P2-xv6-syscall Solution

Created two new system calls in the xv6 OS. These system calls are used to get the number of system calls that have been called from the respective process.

Code Base
You will be using the current version of xv6. You could find a copy of xv6 source code in ~cs537-1/projects/xv6.tar.gz. Copy it into your private directory and untar it.

prompt> cp ~cs537-1/projects/xv6.tar.gz /path/to/your/private/dir
prompt> tar -xvzf xv6.tar.gz
If, for development and testing, you would like to run xv6 in an environment other than the CSL instructional Linux cluster, you may need to set up additional software. You can read these instructions for the MacOS build environment. (Links to an external site.) Note that we will run all of our tests and do our grading on the instructional Linux cluster so you should always ensure that the final code you handin works on those machines.

After you have obtained the source files, you can run make qemu-nox to compile all the code and run it using the QEMU emulator. Test out the unmodified code by running a few of the existing user-level applications, like ls and forktest. With ls inside the emulator, you'll be able to see a few other applications that are available (as well as files that have been created within the xv6 environment).

To quit the emulator, type Ctl-a x.

You will want to be familiar with the Makefile and comfortable modifying it. In particular, see the list of existing UPROGS. See the different ways of running the environment through make (e.g., qemu-nox or qemu-nox-gdb). 

Find where the number of CPUS is set and change this to be 1.

For additional information about xv6, we strongly encourage you to look through the code while reading this book (Links to an external site.) by the xv6 authors. 

Or, if you prefer to watch videos, the last ten minutes of the first video (Links to an external site.) plus a 2nd video from a previous discussion section (Links to an external site.) describe some of the relevant files. Note that the code and project in the videos do not exactly match what you are doing. We always recommend that you look at the actual code yourself while either reading or watching (perhaps pausing the video as needed).

Debugging
Your first task is to demonstrate that you can use gdb to debug xv6 code. You should show the integer value that fdalloc() (in sysfile.c) returns the first time it is called after a process has been completely initialized.

To do this, you can follow these steps:

In one window, start up qemu-nox-gdb (using make). In another window where you are logged into the same machine, start up gdb (it will attach to the qemu process) and continue it until xv6 finishes its bootup process and gives you a prompt. Now, interrupt (Ctrl+C for Windows) gdb and set a breakpoint in the fdalloc() routine. Continue gdb. Then, run the stressfs user application at the xv6 prompt since this will cause fdalloc() to be called. Your gdb process should now have stopped in fdalloc. Now, step (or, probably next) through the C code until gdb reaches the point just before fdalloc() returns and print the value that will be returned (i.e., the value of fd). Now immediately quit gdb and run whoami to display your login name. 

To sanity-check your results, you should think about the value you expect fdalloc() to return in these circumstances to make sure you are looking at the right information. What is the first fd number returned after stdin, stdout, and stderr have been set up? 

If gdb gives you the error message that fd has been optimized out and cannot be displayed, make sure that your Makefile uses the flag "-Og" instead of "-O2". Debugging is also a lot easier with a single CPU, so if you didn't do this already: in your Makefile find where the number of CPUS is set and change this to be 1.

Take a screenshot showing your gdb session with the returned value of fd printed and your login name displayed. Submit this screenshot to Canvas.

System Calls and User-Level Application
You will add two related system calls to xv6:

int getnumsyscalls(int pid) returns the number of system calls that the process identified by pid has completed (not just initiated); calls to getnumsyscalls(), getnumsyscallsgood(), fork(), exec(), and sbrk() should not be counted in that total*. Returns -1 if pid is not a valid pid.
int getnumsyscallsgood(int pid) returns the number of system calls that the process identified by pid has completed successfully (i.e., with a return code that is not -1); calls to getnumsyscalls(), getnumsyscallsgood(), fork(), exec(), and sbrk() should not be counted in this total. Returns -1 if pid is not a valid pid.
So that you can test your system calls, you should also create a user-level application with the following behavior:

syscalls N g. This program takes two arguments: N, which is the total number of system calls that it should make, and g, which is the number of those which should be successful. Note that N and g should >= 1 because you always need a syscall getpid() to know the pid of the current running process. You can choose to make any system calls that you know will be successful or unsuccessful; you can perform those system calls in any order you choose. After this work has been done, it should print out two values: the value returned by getnumsyscalls(mypid) and the value returned by getnumsyscallsgood(mypid). For example, if you run the program as "syscalls 20 5" the output should be exactly "20 5\n". You can handle errors (e.g., an incorrect number of arguments or the number of good system calls is greater than the total number of calls however you choose) but we won't test it. 
You must use the names of the system call and the application exactly as specified!

*Note:

Typically, fork(), exec(), and sbrk() will be called during the process creation (before the main() of the user-level application syscalls  is invoked); for simplicity, we exclude them here. You should also be careful when calling some library functions as they may also invoke syscalls (e.g. malloc() might call sbrk()). Try to avoid them before printing the final result.
A valid pid indicates any pid that is in-use which includes the zombie processes. A process is in the zombie state after it exits and before it is waited for by its parent process.
Implementation Hints and Details
The primary files you will want to examine in detail include syscall.c, sysproc.c, proc.h, and proc.c. You might also want to take a look at usys.S, which (unfortunately) is written in assembly.

To add a system call, find some other very simple system call that also takes an integer parameter, like sys_kill, copy it in all the ways you think are needed, and modify it so it doesn't do anything and has the new name. Compile the code to see if you found everything you need to copy and change. You probably won't find everything the first time you try.

Then think about the changes that you will need to make so your system calls act like themselves.

How will you find out the pid that has been passed to your two system calls? This is the same as what sys_kill() must do. 
How will you track the two counters for each process? You will want to add both of these new counters to a structure that already exists for each process. Your new system calls will simply return the value of one of the two counters. 
Make sure you increment the appropriate counter after the system call (except for those we have said to exclude) has returned and check its return value to make sure it isn't -1. 
How will you find the data structures for the specified process? You'll need to look through the ptable.proc data structure to find the process with the matching pid. 
For this project, you do not need to worry about concurrency or locking. [updated Feb. 12] You still need to have lock acquire and lock release code (just as what kill() does in proc.c), but you are not required to fully understand them. See this piazza note (Links to an external site.) for more details.

To create the user-level application syscalls we again suggest copying one of the straight-forward utilities that already exist. 

Good luck! While the xv6 code base might seem intimidating at first, you only need to understand very small portions of it for this project. This project is very doable!