基于mykernel2.0编写一个操作系统内核

实验要求

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；

实验环境搭建

　　本次实验使用虚拟机Vmware15，Ubuntu18.01操作系统。

　　按照如下命令依次执行：

　　

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  libncurses5-dev bison flex libssl-dev libelf-dev
sudo apt install qemu 
make defconfig 
make -j$(nproc)

在下载mykernel-2.0_for_linux-5.4.34.patch时可能出现无法连接或者拒绝访问的情况，可以修改hosts文件，具体参照这篇博文https://www.cnblogs.com/sinferwu/p/12726833.html。

执行到这里环境基本搭建成功，然后可以启动mykernel进行验证，执行以下代码

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

然后出现如下界面：

可以看到qemu在不停执行，具体我们可以看一下代码：

可以看出mymain.c中的代码在不停地执⾏。同时有⼀个中断处理程序的上下⽂环境，周期性地产⽣的时钟中断信号，能够触发myinterrupt.c中的代码。

基于mykernel 2.0编写一个操作系统内核

在https://github.com/mengning/mykernel网址下载孟宁老师的源码，然后将mypcb.h，myinterrupt.c和mymain.c这三个文件拷贝到mykernel目录下，即要覆盖之前的mykernel文件夹下mymain.c和myinterrupt.c，并新增头文件mypcb.h。

mypcb.h定义进程控制块

/*
 *  linux/mykernel/mypcb.h
 *
 *  Kernel internal PCB types
 *
 *  Copyright (C) 2013  Mengning
 *
 */

#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);

pid：进程号

state：进程状态，在模拟系统中，所有进程控制块信息都会被创建出来，其初始化值就是-1，如果被调度运行起来，其值就会变成0

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，模拟系统中所有的PCB是以链表的形式组织起来的。

mymain.c：初始化各个进程并启动0号进程

/*
 *  linux/mykernel/mymain.c
 *
 *  Kernel internal my_start_kernel
 *  Change IA32 to x86-64 arch, 2020/4/26
 *
 *  Copyright (C) 2013, 2020  Mengning
 *  
 */
#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
	"     /* 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*/
    );
} 

int i = 0;

void my_process(void)
{    
    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);
        }     
    }
}

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。

myinterrupt.c：时钟中断处理和进程调度算法

/*
 *  linux/mykernel/myinterrupt.c
 *
 *  Kernel internal my_timer_handler
 *  Change IA32 to x86-64 arch, 2020/4/26
 *
 *  Copyright (C) 2013, 2020  Mengning
 *
 */
#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<<<
");
        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;    
}

mykernel提供了时钟中断机制，周期性执行my_time_handler中断处理程序。

最后重新编译，在linux-5.4.34目录下执行如下命令：

make defconfig
make -j$(nproc)
qemu-system-x86_64 -kernel arch/x86/boot/bzImage

可以看到效果：

从process0到process3交替执行

实验总结

通过本次实验，我对操作系统的时间轮转机制和中断机制有了更深的了解，对底层的实现有了更加清晰的认识，实在获益良多。