System Calls(pending the challenge)

In the lab, we will get deeper into OS than the previous one. We are about to implement the ‘bridge’ between user and kernel mode, then implement two specific syscall to be called by user’s program.

Lab

The two labs are symmetry overall, so I just describe how to do a system call.

To make our own syscalls,we should know how the syscall works between user & kernel mode.

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						   user function
							     ↓
					      wrapped interface
-----------------------------↓(ecall)--------------------------------------
						     syscall()
						     	 ↓
						kernel sys_(syscall's name)

In this framework, there are some points:

  • How to use ecall?
    • We first declare a wrapped function in user/user.h, then implement it in user/usys.S which are assembly code generated by user/usys.pl, the assembly code are used to load the syscall number into a7 register.
  • In kernel mode how to fetch argument?
    • Refer to other functions in kernel/sysproc.c, we can use argint() in kernel/syscall.c to fetch argument from stack
  • syscall() in kernel/syscall.c , what’s its function?
    • It precedes after ecall, fetches the a7 then executes corresponding syscall, then put the return value into a0

Trace

  • Remember to put your new syscall number into every part a syscall can involve with, especially don’t forget set the function point entry in syscall.c‘s array.

  • The function uses mask to mark all system call it wants to trace, so each bit on mask is a syscall you should check. Use mask & 1 << syscallnum to check it.

![Lab2-1](Lab2 Syscalls/Lab2-1.png)

Sysinfo

  • The way kernel mode gives back argument to user mode is to call copyout which maps return args’ virtual memory back to physical memory then revises it to target value.
  • The collections of free memory and processes is not too hard, we can just iterate the freemem linklist and process array.

![lab2-2](Lab2 Syscalls/lab2-2.png)

Conclusion

In this lab, we breakthrough the barrier between u/k mode, the whole detailed system call process is in front of us. But there are still many information we need to get into, mmap, pages management , process management, etc.

Also, I have not started challenges, because I am about to begin my graduate life in NUS, hopefully I can go back here and finish them someday!