trap.c

#include "types.h"
#include "param.h"
#include "memlayout.h"
#include "riscv.h"
#include "spinlock.h"
#include "proc.h"
#include "defs.h"
#include "watchdog.h"

struct spinlock tickslock;
uint ticks;

extern char trampoline[], uservec[], userret[];

// in kernelvec.S, calls kerneltrap().
void kernelvec();

extern int devintr();

static const char *
scause_desc(uint64 stval);

void
trapinit(void)
{
  initlock(&tickslock, "time");
}

// set up to take exceptions and traps while in the kernel.
void
trapinithart(void)
{
  w_stvec((uint64)kernelvec);
}

int handle_page_fault(struct proc* p, uint64 scause, uint64 stval, uint64 sepc){
  uint64 addr = PGROUNDDOWN(stval);
  acquire(&p->vma_lock);
  int flags = allocate_if_possible(p->pagetable, p, addr);
  release(&p->vma_lock);
  if(flags < 0){
    if(flags == ENOVMA){
      printf("Could not find VMA associated with addr=%p\n", addr);
    } else if (flags == ENOMEM){
      printf("No more memory could be allocated from the kernel\n");
    } else if (flags == ENOFILE){
      printf("Could not read file associated with memory area\n");
    } else if (flags == EMAPFAILED){
      printf("mappages failed for an unknown reason\n");
     } else if (flags == EBADPERM){
      printf("Bad permission addr=%p, scause=%p\n", addr, scause);
    }

    proc_vmprint(p);
    printf("unrecoverable page fault by pid=%d at sepc=%p stval=%p scause=%x\n",
           p->pid, sepc, stval, scause);
    p->killed = 1;
    return -1;
  }
  return 0;
}

//
// handle an interrupt, exception, or system call from user space.
// called from trampoline.S
//
void
usertrap(void)
{
  int which_dev = 0;

  if((r_sstatus() & SSTATUS_SPP) != 0)
    panic("usertrap: not from user mode");

  // send interrupts and exceptions to kerneltrap(),
  // since we're now in the kernel.
  w_stvec((uint64)kernelvec);

  struct proc *p = myproc();
  
  // save user program counter.
  p->tf->epc = r_sepc();

  uint64 scause = r_scause();

  if(scause == 8){
    // system call

    if(p->killed)
      exit(-1);

    // sepc points to the ecall instruction,
    // but we want to return to the next instruction.
    p->tf->epc += 4;

    // an interrupt will change sstatus &c registers,
    // so don't enable until done with those registers.
    intr_on();

    syscall();
  } else if (scause == 0xf || scause == 0xd) {
    handle_page_fault(p, scause, r_stval(), r_sepc());
  } else if((which_dev = devintr()) != 0){
    // ok
  } else {
    printf("usertrap(): unexpected scause %p (%s) pid=%d\n", r_scause(), scause_desc(r_scause()), p->pid);
    printf("            sepc=%p stval=%p\n", r_sepc(), r_stval());
    p->killed = 1;
  }

  if(p->killed)
    exit(-1);

  // give up the CPU if this is a timer interrupt.
  if(which_dev == 2)
    yield();

  usertrapret();
}

//
// return to user space
//
void
usertrapret(void)
{
  struct proc *p = myproc();

  // turn off interrupts, since we're switching
  // now from kerneltrap() to usertrap().
  intr_off();

  // send syscalls, interrupts, and exceptions to trampoline.S
  w_stvec(TRAMPOLINE + (uservec - trampoline));

  // set up trapframe values that uservec will need when
  // the process next re-enters the kernel.
  p->tf->kernel_satp = r_satp();         // kernel page table
  p->tf->kernel_sp = p->kstack + PGSIZE; // process's kernel stack
  p->tf->kernel_trap = (uint64)usertrap;
  p->tf->kernel_hartid = r_tp();         // hartid for cpuid()

  // set up the registers that trampoline.S's sret will use
  // to get to user space.
  
  // set S Previous Privilege mode to User.
  unsigned long x = r_sstatus();
  x &= ~SSTATUS_SPP; // clear SPP to 0 for user mode
  x |= SSTATUS_SPIE; // enable interrupts in user mode
  w_sstatus(x);

  // set S Exception Program Counter to the saved user pc.
  w_sepc(p->tf->epc);

  // tell trampoline.S the user page table to switch to.
  uint64 satp = MAKE_SATP(p->pagetable);

  // jump to trampoline.S at the top of memory, which 
  // switches to the user page table, restores user registers,
  // and switches to user mode with sret.
  uint64 fn = TRAMPOLINE + (userret - trampoline);
  ((void (*)(uint64,uint64))fn)(TRAPFRAME, satp);
}

// interrupts and exceptions from kernel code go here via kernelvec,
// on whatever the current kernel stack is.
void 
kerneltrap()
{
  int which_dev = 0;
  uint64 sepc = r_sepc();
  uint64 sstatus = r_sstatus();
  uint64 scause = r_scause();
  
  if((sstatus & SSTATUS_SPP) == 0)
    panic("kerneltrap: not from supervisor mode");
  if(intr_get() != 0)
    panic("kerneltrap: interrupts enabled");

  if((which_dev = devintr()) == 0){
    printf("scause %p (%s)\n", scause, scause_desc(scause));
    printf("sepc=%p stval=%p\n", r_sepc(), r_stval());
    panic("kerneltrap");
  }

  // give up the CPU if this is a timer interrupt.
  if(which_dev == 2 && myproc() != 0 && myproc()->state == RUNNING)
    yield();

  // the yield() may have caused some traps to occur,
  // so restore trap registers for use by kernelvec.S's sepc instruction.
  w_sepc(sepc);
  w_sstatus(sstatus);
}

void
clockintr()
{
  acquire(&watchdog_lock);
  acquire(&tickslock);
  if (watchdog_time && ticks - watchdog_value > watchdog_time){
    panic("watchdog !!!");
  }
  ticks++;
  wakeup(&ticks);
  release(&tickslock);
  release(&watchdog_lock);
}

// check if it's an external interrupt or software interrupt,
// and handle it.
// returns 2 if timer interrupt,
// 1 if other device,
// 0 if not recognized.
int
devintr()
{
  uint64 scause = r_scause();

  if((scause & 0x8000000000000000L) &&
     (scause & 0xff) == 9){
    // this is a supervisor external interrupt, via PLIC.

    // irq indicates which device interrupted.
    int irq = plic_claim();

    if(irq == UART0_IRQ){
      uartintr();
    } else if(irq == VIRTIO0_IRQ || irq == VIRTIO1_IRQ ){
      virtio_disk_intr(irq - VIRTIO0_IRQ);
    } else {
      // the PLIC sends each device interrupt to every core,
      // which generates a lot of interrupts with irq==0.
    }

    if(irq)
      plic_complete(irq);

    return 1;
  } else if(scause == 0x8000000000000001L){
    // software interrupt from a machine-mode timer interrupt,
    // forwarded by timervec in kernelvec.S.

    if(cpuid() == 0){
      clockintr();
    }
    
    // acknowledge the software interrupt by clearing
    // the SSIP bit in sip.
    w_sip(r_sip() & ~2);

    return 2;
  } else {
    return 0;
  }
}

static const char *
scause_desc(uint64 stval)
{
  static const char *intr_desc[16] = {
    [0] "user software interrupt",
    [1] "supervisor software interrupt",
    [2] "<reserved for future standard use>",
    [3] "<reserved for future standard use>",
    [4] "user timer interrupt",
    [5] "supervisor timer interrupt",
    [6] "<reserved for future standard use>",
    [7] "<reserved for future standard use>",
    [8] "user external interrupt",
    [9] "supervisor external interrupt",
    [10] "<reserved for future standard use>",
    [11] "<reserved for future standard use>",
    [12] "<reserved for future standard use>",
    [13] "<reserved for future standard use>",
    [14] "<reserved for future standard use>",
    [15] "<reserved for future standard use>",
  };
  static const char *nointr_desc[16] = {
    [0] "instruction address misaligned",
    [1] "instruction access fault",
    [2] "illegal instruction",
    [3] "breakpoint",
    [4] "load address misaligned",
    [5] "load access fault",
    [6] "store/AMO address misaligned",
    [7] "store/AMO access fault",
    [8] "environment call from U-mode",
    [9] "environment call from S-mode",
    [10] "<reserved for future standard use>",
    [11] "<reserved for future standard use>",
    [12] "instruction page fault",
    [13] "load page fault",
    [14] "<reserved for future standard use>",
    [15] "store/AMO page fault",
  };
  uint64 interrupt = stval & 0x8000000000000000L;
  uint64 code = stval & ~0x8000000000000000L;
  if (interrupt) {
    if (code < NELEM(intr_desc)) {
      return intr_desc[code];
    } else {
      return "<reserved for platform use>";
    }
  } else {
    if (code < NELEM(nointr_desc)) {
      return nointr_desc[code];
    } else if (code <= 23) {
      return "<reserved for future standard use>";
    } else if (code <= 31) {
      return "<reserved for custom use>";
    } else if (code <= 47) {
      return "<reserved for future standard use>";
    } else if (code <= 63) {
      return "<reserved for custom use>";
    } else {
      return "<reserved for future standard use>";
    }
  }
}