  Retrofit resource accounting into OS (V-Servers)  Virtual machines (Xen, VMware)  Virtualize hardware interface  Each app gets to choose its own OS  (e.g. apps have their own virt. CPU, physical memory, disk)  Drawbacks of this approach?  Very heavy-weight  A lot of redundant state (e.g. kernel, libraries, executables)  Might not scale well

34. Amazon EC2 Anyone know what EC2 uses? Xen

35. Xen challenges  Kind of the opposite approach of V-Servers  V-Servers: start with OS, virtualize  Xen: start with VM, “para-virtualize”  Goals  Performance isolation  Support many operating systems  Reduce performance overhead of virtualization

36. Para-virtualization  Full virtualization  Fool OS into thinking it has access to hardware  Para-virtualization  Expose real and virtual resources to OS  Why do we need para-virtualization?  Mostly because X86 makes full virtualization hard

37. Why para-virtualize?  Limitations of x86  Privileged instructions fail silently  VMM must execute these instructions  Cannot rely on traps to VMM  How does VMware deal with this?  At run-time rewrite guest kernel binary  Insert traps into the VMM, when necessary

38. Why para-virtualize?  Limitations of x86  Timing issues  May want to expose “real time” to OS  TCP time outs, RTT estimates  Support for performance optimizations  Superpages  Page coloring

39. VMware architecture Host Machine Host OS VM AppTarget OS Target App Target App Virtual Machine Monitor VM Driver Host App VMM World Host World

40. SimOS architecture Host Machine Host OS SimOS Target OS Target App Target App Host App Host App

41. Xen architecture Host Machine Guest OS Guest App Host App XenDomain 0 Guest OS Guest App

42. X86_32 address space Kernel User 4GB 3GB 0GB Xen S S U All address spaces All of a VM’s address spaces Each guest app When does the hypervisor need to flush the TLB? When a new guest VM or guest app needs to be run. When are each set of virtual addresses are valid?

43. Xen physical memory  Allocated by hypervisor when VM is created  Why can’t we allow guests to update PTBR?  Might map virtual addrs to physical addrs they don’t own  VMware and Xen handled this differently  VMware maintains “shadow page tables”  Xen uses “hypercalls”  (update: Xen and VMware support both mechanisms now)

44. VMware guest page tables Guest OS VMM Hardware Virtual → Machine Shadow page table Update PTE How does VMM grab control when PTE is updated? Marks PTE pages read-only, generates page fault. MMU

45. Xen physical memory  Guest OSes allocate and manage own PTs  “Hypercall” to change PT base  Like a system call between guest OS and Xen  Xen must validate PT updates before use  What are the validation rules? 1. Guest may only map phys. pages it owns 2. PT pages may only be mapped RO

46. Xen guest page tables Guest OS VMM Hardware Virtual → Machine Update PTE hypercall 1) Validation check 2) Perform update MMU

47. Para-virtualized CPU  Hypervisor runs at higher privilege than guest OS  Why is having only two levels a problem?  Guest OSes must be protected from guest applications  Hypervisor must be protected from guest OS  What do we do if we only have two privilege levels?  OS shares lower privilege level with guest applications  Run guest apps and guest OS in different address spaces  Why would this be slow?  VMM must flush the TLB on system calls, page faults

48. X86_32 address space Kernel User 4GB 3GB 0GB Xen S S U Ring 0 Ring 1 Ring 3 What does this assume? Guest OS doesn’t need ring 2 (e.g. OS/2).

49. Para-virtualized CPU  Hypervisor runs at higher privilege than guest OS  Ring 0  hypervisor, ring 1  guest OS, ring 3  guest apps  Handling exceptions  Guest registers handlers with Xen (must modify guest)  System calls  Guests register "fast" handler with Xen  Xen validates handler, inserts in CPU’s handler table  No need to go to ring 0 to execute  What handler cannot be executed directly and why?  Page fault handler must read register CR2 (only allowed in ring 0)  CR2 is where the fault-generating address is stored

50. Para-virtualization  Pros  Better performance  Scales better than full virtualization  Cons  OS needs (minor) changes  How well does it actually scale? Unclear at this point  Impure abstractions  Is it important to provide good abstractions?  I say yes  Bad interfaces lead to code complexity, maintainability issues