基于mykernel 2.0编写一个操作系统内核
一、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.要分析操作系统内核核心功能及运行工作机制
二、实验环境
VMware Workstation Pro 15
ubuntu18.04.4
三、实验内容
1.按照https://github.com/mengning/mykernel 的说明配置mykernel 2.0
参照如下代码
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 make -j$(nproc) sudo apt install qemu qemu-system-x86_64 -kernel arch/x86/boot/bzImage
配置完成后，可以看到，时钟中断处理程序my_timer_handler在周期性执行。
通过分析源码：
mymain.c
void __init my_start_kernel(void)
{
int i = 0;
while(1)
{
i++;
if(i%100000 == 0)
printk(KERN_NOTICE "my_start_kernel here %d \n",i);
}
}
my interrupt.c
void my_timer_handler(void) { printk(KERN_NOTICE "\n>>>>>>>>>>>>>>>>>my_timer_handler here<<<<<<<<<<<<<<<<<<\n\n"); }
mymain.c不断循环执行，当 i 能够被 100000 整除 就输出 “my_start_kernel here”
myinterrupt.c是中断处理程序，内核执行时有一个中断处理程序的上下文环境，在固定的时间间隔都发生一次中断，也是说每秒发生该中断的频率都是固定的。该频率是常量HZ，该值一般是在100 ~ 1000之间，关中断时调用触发myinterrupt.c，输出“>>>>>>>>>>>>>>my_timer_handler here<<<<<<<<<<<<<<<”
2.基于mykernel 2.0编写一个操作系统内核
（1）添加进程控制块文件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);
IP:指令指针
SP：堆栈指针
PID:进程控制符
state：定义了进程的三种状态。-1就绪，0运行，>0阻塞
stack[]：进程使用的堆栈
thread：进程当前正在执行的线程
task_entry：进程入口函数
next：指向下一个进程PCB的指针
（2）修改mymain.c中的代码
修改后代码如下：
#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); } } }
首先调用my_start_kernel函数，创建0号进程后，创建其他多个进程，并通过next指针进行连接。接下来通过汇编语言分配系统资源：
movq %1,%%rsp: 将待执行的进程的栈顶指针赋值给RSP寄存器
pushq %1:将当前进程的栈顶指针压入堆栈，用于恢复。
pushq %0：将当前进程的指令指针压入堆栈
ret：将压栈的进程指令指针保存到RIP寄存器中
然后在my_process函数中，模拟了时间片轮转调度，每当i增加10000000时，根据my_need_sched变量，如果为1，当前进程进行调度。即运行完一个时间片后，让出CPU。
（3）修改myinterrupt.c中的代码
修为改：
#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; }
其中my_timer_handler函数的功能为周期性将my_need_sched置为1，标志进程需要进程调度。实际的调度代码为my_schedule函数。
对核心代码的理解：
pushq %%rbp:将当前进程的栈底指针压入堆栈
movq %%rsp,%0: 将RSP寄存器的值保存到前一个进程的栈顶指针中
movq %2,%%rsp: 将RSP寄存器的值更新为下一进程的栈顶地址
movq $1f,%1: 将RIP寄存器中的值保存到前一个进程的指令指针中
pushq %3: 将下一个进程的指令指针压入堆栈
ret: 将之前压栈的进程指令指针保存到RIP寄存器中
1:开始执行下一个进程
popq %%rbp: 将之前进程保存的栈底地址出栈，赋值给RBP寄存器
（4）重新编译后，启动QEMU观察结果
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