基于mykernel 2.0编写一个操作系统内核

一、实验环境

二、实验内容

按照 https://github.com/mengning/mykernel 的说明配置mykernel 2.0。

对mykernel文件夹中的mymain.c，myinterrupt.c，mypcb.h进行修改编写一个简单的操作系统内核。

进程控制块和线程的数据结构在mypcb.h中定义。

#define MAX_TASK_NUM        4
#define KERNEL_STACK_SIZE   1024*2
/* CPU-specific state of this task */
struct Thread {
    unsigned long        ip;
    unsigned long        sp;
};

typedef struct PCB{
    int pid;
    volatile long state;    /* -1 unrunnable, 0 runnable, >0 stopped */
    unsigned long stack[KERNEL_STACK_SIZE];
    /* CPU-specific state of this task */
    struct Thread thread;
    unsigned long    task_entry;
    struct PCB *next;
}tPCB;

void my_schedule(void);

ip储存线程的入口地址，sp储存线程的堆栈栈顶地址。

pid为进程编号，state表示进程的状态，即是否可运行，stack[KERNEL_STACK_SIZE]为进程的堆栈，task_entry储存进程入口的地址，*next储存下一个进程的地址，所有进程控制块以一个单链表储存。

mymain.c中的void __init my_start_kernel(void)函数是操作系统的入口，完成进程控制块的初始化，并调用第一个进程，void my_process(void)函数是进程的主体，这里四个进程均用此函数。

#include <linux/types.h>
#include <linux/string.h>
#include <linux/ctype.h>
#include <linux/tty.h>
#include <linux/vmalloc.h>

#include "mypcb.h"

tPCB task[MAX_TASK_NUM];
tPCB * my_current_task = NULL;
volatile int my_need_sched = 0;

void my_process(void);

void __init my_start_kernel(void)
{
    int pid = 0;
    int i;
    /* Initialize process 0*/
    task[pid].pid = pid;
    task[pid].state = 0;/* -1 unrunnable, 0 runnable, >0 stopped */
    task[pid].task_entry = task[pid].thread.ip = (unsigned long)my_process;
    task[pid].thread.sp = (unsigned long)&task[pid].stack[KERNEL_STACK_SIZE-1];
    task[pid].next = &task[pid];
    /*fork more process */
    for(i=1;i<MAX_TASK_NUM;i++)
    {
        memcpy(&task[i],&task[0],sizeof(tPCB));
        task[i].pid = i;
        task[i].thread.sp = (unsigned long)(&task[i].stack[KERNEL_STACK_SIZE-1]);
        task[i].next = task[i-1].next;
        task[i-1].next = &task[i];
    }
    /* start process 0 by task[0] */
    pid = 0;
    my_current_task = &task[pid];
    asm volatile(
        "movq %1,%%rsp\n\t"     /* set task[pid].thread.sp to rsp */
        "pushq %1\n\t"             /* push rbp */
        "pushq %0\n\t"             /* push task[pid].thread.ip */
        "ret\n\t"                 /* pop task[pid].thread.ip to rip */
        : 
        : "c" (task[pid].thread.ip),"d" (task[pid].thread.sp)    /* input c or d mean %ecx/%edx*/
    );
} 

int i = 0;

void my_process(void)
{    
    while(1)
    {
        i++;
        if(i%120000000 == 0)
        {
            printk(KERN_NOTICE "process %d starts\n",my_current_task->pid);
            if(my_need_sched == 1)
            {
                my_need_sched = 0;
                my_schedule();
            }
            printk(KERN_NOTICE "process %d ends\n",my_current_task->pid);
        }     
    }
}

对于其中的汇编语言部分，先将sp中的堆栈栈顶地址存入rsp寄存器，再将其压栈（栈底地址），再将ip压栈，最后用ret指令出栈，将刚刚压入的ip值弹出到rip寄存器，即完成了对于第一个进程的调用。

myinterrupt.c中void my_timer_handler(void)函数为时钟中断，每隔一定时间将my_need_sched标志置1，void my_schedule(void)函数为进程调度函数。

#include <linux/types.h>
#include <linux/string.h>
#include <linux/ctype.h>
#include <linux/tty.h>
#include <linux/vmalloc.h>

#include "mypcb.h"

extern tPCB task[MAX_TASK_NUM];
extern tPCB * my_current_task;
extern volatile int my_need_sched;
volatile int time_count = 0;

/*
 * Called by timer interrupt.
 * it runs in the name of current running process,
 * so it use kernel stack of current running process
 */
void my_timer_handler(void)
{
    if(time_count%1000 == 0 && my_need_sched != 1)
    {
        printk(KERN_NOTICE ">>>my_timer_handler here<<<\n");
        my_need_sched = 1;
    } 
    time_count ++ ;  
    return;      
}

void my_schedule(void)
{
    tPCB * next;
    tPCB * prev;

    if(my_current_task == NULL 
        || my_current_task->next == NULL)
    {
        return;
    }
    printk(KERN_NOTICE ">>>my_schedule<<<\n");
    /* schedule */
    next = my_current_task->next;
    prev = my_current_task;
    if(next->state == 0)/* -1 unrunnable, 0 runnable, >0 stopped */
    {        
        my_current_task = next; 
        printk(KERN_NOTICE ">>>switch %d to %d<<<\n",prev->pid,next->pid);  
        /* switch to next process */
        asm volatile(    
            "pushq %%rbp\n\t"         /* save rbp of prev */
            "movq %%rsp,%0\n\t"     /* save rsp of prev */
            "movq %2,%%rsp\n\t"     /* restore  rsp of next */
            "movq $1f,%1\n\t"       /* save rip of prev */    
            "pushq %3\n\t" 
            "ret\n\t"                 /* restore  rip of next */
            "1:\t"                  /* next process start here */
            "popq %%rbp\n\t"
            : "=m" (prev->thread.sp),"=m" (prev->thread.ip)
            : "m" (next->thread.sp),"m" (next->thread.ip)
        ); 
    }  
    return;    
}

其中的汇编部分，先将当前的栈底地址，即rbp寄存器的值压栈，再将当前栈顶地址，即rsp寄存器的值存入当前线程的sp中，至此完成了对当前堆栈的保存。

接着将新线程的sp移入rsp，将当前线程的下一条命令地址（$1f代表该地址，也即my_schedule函数后的第一条命令）存入当前线程的ip中，完成了当前进程入口的保存。

然后将新进程的ip压栈，用ret指令将其弹出至rip寄存器，完成新进程的入口的调用。

最后将新进程的堆栈栈顶的值（之前保存的栈底地址）弹出至rbp寄存器，完成新进程的堆栈的调用。

编译内核，用qemu-system-x86_64 -kernel arch/x86/boot/bzImage命令运行，结果如下。