Submit your solutions before the beginning of the next lecture to any course staff.
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 $ cAnd now print out the processes (if any) on both cpus:
$ p cpus[0].proc $ p cpus[1].procon 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].namewhat 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?
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.
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.