Lab4

Part2 –Backtrace

backtrace is the method that prints the address of call stack. To implement backtrace, we should know some key points :

  • In RISC-V, stack grows from high to low.
  • fp points to the start of a function(highest address), then Return Address (offset by -8), then previous frame pointer address(offset by -16)
  • We should print the address of Return Address(means the address of code)
  • Stack size are 1 PGSIZE and aligned.

So we just need to fetch the frame pointer by the follower assembly code:

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static inline uint64
r_fp()
{
  uint64 x;
  asm volatile("mv %0, s0" : "=r" (x) );
  return x;
}

Then recursively fetch the previous frame pointer, note we should use check the boundary of one page.

Part3 – Alarm

In this part, the alarm are like a callback function but trigged by timer(Event driven).
So sys_sigalarm are the setter of this scheme, first paramter is the count down number and second is the hanlder of callback function. ‘sys_sigreturn’ is the return function that reloads context of origin function(we will see in the lab)
Firstly, we should register two functions in /user and make a syscall reference for them, then we go to /kernel to complete these two syscalls. In the sys_sigalarm, we should keep the tick intervals and the function pointer in the struct proc for later call. Then sys_sigreturn, for now it just return 0 is ok.

Note that in the /kernel/trap.c:usertrap(), it handles the all exceptions (syscall, timer interupt…). Once it is called by an interupt of timer, we should increase the tick number in the struct proc and once ticks == interval, go to handler.

By modifing the address of pc, we could jump to the handler. But the way jump back to the origin code should be considered carefully – The lab hints provide us a process to return, which is the sys_sigreturn. We should reload all the origin(as many as possible to keep the context safy) in it then jump back to origin place(by pc register)

Hints

  • p->trapframe->epc points to the return address once the process jumps from kernel
  • Record as many as possible register in the proc especially pc, sp, ra and so on.
  • add backtrace() in panic() so that whenever a panic happens, it will print a backtrace information.