miniriscvos 05Preemptive

void timer_init()
{
  // each CPU has a separate source of timer interrupts.
  int id = r_mhartid();

  // ask the CLINT for a timer interrupt.
  // int interval = 1000000; // cycles; about 1/10th second in qemu.

  *(reg_t *)CLINT_MTIMECMP(id) = *(reg_t *)CLINT_MTIME + interval;

  // prepare information in scratch[] for timervec.
  // scratch[0..2] : space for timervec to save registers.
  // scratch[3] : address of CLINT MTIMECMP register.
  // scratch[4] : desired interval (in cycles) between timer interrupts.
  reg_t *scratch = &timer_scratch[id][0];
  scratch[3] = CLINT_MTIMECMP(id);
  scratch[4] = interval;
  w_mscratch((reg_t)scratch);

  // enable machine-mode timer interrupts.
  w_mie(r_mie() | MIE_MTIE);
}

qemu-system-riscv64 -M virt -kernel kernel.img -bios none -serial stdio -display none
OS start
OS start loop 
OS: Activate next task
Task0: Created!
Task0: Running...
Task0: Running...
Task0: Running...
Task0: Running...
Task0: Running...
Task0: Running...
Task0: Running...
Task0: Running...
Task0: Running...
Task0: Running...
Sync exceptions! ,  code = 7
OOPS! What can I do!

reg_t trap_handler(reg_t epc, reg_t cause)
{
  reg_t return_pc = epc;
  reg_t cause_code = cause & 0xfff;

  if (cause & 0x80000000)
  {
    /* Asynchronous trap - interrupt */
    switch (cause_code)
    {
    case 3:
      lib_puts("software interruption!\n");
      break;
    case 7:
      lib_puts("timer interruption!\n");
      // disable machine-mode timer interrupts.
      w_mie(~((~r_mie()) | (1 << 7)));
      timer_handler();
      return_pc = (reg_t)&os_kernel;
      // enable machine-mode timer interrupts.
      w_mie(r_mie() | MIE_MTIE);
      break;
    case 11:
      lib_puts("external interruption!\n");
      break;
    default:
      lib_puts("unknown async exception!\n");
      break;
    }
  }
  else
  {
    /* Synchronous trap - exception */
    // https://juejin.cn/post/7006347707144994853
    lib_printf("Sync exceptions! ,  code = %d\n", cause_code);
    lib_puts("OOPS! What can I do!");
    while (1)
    {
      /* code */
    }
  }
  return return_pc;
}

 原来是trap_vector 处理有问题

.globl trap_vector
# the trap vector base address must always be aligned on a 4-byte boundary
.align 4
trap_vector:
        # save context(registers).
        csrrw   t6, mscratch, t6        # swap t6 and mscratch
        reg_save t6
        csrw    mscratch, t6
        # call the C trap handler in trap.c
        csrr    a0, mepc
        csrr    a1, mcause
        call    trap_handler

        # trap_handler will return the return address via a0.
        csrw    mepc, a0

        # load context(registers).
        csrr    t6, mscratch
        reg_load t6
        mret

 更改timervec

#include "timer.h"
#include "lib.h"

// a scratch area per CPU for machine-mode timer interrupts.
reg_t timer_scratch[NCPU][5];

#define interval 20000000 // cycles; about 2 second in qemu.
// assembly code in kernelvec.S for machine-mode timer interrupt.
extern void timervec();
void timer_init()
{
  // each CPU has a separate source of timer interrupts.
  int id = r_mhartid();

  // ask the CLINT for a timer interrupt.
  // int interval = 1000000; // cycles; about 1/10th second in qemu.

  *(reg_t *)CLINT_MTIMECMP(id) = *(reg_t *)CLINT_MTIME + interval;

  // prepare information in scratch[] for timervec.
  // scratch[0..2] : space for timervec to save registers.
  // scratch[3] : address of CLINT MTIMECMP register.
  // scratch[4] : desired interval (in cycles) between timer interrupts.
  reg_t *scratch = &timer_scratch[id][0];
  scratch[3] = CLINT_MTIMECMP(id);
  scratch[4] = interval;
  w_mscratch((reg_t)scratch);

  //set the machine-mode trap handler.
  w_mtvec((reg_t)timervec);
    //
    //     // enable machine-mode interrupts.
  w_mstatus(r_mstatus() | MSTATUS_MIE);
  // enable machine-mode timer interrupts.
  w_mie(r_mie() | MIE_MTIE);
}

static int timer_count = 0;

void timer_handler()
{
  lib_printf("timer_handler: %d\n", ++timer_count);
  int id = r_mhartid();
  *(reg_t *)CLINT_MTIMECMP(id) = *(reg_t *)CLINT_MTIME + interval;
}