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MIT 6.828 HW5 xv6 CPU alarm

Notes of MIT 6.828 HW5

Add a new alarm(interval, handler) system call.
If an application calls alarm(n, fn), then after every n ticks of CPU time that the program consumes, the kernel will cause application function fn to be called. When fn returns, the application will resume where it left off.

First, we modify the struct proc.

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struct proc {
  uint sz;                     // Size of process memory (bytes)
  pde_t* pgdir;                // Page table
  char *kstack;                // Bottom of kernel stack for this process
  enum procstate state;        // Process state
  int pid;                     // Process ID
  struct proc *parent;         // Parent process
  struct trapframe *tf;        // Trap frame for current syscall
  struct context *context;     // swtch() here to run process
  void *chan;                  // If non-zero, sleeping on chan
  int killed;                  // If non-zero, have been killed
  struct file *ofile[NOFILE];  // Open files
  struct inode *cwd;           // Current directory
  char name[16];               // Process name (debugging)
  // fields for alarm
  int alarmticks;
  int current_tick;
  void (*alarmhandler)();
};

We also need to modify allocproc() in proc.c to initialize the fields we added.

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p->current_tick = 0;
p->alarmticks = -1;     // initialize alarmticks to -1

In syscall.c, add a function sys_alarm.

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int
sys_alarm(void)
{
  int ticks;
  void (*handler)();

  if(argint(0, &ticks) < 0)
    return -1;
  if(argptr(1, (char**)&handler, 1) < 0)
    return -1;
  myproc()->alarmticks = ticks;
  myproc()->alarmhandler = handler;
  return 0;
}

In trap.c, add code handle case IRQ_TIMER.
In kernel mode, we can’t directly call alarmhandler. However, we need to modify the trapframe.

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case T_IRQ0 + IRQ_TIMER:
  if(cpuid() == 0){
    acquire(&tickslock);
    ticks++;
    wakeup(&ticks);
    release(&tickslock);
  }
  lapiceoi();

  // alarm 
  struct proc* p = myproc();
  if(p != 0 && (tf->cs & 3) == 3) {
    p->current_tick++;
    if (p->current_tick == p->alarmticks) {
      p->current_tick = 0;
      // push old eip
      tf->esp -= 4;
      *(uint *)(tf->esp) = tf->eip;
      // set eip to alarmhandler
      tf->eip = (uint)p->alarmhandler;;
    }
  }

  break;
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