Lab1：基于 mykernel 2.0 编写一个操作系统内核

一、按照 https://github.com/mengning/mykernel 的说明配置 mykernel 2.0，熟悉 Linux 内核的编译

配置环境

wget https://raw.github.com/mengning/mykernel/master/mykernel-2.0_for_linux-5.4.34.patch
sudo apt install axel
axel -n 20 https://mirrors.edge.kernel.org/pub/linux/kernel/v5.x/linux-5.4.34.tar.xz
xz -d linux-5.4.34.tar.xz
tar -xvf linux-5.4.34.tar
cd linux-5.4.34
patch -p1 < ../mykernel-2.0_for_linux-5.4.34.patch
sudo apt install build-essential gcc-multilib
sudo apt install qemu # install QEMU
sudo apt install libncurses5-dev bison flex libssl-dev libelf-dev
make defconfig # Default configuration is based on 'x86_64_defconfig'
make -j$(nproc)
qemu-system-x86_64 -kernel arch/x86/boot/bzImage

进入 mykernel 目录可以查看 mymain.c 和 myinterrupt.c 代码。

mymain.c 中主要是一个 while 循环，模拟 CPU 执行 C 代码的上下文环境，而周期性产⽣的时钟中断信号能够触发 myinterrupt.c 中的代码。这样就通过 Linux 内核代码模拟
了⼀个具有时钟中断和 C 代码执⾏环境的硬件平台 。

 

 

二、基于 mykernel 2.0 编写一个操作系统内核，参照 https://github.com/mengning/mykernel 提供的范例代码

1、mypcb.h 

添加 mypcb.h 文件，定义 pcb 结构

//最大任务数
#define MAX_TASK_NUM        4
#define KERNEL_STACK_SIZE   1024*2

//存储ip和sp
struct Thread {
    unsigned long    ip;
    unsigned long    sp;
};

//PCB结构
typedef struct PCB{
    int pid; //进程id
    volatile long state;
    unsigned long stack[KERNEL_STACK_SIZE]; //进程堆栈
    struct Thread thread; //线程
    unsigned long task_entry; //进程入口地址
    struct PCB *next; //下一个进程控制块地址
}tPCB;

void my_schedule(void); //调度函数

 

2、myinterrupt.c

myinterrupt.c 文件中定义了进程如何切换

#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%10000000 == 0)
        {
            printk(KERN_NOTICE "this is process %d -\n",my_current_task->pid);
            if(my_need_sched == 1)
            {
                my_need_sched = 0;
                my_schedule();
            }
            printk(KERN_NOTICE "this is process %d +\n",my_current_task->pid);
        }     
    }
}

 

3、mymain.c

mymain.c 是内核运行的程序，有一个中断处理程序的上下文环境，周期性地产生中断信号来触发 myinterrupt.c

对代码进行修改，在原来的基础上增加了进程管理的代码

#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;    
}

 

重新编译运行

 

三、简要分析操作系统内核核心功能及运行工作机制

进程调度算法

　　基于时间片，每次执行完一个时间片后会触发一个时钟中断进行进程调度。初始化将第一个进程的 id 指向函数 my_process 的地址，进程以循环链表的方式进行存储。指令寄存器 ip 初始化为第一个进程的 id 值，执行 my_process 函数。

进程切换

　　1、当前栈基址寄存器 rbp 入栈，保存当前进程的栈底地址

　　2、栈寄存器 rsp 的值保存到当前线程的 sp 中，保存当前进程的栈顶地址

　　3、将 rsp 指向下一个进程的 sp，完成当前进程和下一个进程的堆栈切换

　　4、保存当前进程的 ip 字段放入 rip 寄存器中

　　5、将即将执行的下一个进程的 ip 放入指令寄存器进行执行

　　6、将下一个进程栈基址从堆栈中恢复到 rbp 寄存器中