6.S081-Lab Util report

Lab Util report

Boot Xv6 (easy)

没啥复杂的，按照教程来就可以了，基本没踩坑。

Sleep

描述：Implement the UNIX program sleep for xv6; your sleep should pause for a user-specified number of ticks. A tick is a notion of time defined by the xv6 kernel, namely the time between two interrupts from the timer chip. Your solution should be in the file user/sleep.c.

指导（可以称得上是保姆级了。。。）：

• Before you start coding, read Chapter 1 of the xv6 book.
  *Look at some of the other programs in user/ (e.g., user/echo.c, user/grep.c, and user/rm.c) to see how you can obtain the command-line arguments passed to a program.
• If the user forgets to pass an argument, sleep should print an error message.
• The command-line argument is passed as a string; you can convert it to an integer using atoi (see user/ulib.c).
• Use the system call sleep.
• See kernel/sysproc.c for the xv6 kernel code that implements the sleep system call (look for sys_sleep), user/user.h for the C definition of sleep callable from a user program, and user/usys.S for the assembler code that jumps from user code into the kernel for sleep.
• Make sure main calls exit() in order to exit your program.
• Add your sleep program to UPROGS in Makefile; once you’ve done that, make qemu will compile your program and you’ll be able to run it from the xv6 shell.
• Look at Kernighan and Ritchie’s book The C programming language (second edition) (K&R) to learn about C.

按照教程即可。代码如下：

#include "kernel/types.h"
#include "kernel/stat.h"
#include "user/user.h"

int main(int argc, char *argv[])
{
   
   
    int t;
	if (argc <= 1 || argc > 2) {
   
   
		fprintf(2, "usage: sleep <seconds>\n");
        exit(1);
	}
    t = atoi(argv[1]);
    sleep(t);
	exit(0);
}

实现sleep命令没啥好讲的，实际上好玩的是sleep syscall的实现，即如何从用户态陷入内核空间，进而完成sleep命令的。

首先在user.h中声明了sleep这个函数，在usys.S中看到，首先.global sleep在全局符号表中注册函数名sleep，之后调用sleep这个系统调用时就会执行以下这段riscv汇编代码：

.global sleep
sleep:
 li a7, SYS_sleep
 ecall
 ret

这段汇编的意思也很明确，就是把SYS_sleep的值放入a7寄存器中，记录系统调用的类型，之后的ecall指令产生一个异常，此时陷入内核态，然后查看kernel/syscall.c可知SYS_sleep和sys_sleep存在映射关系，syscall函数可以根据SYS_<name>值的不同执行不同的sys_<name>函数(kernel/syscall.c:140)。

void
syscall(void)
{
   
   
  int num;
  struct proc *p = myproc();

  num = p->trapframe->a7;
  if(num > 0 && num < NELEM(syscalls) && syscalls[num]) {
   
   
    p->trapframe->a0 = syscalls[num]();
  } else {
   
   
    printf("%d %s: unknown sys call %d\n",
            p->pid, p->name, num);
    p->trapframe->a0 = -1;
  }
}

反正目前看kernel/syscall.c里的syscall实现，是通过中断实现的(trapframe)，具体的实现会等到后面的实验再详细解释。

以上是一个基本的理解，更详细的了解估计在后面的课程中会涉及。

参考：

• riscv特权等级介绍：riscv特权等级
• riscv指令手册：riscv-spec

PingPong

描述：Write a program that uses UNIX system calls to ‘‘ping-pong’’ a byte between two processes over a pair of pipes, one for each direction. The parent should send a byte to the child; the child should print “: received ping”, where is its process ID, write the byte on the pipe to the parent, and exit; the parent should read the byte from the child, print “: received pong”, and exit. Your solution should be in the file user/pingpong.c.

指导：

• Use pipe to create a pipe.
• Use fork to create a child.
• Use read to read from the pipe, and write to write to the pipe.
• Use getpid to find the process ID of the calling process.
• Add the program to UPROGS in Makefile.
• User programs on xv6 have a limited set of library functions available to them. You can see the list in user/user.h; the source (other than for system calls) is in user/ulib.c, user/printf.c, and user/umalloc.c.

之前实现过unix的版本，直接把代码copy稍作改动即可。

#include "kernel/types.h"
#include "kernel/stat.h"
#include "user/user.h"


int main()
{
   
   
    int p[2], q[2];
    pipe(p), pipe(q);
    if (fork() > 0) {
   
   
        close(p[0]);
        close(q[1]);
        char x[1];
        write(p[1], "0", 1);
        read