Lecture: interrupts Q&A

recap

• user-kernel switch/communication: ring 0 ↔ ring 3
  • how can kernel take back control
  • how can user request help from kernel
  • how can kernel notify user
• hardware support
  • IDT register %idtr
  • task register for the stack switch
  • exceptions, device interrupts
  • instructions: int, iret

exception handling

• user-level exception handling
  • review exercise: CPU alarm
  • compare to Unix signals
  • see also: Hardware and Software Support for Efficient Exception Handling, Thekkath and Levy
  • will implement this for COW fork in lab 4
• question from last week: change div by zero to return 42 instead of crash
  • how to handle vector 0
    • skip the idiv instruction - how?
    • emulate the result - what registers does idiv put result in?
  • basic idea for virtual machines: trap-and-emulate
  • how to map memory from a remote machine

fast system calls

• what’s the cost of a syscall
  • IDT lookup
  • stack switch
  • state push/pop: error code (optional), eip, cs, eflags, esp, ss
• how to reduce the cost
  • dedicated instructions
    • dedicated register for syscall entry: no IDT lookup
    • save user ip & eflags in registers: no stack switch/push
    • x86_64: syscall/sysret (Linux)
  • don’t switch to kernel
    • vDSO: gettimeofday
    • library OS/unikernel - see the exokernel paper
  • many other approaches: batching, downloading code
• programming model
  • consider recv(fd, buf, len, flags): syscall for receiving a network packet
  • what should the kernel do if there’s no data
    • block
    • return with “try-again”
    • return right away; later upcall to user space when data arrive
  • C10K problem: pros and cons