基于mykernel 2.0编写一个操作系统内核
1. mykernel 下载并安装
参考孟老师PPT上的教程，以下为终端命令行详细步骤：
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-get install build-essential gcc-multilib
sudo apt install qemu # install QEMU
sudo apt-get install libncurses-dev bison flex libssl-dev libelf-dev
make defconfig # Default configuration is based on 'x86_64_defconfig'
make -j$(nproc)
qemu-system-x86_64 -kernel arch/x86/boot/bzImage
核心代码为 ./linux-5.4.34/mykernel 文件夹中的 mymain.c 和 myinterrupt.c 文件
综上，我们可以了解到，在 mykernel 系统启动后，首先会调用 my_start_kernel 函数，不断循环打印，然后周期性的调用 my_timer_handler 函数。
因此，内核的编写围绕以上两个函数展开即可。
2. mykernel 系统内核
（1）参考 PPT 以及老师上课的讲解，在 mykernel 文件夹中添加 mypcb.h （进程描述头文件），并修改 mymain.c 和 myinterrupt.c 中的代码
（2）终端回退到 ./linux-5.4.34 文件夹下，输入如下命令：
make defconfig # Default configuration is based on 'x86_64_defconfig' make -j$(nproc) qemu-system-x86_64 -kernel arch/x86/boot/bzImage
可以看到如下输出：
mykernel 一直在虚拟进程 process0 到 process3 之间轮回切换，从而实现一个简单的时间片轮转操作系统内核。
3.关键代码分析
（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);
在该头文件中，定义了进程的基本描述信息 栈顶指针sp 和 指令指针ip，也对进程控制块的数据结构做了相关描述。
pid：进程标示
state：进程所处的不同状态，进程刚创建是为-1，表示未运行；被调用时，为0
stack[KERNEL_STACK_SIZE]：用数组模拟进程的堆栈
thread：进程的基本描述信息
task_entry：进程入口函数地址
next：指向下一个进程的指针
（2）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\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);
}
}
}
核心函数为__init my_start_kernel，实现了所有进程（在本模拟系统中统一为 my_process 函数）的创建和初始化，而内嵌如下一段汇编代码
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*/
);
该段汇编代码将0号进程的 ip 和 sp 分别存入 rip 和 rsp 寄存器，实现0号进程的加载。
（3）myinterrupt.c
#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<<<\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;
}
my_timer_handler函数实现了进程的周期性调用，每循环1000次，就通知进程去执行调度函数 my_schedule，关键的汇编代码如下
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)
);
上述代码，将旧进程的 ip、sp等指针信息保存，并将新进程的 ip、sp 存入相应寄存器，从而实现不同进程之前的切换。
4.总结
通过手动实现 mykernel 操作系统的内核，我学习到了进程的创建、加载以及不同进程之间切换的相关知识。通过内嵌在c语言中的汇编代码，直接对操作系统底层做相应操作，透过现象看本质，着实受益良多。
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