一、 实验要求
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按照https://github.com/mengning/mykernel 的说明配置mykernel 2.0,熟悉Linux内核的编译;
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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
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简要分析操作系统内核核心功能及运行工作机制
二、实验步骤
2.1 配置mykernel 2.0
2.1.1 wget https://raw.github.com/mengning/mykernel/master/mykernel-2.0_for_linux-5.4.34.patch (这里wget未成功,显示拒绝连接,直接使用了群里的文件)
2.1.2 sudo apt install axel
2.1.3 axel -n 20 https://mirrors.edge.kernel.org/pub/linux/kernel/v5.x/linux-5.4.34.tar.xz
2.1.4 xz -d linux-5.4.34.tar.xz
2.1.5 tar -xvf linux-5.4.34.tar
2.1.6 cd linux-5.4.34
2.1.7 patch -p1 < ../mykernel-2.0_for_linux-5.4.34.patch
2.1.8 sudo apt install build-essential libncurses-dev bison flex libssl-dev libelf-dev
2.1.9 make defconfig # Default configuration is based on 'x86_64_defconfig'
2.1.10 make -j$(nproc) (耗时较长)
2.1.11 sudo apt install qemu
2.1.12 qemu-system-x86_64 -kernel arch/x86/boot/bzImage
在mykernel目录下可以看到mymain.c以及myinterrupt.c程序代码,如下图所示:
可以看到,该程序在运行时不断计时,每当满100000时,打印出“my_start_kernel here”,同时有一个中断处理程序的上下文环境,周期性地产生时钟中断信号,能够触发myinterrupt.c中的代码。
在中断处理程序中,每隔一段时间输出“>>>>>>my_timer_handler here<<<<<<”,与QEMU虚拟机中所示结果相同,模拟了一个具有时钟中断和C代码执行环境的硬件平台。
2.2 基于mykernel 2.0编写一个操作系统内核
首先在mykernel目录下增加一个mypcb.h头文件,用来定义PCB(进程控制块),如图所示:
该文件定义了进程号,进程状态码,使用的堆栈,线程信息,入口函数以及指向下一个pcb的指针,此外还定义了最大进程数和堆栈空间,封装了指令指针ip和堆栈指针sp在结构体Thread中。
然后修改mymain.c文件:
#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); } } }
该文件中,__init my_start_kernel作为内核代码入口,从0号进程开始初始化pcb中的进程变量,再通过汇编代码完成进程的启动和切换。
my_process函数作为进程代码,模拟了一个简单的时间片的进程,每完成10000000次计数将my_need_sched置0,并重新调用my_schedule函数。
接下来修改myinterrupt.c文件:
#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<<< "); 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 $1f,%1 " /* save rip of prev */ "pushq %3 " "ret " /* restore rip of next */ "1: " /* next process start here */ "popq %%rbp " : "=m" (prev->thread.sp),"=m" (prev->thread.ip) : "m" (next->thread.sp),"m" (next->thread.ip) ); } return;
my_timer_handler用来记录时间片,每完成1000次计数,就进行进程切换。此外,在my_schedule中也增加了进程切换的代码。
执行:
make clean make defconfig make -j$(nproc) qemu-system-x86_64 -kernel arch/x86/boot/bzImage
2.3简要分析操作系统内核核心功能及运行工作机制
asm volatile( "movq %1,%%rsp " "pushq %1 " "pushq %0 " "ret " : : "c" (task[pid].thread.ip),"d" (task[pid].thread.sp) );
- RSP寄存器指向原堆栈的栈顶,%1指后面的task[pid].thread.sp
- 压栈当前进程RBP寄存器
- 压栈当前进程RIP寄存器,%0指task[pid]. thread.ip
- ret命令正好可以让压栈的进程RIP保存到RIP寄存器中
asm volatile( "pushq %%rbp " "movq %%rsp,%0 " "movq %2,%%rsp " "movq $1f,%1 " "pushq %3 " "ret " "1: " "popq %%rbp " : "=m" (prev->thread.sp),"=m" (prev->thread.ip) : "m" (next->thread.sp),"m" (next->thread.ip) );
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pushq %%rbp 保存prev进程(进程0)当前RBP寄存器的值到堆栈
- movq %%rsp,%0 保存prev进程(进程0)当前RSP寄存器的值到prev->thread.sp(%0)
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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
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movq $1f,%1 保存prev进程当前RIP寄存器值到prev->thread.ip(%1),这⾥$1f是指标号1
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pushq %3 把即将执⾏的next进程的指令地址next->thread.ip(%3)⼊栈
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ret 将压⼊栈中的next->thread.ip放⼊RIP寄存器,程序jianjie直接使用RIP寄存器,通过ret间接改变
- 1: 一个地址,与上文$1f相对应
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popq %%rbp 将next进程堆栈基地址从堆栈中恢复到RBP寄存器中
3.总结
本实验作为Linux学习过程中的第一个实验,通过编写一个简单的计算机操作系统内核,完成了基于时间片的进程轮换。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,就这样通过虚拟化的进程概念实现了多道程序在同⼀个物理CPU上并发执⾏。同时也进一步加深了对汇编代码的理解,也为后续中断的学习打下了基础。