一 实现trace功能
1 题目要求
In this assignment you will add a system call tracing feature that may help you when debugging later labs. You’ll create a new trace system call that will control tracing. It should take one argument, an integer “mask”, whose bits specify which system calls to trace. For example, to trace the fork system call, a program calls trace(1 << SYS_fork), where SYS_fork is a syscall number from kernel/syscall.h. You have to modify the xv6 kernel to print out a line when each system call is about to return, if the system call’s number is set in the mask. The line should contain the process id, the name of the system call and the return value; you don’t need to print the system call arguments. The trace system call should enable tracing for the process that calls it and any children that it subsequently forks, but should not affect other processes.
We provide a trace user-level program that runs another program with tracing enabled (see user/trace.c). When you’re done, you should see output like this:
$ trace 32 grep hello README
3: syscall read -> 1023
3: syscall read -> 966
3: syscall read -> 70
3: syscall read -> 0
$
$ trace 2147483647 grep hello README
4: syscall trace -> 0
4: syscall exec -> 3
4: syscall open -> 3
4: syscall read -> 1023
4: syscall read -> 966
4: syscall read -> 70
4: syscall read -> 0
4: syscall close -> 0
$
$ grep hello README
$
$ trace 2 usertests forkforkfork
usertests starting
test forkforkfork: 407: syscall fork -> 408
408: syscall fork -> 409
409: syscall fork -> 410
410: syscall fork -> 411
409: syscall fork -> 412
410: syscall fork -> 413
409: syscall fork -> 414
411: syscall fork -> 415
...
$
trace是一个运行在用户空间的程序。 你需要实现一个用户空间程序 trace，它将启动另一个程序并启用跟踪功能。
trace调用时，会传入一个int类型的mask值，来指明需要对哪些系统调用trace。
trace程序的函数调用流程：
二 实现sys_info功能
在系统调用函数sys_info中获取到的数据要拷贝到用户空间。
在xv6操作系统中，trapframe 结构体用于保存当发生陷阱（trap）时的寄存器状态，以便之后可以恢复执行。对于RISC-V架构而言，a0 到 a7 寄存器通常用来传递系统调用参数和返回值。
To be continue: 研究下如何将sysinfo结构体从kernel空间拷贝到user空间