MIT6.S081笔记：lecture 3

OS design

OS organization

Strawman design: NO OS

• Application interact directly with the OS:

  

• OS acts as a library:

  

• which is not a good design, because it doesn’t have enforced multiplexing and strong memory isolation

Unix interface

• abstract the hardware resources to provide strong isolation
• processes: instead CPU: fork()
  • OS transparently allocates cores
  • enforce that processes give them up
• memory: instead physical memory: exec(), brk()
  • Each process has its own memory
  • OS decides where to place app in memory
  • OS enforces isolation between apps
  • OS stores image in file system (loaded with exec)
• files: instead disk block: open(), read(), write()
  • OS provides convenient names
  • OS allows files to be shared by apps and users
• Pipes: instead shared physical memory: pipe()
  • OS buffers data
  • OS stops sender if it transmits too fast

OS should be defensive

• app can not crash the operating system
• app can not break out of its isolation
• it means Strong Isolation between apps and OS
  • hardware support
    • user/kernel mode
    • virtual memory

User/Kernel mode

• kernel mode: privileged instructions
  • changing back to user mode
  • programming a timer chip
  • controlling the virtual memory
• user mode: unprivileged instructions

CPU provide virtual memory

page table

• map virtual addr to physical addr

• defines what physical memory an app can access

• process has its own page table

System calls

• apps need to communicate with kernel
• solutions add instruction to change mode in controlled way:
  • ecall
  • enter kernel at pre-agreed instruction address

Entering the kernel

ecall

• fork();
  • ecall sys_fork -> syscall
• write()
  • ecall sys_write -> syscall

Trusted computing base

• kernel must have no bugs

• kernel must treat process as malicious

• security mindset

Monolithic kernels

• kernel is one big program with everything
• Kernel interface == system call interface
• Pros:
  • good performance
  • easy for subsystems to cooperate
• cons:
  • Interactions are complex
  • No isolation within

Microkernels

• Runs OS services as ordinary user programs
• kernel implements minimal mechanism to run services in user space
  • IPC
  • process with memory
• Kernel interface != system call interface
• Pro:
  • more isolation
  • easy to debug
• Con:
  • Hard to get good performance
  • complexity