Lecture 4

Yield mechanisms

The timer interrupt case (INT_IRQ + INT_TIMER in kernel.cc) calls proc::yield_noreturn(). Change it to call proc::yield() instead.

The SYSCALL_YIELD case (in kernel.cc) calls proc::yield(). Change it to call proc::yield_noreturn() instead.

Changing the timer interrupt to call proc::yield() is simple:

kernel.cc:95

    // this->regs_ = regs;
    // this->yield_noreturn();
    this->yield();
    break;

There’s no need to store an explicit resume point. When proc::exception returns (after this->yield() resumes and returns), the interrupted process will resume.

To use proc::yield_noreturn() in SYSCALL_YIELD, we must set the system call’s return value by modifying regs explicitly. System calls, unlike general exceptions, have return values, and it’s important to get them right.

kernel.cc:150

    // this->yield();
    // return 0; -- sets reg_rax
    this->regs_ = regs;
    regs->reg_rax = 0;
    this->yield_noreturn(); // NB does not return
    // This comment will never be reached!
    // This one either!
    break;

System calls and information leaks

The syscall_entry implementation can leak information from the kernel. Explain how, and explain whether and why this is a problem. Find a good reference online to a similar issue in Linux or another kernel.