CS 537 9/14

Virtualization

Build an illusion (as many CPU & RAM as you want)

Goals

• Efficiency
• Security
• Isolation

Abstraction

What changes when a program runs?

• registers
• memory
• I/O

CPU: How to Virtualize

• Run $N$ proceses “at once” even though we have $M$ CPUs $(N>M)$
• General idea:
  • If 1 CPU, 2 processes (A, B)
  • Run as ABABABAB… (interleave) aka “time sharing”
• Mechanisms: low-level how
• Policies: whilch process to run?

First Attempt: Direct Execution

Boottime: (Start up)

OS is the first program to run

• Set up:
  • free list (to track memory)
  • process list (etc.)

Want: Run One Program

• alloc entry on process list
• alloc some memory
• load the program (from disk) into memory (code, static)
• jump to main of the prgram & start running

Obersavation

• Only one thing running at at time

Problems

1. What if process $P$ wants to do something restricted? (e.g., issue a read to disk)
   Hardware Support
   1. Modes of Opertaion - User mode / Kernel mode
      • Bit: 0 or 1 => which modes
   2. Transition between modes
      Instructions:
      • user mode -> kernel mode: trap
      • kernel mode -> user mode: return-from-trap
      • Set up trap handlers
2. What if OS wants to stop $P_a$, and run $P_b$?
3. What if $P$ does something that is “slow”? (I/O)

Timeline

• OS @ boot
  Set up
  • trap handlers
  • free list (to track memory)
  • process list (etc.)
• Process @ run time
  • issue system calls (func written in asm that calls trap)