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
• SETGATE(gate, istrap, sel, off, dpl) in xv6/JOS
  • does your kernel allow interrupts in kernel
  • does your kernel allow user to invoke int N

More on system calls

• suppose your OS has a recv(fd, buf, len, flags) syscall
  • user space invokes it to receive a network packet
  • what should the kernel do if there’s no data
    • block
    • return with “try-again”
    • return right away; later up-call to user space when data arrive
  • pros and cons
• what’s the cost of a syscall & how to reduce the cost
  • don’t switch to kernel
    • library OS - see the exokernel paper
    • vDSO: gettimeofday
  • dedicated instructions
    • x86_64: syscall/sysret (Linux)
    • arm: svc/eret
    • riscv: ecall/*ret
  • many other approaches: batching, downloading code