基于mykernel2.0编写一个操作系统内核

一. 实验准备

详细要求

基于mykernel 2.0编写一个操作系统内核

按照https://github.com/mengning/mykernel 的说明配置mykernel 2.0，熟悉Linux内核的编译；

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

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

发行版本：Ubuntu 18.04.4 LTS

处理器：Intel® Core™ i7-8850H CPU @ 2.60GHz × 3

图形卡：Parallels using AMD® Radeon pro 560x opengl engine

GNOME：3.28.2

二. 实验过程

Set up mykernel 2.0

依次执行如下的指令

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

执行成功并运行后可以看到如下的界面

qemu窗口中输出的内容来自代码mymain.c和myinterrupt.c。先查看一下代码内容

mymain.c

#ifdef CONFIG_X86_LOCAL_APIC
#include <asm/smp.h>
#endif


void __init my_start_kernel(void)
{
    int i = 0;
    while(1)
    {
        i++;
        if(i%100000 == 0)
            pr_notice("my_start_kernel here  %d 
",i);
            
    }
}

nyinterrupt.c

/*
 * Called by timer interrupt.
 */
void my_timer_handler(void)
{
	pr_notice("
>>>>>>>>>>>>>>>>>my_timer_handler here<<<<<<<<<<<<<<<<<<

");
}

首先在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，在Linux内核中是struct tast_struct结构体

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


#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;


void my_schedule(void);

修改mymain.c中的my_start_kernel函数，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);
        }
    }
}

对myinterrupt.c进行修改，my_timer_handler用来记录时间片，时间片消耗完之后完成调度。

#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;
}

更改后的运行结果：

进程切换过程中进程0和进程1的堆栈和相关寄存器的变化过程大致如下：

pushq %%rbp 保存prev进程（本例中指进程0）当前RBP寄存器的值到prev进程的堆栈；
movq %%rsp,%0 保存prev进程（本例中指进程0）当前RSP寄存器的值到prev->thread.sp，这时RSP寄存器指向进程的栈顶地址，实际上就是将prev进程的栈顶地址保存；%0、%1...指这段汇编代码下面输入输出部分的编号。
movq %2,%%rsp 将next进程的栈顶地址next->thread.sp放入RSP寄存器，完成了进程0和进程1的堆栈切换。
movq $1f,%1 保存prev进程当前RIP寄存器值到prev->thread.ip，这里$1f是指标号1。
pushq %3 把即将执行的next进程的指令地址next->thread.ip入栈。
ret 就是将压入栈中的next->thread.ip放入rip寄存器，rip寄存器现在存储next进程的指令。
1: 标号1是一个特殊的地址位置，该位置的地址是$1f。
popq %%rbp 将next进程堆栈基地址从堆栈中恢复到RBP寄存器中。

三. 总结

这次实验主要做了如下的事情：

学习并完成实验环境的配置的搭建
学习并了解Linux内核编译相关知识
通过实践加深对编译的学习与体会
基于mykernel 2.0编写一个操作系统内核
思考代码执行的各项原理