基于mykernel2.0编写一个操作系统内核

1. 实验环境配置-mykernel 2.0(参考https://github.com/mengning/mykernel )

（1）本机环境：VMware®+ Ubuntu18.04.1 LTS

（2）在Ubuntu环境中，右键打开terminal（默认在/home/user目录下，比如我就是/home/cj），按照下面的顺序配置实验环境

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 libncurses-dev bison flex libssl-dev libelf-dev
make defconfig 10 make -j$(nproc) 
sudo apt install qemu 12 qemu-system-x86_64 -kernel arch/x86/boot/bzImage

 我的会拒接连接，我也不知道为什么，就直接用了群里同学发的这个文件，存放目录如下图

sudo apt install axel

axel:多线程下载工具

sudo apt install build-essential gcc-multilib
sudo apt install qemu #模拟器
sudo apt install libncurses5-dev bison flex libssl-dev libelf-dev#编译内核的工具
make allnoconfig
make -j
sudo apt install qemu 12 qemu-system-x86_64 -kernel arch/x86/boot/bzImage

配置成功后结果如下，从qemu窗口中您可以看到my_start_kernel在执行，同时my_timer_handler时钟中断处理程序周期性执行

 

 按照上述流程，在模拟器编译之后，对于linux内核的编译，我们都有了基本的熟悉。

2：基于mykernel 2.0编写一个操作系统内核，参照https://github.com/mengning/mykernel 在mykernel目录下增加一个mypcb.h 头文件，用来定义进程控制块（Process Control Block），也就是进程结构体的定义。 

/*
 *  linux/mykernel/mypcb.h
 */

//最大的任务数
#define MAX_TASK_NUM        4
#define KERNEL_STACK_SIZE   1024*8


/* 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 */
    char stack[KERNEL_STACK_SIZE];
    /* CPU-specific state of this task */
    struct Thread thread;
    unsigned long   task_entry;
    struct PCB *next;
}tPCB;

//调度函数
void my_schedule(void);

结构体Thread 结构体，用于存储当前进程中正在执行的线程的ip和sp，PCB结构体中的各个字段含义如下

pid：进程号

state：进程状态，-1表示就绪态，0表示运行态，大于0表示阻塞态

stack：进程使用的堆栈

thread：当前正在执行的线程信息

task_entry：进程入口函数

next：指向下一个PCB，系统中所有的PCB是以链表的形式组织起来的。

3）对mymain.c中的my_start_kernel函数进行修改，并在mymain.c中实现了my_process函数，用来作为进程的代码模拟一个个进程，时间片轮转调度。

#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].state = -1;
        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
	"  /* set task[pid].thread.sp to rsp */
        "pushq %1
	"          /* push rbp */
        "pushq %0
	"          /* push task[pid].thread.ip */
        "ret
	"              /* pop task[pid].thread.ip to rip */
        :
        : "c" (task[pid].thread.ip),"d" (task[pid].thread.sp)   /* input c or d mean %ecx/%edx*/
    );
}

void my_process(void)
{
    int i = 0;
    while(1)
    {
        i++;
        if(i%10000000 == 0)
        {
            printk(KERN_NOTICE "this is process %d -
",my_current_task->pid);
            if(my_need_sched == 1)
            {
                my_need_sched = 0;
                my_schedule();
            }
            printk(KERN_NOTICE "this is process %d +
",my_current_task->pid);
        }
    }
}

（4）对myinterrupt.c的修改，my_timer_handler用来记录时间片，时间片消耗完之后完成调度。并在该文件中完成，my_schedule(void)函数的实现

#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.
 */
void my_timer_handler(void)
{
    if(time_count%1000 == 0 && my_need_sched != 1)
    {
        printk(KERN_NOTICE ">>>my_timer_handler here<<<
");
        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<<<
");
    /* 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<<<
",prev->pid,next->pid);
      /* switch to next process */
      asm volatile(
         "pushq %%rbp
	"       /* save rbp of prev */
         "movq %%rsp,%0
	"     /* save rsp of prev */
         "movq %2,%%rsp
	"     /* restore  rsp of next */
         "movq $1f,%1
	"       /* save rip of prev */
         "pushq %3
	"
         "ret
	"               /* restore  rip of next */
         "1:	"                  /* next process start here */
         "popq %%rbp
	"
        : "=m" (prev->thread.sp),"=m" (prev->thread.ip)
        : "m" (next->thread.sp),"m" (next->thread.ip)
      );
    }
    return;
}

（5）重新编译（linux 目录下make命令），再次运行，查看运行结果，可以看见进程的切换

3：简要分析操作系统内核核心功能及运行工作机制

(1)系统启动后，运行mymain.c中的my_start_kernel函数

(2)myinterrupt.c，里面的my_timer_handler 函数会被内核周期性的调用，每调用1000次，就去将全局变量my_need_sched的值修改为1

(3)my_start_kernel中的while循环发现my_need_sched值变为1后，就进行进程的调度，完成进程的切换，如此往复，来实现进程切换。