trap.c

#include "types.h"
#include "defs.h"
#include "param.h"
#include "memlayout.h"
#include "mmu.h"
#include "proc.h"
#include "x86.h"
#include "traps.h"
#include "spinlock.h"

// Interrupt descriptor table (shared by all CPUs).
struct gatedesc idt[256];
extern uint vectors[];  // in vectors.S: array of 256 entry pointers
struct spinlock tickslock;
uint ticks;

void
tvinit(void)
{
  int i;

  for(i = 0; i < 256; i++)
    SETGATE(idt[i], 0, SEG_KCODE<<3, vectors[i], 0);
  // User syscalls get the user DPL; see xv6 book p. 42
  SETGATE(idt[T_SYSCALL], 1, SEG_KCODE<<3, vectors[T_SYSCALL], DPL_USER);

  initlock(&tickslock, "time");
}

void
idtinit(void)
{
  lidt(idt, sizeof(idt));
}

//PAGEBREAK: 41
void
trap(struct trapframe *tf)
{
  if(tf->trapno == T_SYSCALL){
    if(proc->killed)
      exit();
    proc->tf = tf;
    syscall();
    if(proc->killed)
      exit();
    return;
  }

  char *mem;
  switch(tf->trapno){
  case T_IRQ0 + IRQ_TIMER:
    if(cpunum() == 0){
      acquire(&tickslock);
      ticks++;
      wakeup(&ticks);
      release(&tickslock);
    }
    // Per-process ticking, alarm handling, for sys_alarm.
    if (proc && (tf->cs & 3) == DPL_USER) {
      proc->elapsed_ticks++;
      if (proc->alarm_ticks && proc->elapsed_ticks >= proc->alarm_ticks) {
        // Trapframe contains the user-prog's eip and esp
        // at time of interrupt. We modify those values as if
        // the user-prog had called its alarm handler immediately
        // before the interrupt. It's hella sketchy to let the kernel
        // jump to a user-specified address--we should check that it's
        // within the bounds of the proc's address space.

        // Place the original user-prog return address on
        // the user's stack, as if the user-prog issued a `call`
        // instruction.
        tf->esp -= 4;
        *(uint*)(tf->esp) = tf->eip;

        // Make trapret return to the alarmhandler instead
        // of the user-prog. When the alarmhandler calls `ret`,
        // it will jump to the original user-prog return address
        // we placed on the stack above.
        tf->eip = (uint)proc->alarm_fn;

        proc->elapsed_ticks = 0;
      }
    }
    lapiceoi();
    break;
  case T_IRQ0 + IRQ_IDE:  // 46
    ideintr();
    lapiceoi();
    break;
  case T_IRQ0 + IRQ_IDE+1:
    // Bochs generates spurious IDE1 interrupts.
    break;
  case T_IRQ0 + IRQ_KBD:
    kbdintr();
    lapiceoi();
    break;
  case T_IRQ0 + IRQ_COM1:
    uartintr();
    lapiceoi();
    break;
  case T_IRQ0 + 7:
  case T_IRQ0 + IRQ_SPURIOUS:
    cprintf("cpu%d: spurious interrupt at %x:%x\n",
            cpunum(), tf->cs, tf->eip);
    lapiceoi();
    break;
  case T_PGFLT:
    // TODO: Check that the PFLA isn't in the guard page below the stack.
    cprintf("PFLT at: %x\n", rcr2());
    if ((mem = kalloc()) == 0)
      panic("page fault handler OOM\n");

    memset(mem, 0, PGSIZE);

    if(mappages(proc->pgdir, (char*)PGROUNDDOWN(rcr2()), PGSIZE, V2P(mem), PTE_W|PTE_U) < 0){
      kfree(mem);
      panic("page fault handler OOM (2)\n");
    }
    break;

  //PAGEBREAK: 13
  default:
    if(proc == 0 || (tf->cs&3) == 0){
      // In kernel, it must be our mistake.
      cprintf("unexpected trap %d from cpu %d eip %x (cr2=0x%x)\n",
              tf->trapno, cpunum(), tf->eip, rcr2());
      panic("trap");
    }
    // In user space, assume process misbehaved.
    cprintf("pid %d %s: trap %d err %d on cpu %d "
            "eip 0x%x addr 0x%x--kill proc\n",
            proc->pid, proc->name, tf->trapno, tf->err, cpunum(), tf->eip,
            rcr2());
    proc->killed = 1;
  }

  // Force process exit if it has been killed and is in user space.
  // (If it is still executing in the kernel, let it keep running
  // until it gets to the regular system call return on line 44 above.)
  if(proc && proc->killed && (tf->cs&3) == DPL_USER)
    exit();

  // Force process to give up CPU on clock tick.
  // If interrupts were on while locks held, would need to check nlock.
  if(proc && proc->state == RUNNING && tf->trapno == T_IRQ0+IRQ_TIMER)
    yield();

  // Check again if the process has been killed since we yielded
  if(proc && proc->killed && (tf->cs&3) == DPL_USER)
    exit();
}