基于mykernel 2.0编写一个操作系统内核

实验要求

基于mykernel 2.0编写一个操作系统内核

1. 按照https://github.com/mengning/mykernel 的说明配置mykernel 2.0，熟悉Linux内核的编译；
2. 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
3. 简要分析操作系统内核核心功能及运行工作机制

实验过程

搭建环境

1. 下载安装axel

sudo apt install axel

2. 使用axel下载源码

axel -n 20 https://mirrors.edge.kernel.org/pub/linux/kernel/v5.x/linux-5.4.34.tar.xz

3. 解压

xz -d linux-5.4.34.tar.xz
tar -xvf linux-5.4.34.tar
cd linux-5.4.34

4. 下载并打上补丁

wget https://raw.github.com/mengning/mykernel/master/mykernel-2.0_for_linux-5.4.34.patch
patch -p1 < ../mykernel-2.0_for_linux-5.4.34.patch

5. 安装编译环境

sudo apt install build-essential libncurses-dev bison flex libssl-dev libelf-dev

6. 编译

make defconfig 
make -j$(nproc) 

7. 安装QEMU

sudo apt install qemu

8. 运行

qemu-system-x86_64 -kernel arch/x86/boot/bzImage

运行结果

进程切换代码具体实现

在mykernel目录下，增加以下三个文件

1. mypcb.h

#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);

2. myinterrupt.c

#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);
        }     
    }
}

进程调度算法：

该算法是基于时间片的进程调度算法。每次执行完一个时间片之后，会触发一个时钟中断，然后进行进程调度。
初始化时，将第一个进程的ip字段指向函数my_process的地址。进程以循环链表的方式链接起来。
在汇编代码中，将指令寄存器ip的值置为第一个进程的ip值（即my_process函数的地址）然后开始执行my_process函数。

3. mymain.c

#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;	
}

上述代码中的汇编部分完成了进程切换

切换进程过程：

1. 将栈基址寄存器rbp的值压入栈中，保存当前进程的栈底地址；

2. 将栈寄存器rsp的值保存到当前线程的sp字段，保存当前进程的栈顶；

3. 将下一个进程的sp字段放到栈寄存器rsp中；

4. 将1f存放到当前进程的ip字段中；

   （1,2,4步保存了当前进程的现场）

5. 将下一个进程的ip字段放入到指令寄存器ip中

6. 将下一个进程的栈顶弹出到栈基址寄存器rbp中。（栈顶保存的是下一个进程的栈底地址）

输出结果