Homework: xv6 system calls

Submit your solutions before the beginning of the next lecture to any course staff.

Part Zero: which CPU runs the first process?

Xv6 is running on a machine configured with two cpus: CPU0 and CPU1. In this exercise, your job is to figure out on which CPU the first process "/init" is running.

First, run xv6 with make qemu-gdb and gdb kernel, set a break point on function forkret and let gdb hit the break point:

  $ b forkret
  $ c

And now print out the processes (if any) on both cpus:

  $ p cpus[0].proc
  $ p cpus[1].proc

on which cpu the first process is running?

Now, check the address of the process table and process 0 information:

  $ p &ptable.proc
  $ p ptable.proc[0].pid
  $ p ptable.proc[0].name

what conclusion can you draw?

Challenge: note that the code is now in kernel space, can you write some code to change this process to run on cpu0, before it enters user space?

Part One: System call tracing

Your first task is to modify the xv6 kernel to print out a line for each system call invocation. It is enough to print the name of the system call and the return value; you don't need to print the system call arguments.

When you're done, you should see output like this when booting xv6:

...
fork -> 2
exec -> 0
open -> 3
close -> 0
$write -> 1
 write -> 1

That's init forking and execing sh, sh making sure only two file descriptors are open, and sh writing the $ prompt.

Hint: modify the syscall() function in syscall.c.

Challenge: print the system call arguments.

Part Two: Halt system call

Your second task is to add a new system call to xv6. The main point of the exercise is for you to see some of the different pieces of the system call machinery.

Your new system call will halt xv6 by telling QEMU to exit. Here is some code that does that; it needs to run in the kernel:

  char *p = "Shutdown";
  for( ; *p; p++)
    outb(0x8900, *p);

You should create a user-level program that calls your new halt system call; here's some source you should put in halt.c:

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

int
main(int argc, char *argv[])
{
  halt();
  return 0;
}

In order to make your new halt program available to run from the xv6 shell, add _halt to the UPROGS definition in Makefile.

Your strategy for making a halt system call should be to clone all of the pieces of code that are specific to some existing system call, for example the "uptime" system call. You should grep for uptime in all the source files, using grep -n uptime *.[chS].

When you're done, typing halt to an xv6 shell prompt should cause qemu to exit.

Write down a few words of explanation for each of the files you had to modify in the process of creating your halt system call.

Challenge: add a dup2() system call.

Turn in: Your explanations of the modifications for halt.