6.033 | Spring 2018 | Undergraduate
Computer System Engineering
Week 4: Operating Systems Part IV

Lecture 6 Outline

  1. Virtual Machines
    • How to run multiple OSes on one machine?
    • Constraint: Compatibility. Don’t want to change existing kernel code.
    • We’ll run multiple virtual machines (VMs) on a single CPU. Kernel equivalent is the “virtual machine monitor” (VMM).
    • Can run VMM as user-mode app inside host OS, or run VMM on hardware in kernel mode with guest OSes in user mode. We’ll talk about second, but the issues are the same.
    • Role of VMM:
      • Allocate resources.
      • Dispatch events.
      • Deal with instructions from guest OS that require interaction with the physical hardware.
    • Attempt 1: Emulate every single instruction.
      • Problem: Slow.
    • Attempt 2: Guest OSes run instructions directly on CPU.
      • Problem: Dealing with privileged instructions (can’t run in kernel mode; then we’d be back to our original problem).
    • VMM will deal with handling privileged instructions.
  2. Virtual Machine Monitor (VMM) Implementation
    • Trap and emulate
      • Guest OS in user mode.
      • Privileged instructions cause an exception; VMM intercepts these and emulates.
      • If VMM can’t emulate, send exception back up to guest OS.
    • Problems:
      • How to do the emulate
      • How to deal with instructions that don’t trigger an interrupt but that the VMM still needs to intercept.
  3. Virtualizing Memory
    • VMM needs to translate guest OS addresses into physical memory addresses. Three layers: Guest virtual, guest physical, host physical.
    • Approach 1: Shadow pages
      • Guest OS loads PTR; causes interrupt. VMM intercepts.
      • VMM locates guest OS’s page table. Combines guest OS’s table with its own table, constructing a third table mapping guest virtual to host physical.
      • VMM loads host physical addr of this new page table into the hardware PTR.
      • If guest OS modifies its page table, no interrupt thrown. To force an interrupt, VMM marks guest OS’s page table as read-only memory.
    • Approach 2
      • Modern hardware has support for virtualization.
      • Physical hardware (effectively) knows about both levels of tables: Will do lookup in the guest OS’s page table and then the VMM’s page table.
  4. Virtualizing U/K Bit
    • Problem with basic trap-and-emulate: U/K bit involved in some instructions that don’t cause exception (e.g., reading U/K bit, writing it to U).
    • Few solutions:
      • Para-virtualization: Modify guest OS. Hard to do, and goes against our compatibility goal.
      • Binary translation: VMM analyzes code from guest OS and replaces problematic instructions.
      • Hardware support: Some architectures have virtualization support built in. Have special VMM operating mode in addition to the U/K bit.
    • Hardware support is arguably the best. Makes VMM’s job easier.
  5. Monolithic Kernels
    • VMs protect OSes from each other’s faults, protect physical machine from OS faults. Why so many bugs, though?
    • The Linux kernel is, effectively, one large C program. Careful software engineering, but very little modularity within the kernel itself.
    • Bugs come about because of its complexity.
    • Kernel bugs = entire system failure (recall the in-class demo).
    • Even worse: Adversary can exploit these bugs.
  6. Microkernels: Alternative to Monolithic Kernels
    • Put subsystems—file servers, device drivers, etc.—in user programs. More modular.
    • There will still be bugs but:
      • Fewer, because of decreased complexity.
      • A single bug is less likely to crash the entire system.
    • Why isn’t Linux a microkernel, then?
      • High communication cost between modules.
      • Not clear that moving programs to userspace is worth it.
      • Hard to balance dependencies (e.g., sharing memory across modules).
      • Redesign is tough!
      • Spend a year of developer time rewriting the kernel or adding new features?
      • Microkernels can make it more difficult to change interfaces.
    • Some parts of Linux do have microkernel design aspects.
  7. Summary
    • Cool things we do with VMs: Run different OSes on a single machine, move VMs from one physical machine to another.
    • Microkernels and VMs solve orthogonal problems.
      • Microkernels: Split up monolithic designs.
      • VMs: Let us run many instances of an existing OS. They are, in some sense, a partial solution to monolithic kernels (at least we can run these kernels safely). But their goal is to run multiple OSes on a single piece of hardware, not to target monolithic OSes specifically.
    • VMs most commonly implemented with hardware support (a special VMM mode in addition to U/K bit).
Course Info
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As Taught In
Spring 2018
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