Virtual Machines

Lecture Notes for CS 140
Spring 2019
John Ousterhout

• Readings for this topic from Operating Systems: Principles and Practice: Section 10.2.
• What is the abstraction provided by an OS to a process?
  • (Virtual) memory
  • A subset of the instruction set of the underlying machine
  • Most (but not all) of the hardware registers
  • A set of kernel calls with particular arguments for file I/O, etc.
  • Overall: a subset of the facilities of the underlying machine, augmented with extra mechanisms implemented by the operating system.
• What if we implemented a different abstraction for a process, which looks exactly like the underlying hardware:
  • The complete instruction set of the underlying machine
  • Physical memory
  • Memory management unit (page maps, etc.)
  • I/O devices
  • Traps and interrupts
  • No predefined system calls
• This abstraction is called a virtual machine:
  • To a "process", it appears that it has its own private machine.
  • Multiple "processes" can share a single machine, each thinking it's running on its own private machine.
  • The operating system for this is called a virtual machine monitor.
  • Can run a complete operating system inside a virtual machine: called a guest operating system.
  • Each virtual machine can run a different guest operating system.

Implementing virtual machine monitors

• One approach: simulation
  • Write program that simulates instruction execution, like Bochs.
  • Simulate memory, I/O devices also.
  • Examples:
    • Use one large file to hold contents of a "disk"
    • Simulate kernel/user bit, interrupt vectors, etc.
  • Problem: too slow
    • 100x slowdown for CPU/memory
    • 2x slowdown for I/O
• Better approach: use CPU to simulate itself.
  • Run virtual machine guest OS like a user process (in unprivileged mode).
  • Most instructions execute at the full speed of the CPU.
  • Anything "unusual" causes a trap into the virtual machine monitor, which simulates the appropriate behavior.
• Special cases:
  • Privileged instructions (e.g. HALT):
    • Since virtual machine runs in user mode, these cause "illegal instruction" traps into hypervisor.
    • Hypervisor catches these traps, simulates appropriate behavior.
  • Kernel calls in guest OS:
    • User program running under guest OS issues kernel call instruction.
    • Traps always go to hypervisor (not guest OS).
    • Hypervisor analyzes trapping instruction, simulates system call to guest OS:
      • Move trap info from hypervisor stack to stack of guest OS
      • Find interrupt vector in memory of guest OS
      • Switch simulated mode to "kernel"
      • Return out of hypervisor to interrupt handler in guest OS.
    • When guest OS returns from system call, this traps to hypervisor also (illegal instruction in user mode); hypervisor simulates return to guest user level.
  • I/O devices:
    • Guest OS writes to I/O device register
    • Hypervisor has arranged for the containing page to fault
    • Hypervisor takes page fault, recognizes address as I/O device register
    • Hypervisor simulates instruction and its impact on the simulated I/O device
    • When actual I/O operation completes, hypervisor simulates interrupt into the guest OS
    • For better performance, write new device drivers that call directly into the hypervisor (using system calls): paravirtualization.
  • Virtual memory: hypervisor uses page maps to simulate virtual memory mapping in guest OS.
    • Three levels of memory:
      • Guest virtual address space
      • Guest physical address space
      • Machine physical memory: hypervisor must have total control over this
  • Software development:
    • Tests may corrupt the state of the machine
    • Solution:
      • Run tests in a VM
      • Always start tests from a saved VM configuration
      • Discard VM state after tests
      • Results: reproducible tests
• Security: can monitor all communication into and out of VM.
• Heavily used in cloud computing (e.g. Amazon Web Services, Google Cloud).