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；

　　1）本机环境如下：

　　　　虚拟机环境：VMware® Workstation 15 Pro

　　　　Ubuntu环境：

 

 

 　　2）配置mykernel环境：

　　　　依次执行如下命令：　　　

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 # Default configuration is based on 'x86_64_defconfig'
# 使用allnoconfig编译出来qemu无法加载启动，不知道为什么？有明白的告诉我，完整编译太慢了，消耗的资源也多。
make -j$(nproc) # 编译的时间比较久哦
sudo apt install qemu # install QEMU
qemu-system-x86_64 -kernel arch/x86/boot/bzImage

　　3）模拟器运行结果如下：

　　

 

 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

　　1）定义程序控制块PCB：

#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);

　　各字段意义如下：

　　pid：进程id

　　state：进程状态

　　stack：进程堆栈

　　thread：进程对应的线程描述信息

　　task_entry：进程入口地址

　　next：进程链表中的下一进程

　　2）通过my_start_kernel进行进程调度。my_start_kernel采用了简单的时间片轮转调度方法进行实现。

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].stack[KERNEL_STACK_SIZE-1] - 1) = (unsigned long)&task[i].stack[KERNEL_STACK_SIZE-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(
        "movl %1,%%esp\n\t"     /* set task[pid].thread.sp to esp */
        "pushl %1\n\t"             /* push ebp */
        "pushl %0\n\t"             /* push task[pid].thread.ip */
        "ret\n\t"                 /* pop task[pid].thread.ip to eip */
        : 
        : "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）通过my_timer_handler实现中断处理逻辑，my_schedule来处理进程切换的上下文逻辑

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;	
}

　　4）重新编译并运行：

　

 

 3.执行流程分析

mymain.c中，有两个函数，分别是my_start_kernel和my_process，其功能分别如下：

　　my_start_kernel:创建0号进程，随后fork出更多的进程。

　　my_process:代表一个进程，通过死循环不断打印进程pid

myinterrupt.c中的my_schedule方法执行进程切换。进程切换的核心代码如下：

"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)

　首先保存prev进程的栈顶指针和栈底指针，随后将栈顶指针设为next进程的sp，并保存prev的rip寄存器的值。然后将next进程的ip入栈，将rip设为next进程的ip，将rbp设为next进程的栈底。以上操作就完成了进程切换。