基于mykernel 2.0编写一个操作系统内核

基于mykernel 2.0编写一个操作系统内核

1.配置mykernel 2.0，熟悉Linux内核的编译；

实验环境：：Ubuntu 16.04.1（虚拟机上）

一次键入如下代码：

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'
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

在输入 patch -p1 < ../mykernel-2.0_for_linux-5.4.34.patch 时报错，bash: ../mykernel-2.0_for_linux-5.4.34.patch: 没有那个文件或目录，将patch文件复制进根目录后解决。

执行qemu后，结果如下：

2.在mykernel目录下进行程序的编写：

1）在mykernel目录中添加 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）修改mymain.c

#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);
        }    
    }
}

可以看到在my_process中有一个进程切换标志位：my_need_sched，在该标志位为1时进行进程切换，通过改写中断服务程序来定时改变标志位，起到时间片调度的效果。

3）修改myinerrupt.c

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;    
}

my_timer_handler⽤来记录时间⽚，每完成1000次计数，就进行进程切换，在my_schedule中也增加了进程切换的代码。

执行

3.分析内核核心功能

运行工作机制：操作系统的进程在执⾏过程中，当时间⽚⽤完需要进⾏进程切换时，需要先保存当前的进程上下⽂环境，下次进程被调度执⾏时，需要恢复进程上下⽂环境。我们通过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，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，产生进程切换。

这段汇编代码就是进程切换的主要实现方法。分析如下：

1）pushq %%rbp：将rbp寄存器值保存在切换前进程（prev）的堆顶。

2）movq %%rsp,%0：将rsp寄存器值保存在切换前进程的sp变量中。（将prev进程栈顶位置保存）

3）movq %2,%%rsp：将切换后进程（next）的栈顶指针sp放入rsp寄存器。（此时已发生堆栈切换，之后的堆栈操作都是在next进程中的）

4）movq $1f,%1：保存切换前进程的下一条指令地址到ip变量中。这里prev进程的下一条指令就在标号1后面。

5）pushq %3：这里将切换后进程的下一条指令地址ip压栈（rip寄存器程序员没有权限进行写入，需要多一个步骤）

6）ret ：将栈顶的ip，pop出来到rip寄存器（此时进程执行指令也被切换成功）

7）popq %%rbp：将切换后进程的栈顶的堆栈基地址pop出来，放入rbp寄存器