# Lab: Alarm and uthread

This lab will familiarize you with the implementation of system calls and switching between threads of execution. In particular, you will implement new system calls (`sigalarm` and `sigreturn`) and switching between threads in a user-level thread package.

## Warmup: RISC-V assembly

For this lab it will be important to understand a bit of RISC-V assembly.

Add a file user/call.c with the following content, modify the Makefile to add the program to the user programs, and compile (make fs.img). The Makefile also produces a binary and a readable assembly a version of the program in the file user/call.asm.

    #include "kernel/param.h"
    #include "kernel/types.h"
    #include "kernel/stat.h"
    #include "user/user.h"

    int g(int x) {
      return x+3;
    }

    int f(int x) {
      return g(x);
    }

    void main(void) {
      printf(1, "%d %d\n", f(8)+1, 13);
      exit();
    }

Read through user/call.asm and understand it. The instruction manual for RISC-V is in the doc directory (doc/riscv-spec-v2.2.pdf). Here are some questions that you should answer for yourself:

- Which registers contain arguments to functions? Which register holds 13 in the call to `printf`? Which register holds the second argument? Which register holds the third one? Etc.
- Where is the function call to `f` from main? Where is the call to `g`? (Hint: the compiler may inline functions.)
- At what address is the function `printf` located?
- What value is in the register `ra` just after the `jalr` to `printf` in `main`?

## Warmup: system call tracing

In this exercise you will 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. (Note: the output of the shell and the system call trace are intermixed, because the shell uses the write syscall to print its output.)

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

Run the xv6 programs you wrote in earlier labs and inspect the system call trace. Are there many system calls? Which system calls correspond to code in the applications you wrote?

Optional: print the system call arguments.

## Alarm

In this exercise you'll add a feature to xv6 that periodically alerts a process as it uses CPU time. This might be useful for compute-bound processes that want to limit how much CPU time they chew up, or for processes that want to compute but also want to take some periodic action. More generally, you'll be implementing a primitive form of user-level interrupt/fault handlers; you could use something similar to handle page faults in the application, for example.

You should add a new `sigalarm(interval, handler)` system call. If an application calls `sigalarm(n, fn)`, then after every `n` "ticks" of CPU time that the program consumes, the kernel should cause application function `fn` to be called. When `fn` returns, the application should resume where it left off. A tick is a fairly arbitrary unit of time in xv6, determined by how often a hardware timer generates interrupts.

You'll find a file `user/alarmtest.c` in your xv6 repository. Add it to the Makefile. It won't compile correctly until you've added `sigalarm` and `sigreturn` system calls (see below).

`alarmtest` calls `sigalarm(2, periodic)` in `test0` to ask the kernel to force a call to `periodic()` every 2 ticks, and then spins for a while. You can see the assembly code for alarmtest in user/alarmtest.asm, which may be handy for debugging. When you've finished the lab, `alarmtest` should produce output like this:

    $ alarmtest
    test0 start
    ......................................alarm!
    test0 passed
    test1 start
    ..alarm!
    ..alarm!
    ..alarm!
    .alarm!
    ..alarm!
    ..alarm!
    ..alarm!
    ..alarm!
    ..alarm!
    ..alarm!
    test1 passed
    $

The main challenge will be to arrange that the handler is invoked when the process's alarm interval expires. You'll need to modify usertrap() in kernel/trap.c so that when a process's alarm interval expires, the process executes the handler. How can you do that? You will need to understand how system calls work (i.e., the code in kernel/trampoline.S and kernel/trap.c). Which register contains the address to which system calls return?

Your solution will be only a few lines of code, but it may be tricky to get it right. We'll test your code with the version of alarmtest.c in the original repository; if you modify alarmtest.c, make sure your kernel changes cause the original alarmtest to pass the tests.

### test0: invoke handler

Get started by modifying the kernel to jump to the alarm handler in user space, which will cause test0 to print "alarm!". Don't worry yet what happens after the "alarm!" output; it's OK for now if your program crashes after printing "alarm!". Here are some hints:

- You'll need to modify the Makefile to cause `alarmtest.c` to be compiled as an xv6 user program.
- The right declarations to put in `user/user.h` are:

          int sigalarm(int ticks, void (*handler)());
          int sigreturn(void);

- Update user/sys.pl (which generates user/usys.S), kernel/syscall.h, and kernel/syscall.c to allow `alarmtest` to invoke the sigalarm and sigreturn system calls.
- For now, your `sys_sigreturn` should just return zero.
- Your `sys_sigalarm()` should store the alarm interval and the pointer to the handler function in new fields in the `proc` structure, defined in `kernel/proc.h`.
- You'll need to keep track of how many ticks have passed since the last call (or are left until the next call) to a process's alarm handler; you'll need a new field in `struct proc` for this too. You can initialize `proc` fields in `allocproc()` in `proc.c`.
- Every tick, the hardware clock forces an interrupt, which is handled in `usertrap()`; you should add some code here.
- You only want to manipulate a process's alarm ticks if there's a a timer interrupt; you want something like

          if(which_dev == 2) ...

- Only invoke the alarm function if the process has a timer outstanding. Note that the address of the user's alarm function might be 0 (e.g., in alarmtest.asm, `periodic` is at address 0).
- It will be easier to look at traps with gdb if you tell qemu to use only one CPU, which you can do by running

          make CPUS=1 qemu

- You've succeeded if alarmtest prints "alarm!".

### test1(): resume interrupted code

Chances are that alarmtest crashes at some point after it prints "alarm!". Depending on how your solution works, that point may be in test0, or it may be in test1. Crashes are likely caused by the alarm handler (`periodic` in alarmtest.c) returning to the wrong point in the user program.

Your job now is to ensure that, when the alarm handler is done, control returns to the instruction at which the user program was originally interrupted by the timer interrupt. You must also ensure that the register contents are restored to values they held at the time of the interrupt, so that the user program can continue undisturbed after the alarm.

Your solution is likely to require you to save and restore registers---what registers do you need to save and restore to resume the interrupted code correctly? (Hint: it will be many). Several approaches are possible; for this lab you should make the `sigreturn` system call restore registers and return to the original interrupted user instruction. The user-space alarm handler calls sigreturn when it is done. Some hints:

- Have `usertrap` save enough state in `struct proc` when the timer goes off that `sigreturn` can correctly return to the interrupted user code.
- Prevent re-entrant calls to the handler----if a handler hasn't returned yet, the kernel shouldn't call it again.

Once you pass `test0` and `test1`, run usertests to make sure you didn't break any other parts of the kernel.

## Uthread: switching between threads

Download uthread.c and uthread_switch.S into your xv6 directory. Make sure `uthread_switch.S` ends with `.S`, not `.s`. Add the following rule to the xv6 Makefile after the _forktest rule:

    $U/_uthread: $U/uthread.o $U/uthread_switch.o
    	$(LD) $(LDFLAGS) -N -e main -Ttext 0 -o $U/_uthread $U/uthread.o $U/uthread_switch.o $(ULIB)
    	$(OBJDUMP) -S $U/_uthread > $U/uthread.asm

Make sure that the blank space at the start of each line is a tab, not spaces.

Add `_uthread` in the Makefile to the list of user programs defined by UPROGS.

Run xv6, then run `uthread` from the xv6 shell. The xv6 kernel will print an error message about `uthread` encountering a page fault.

Your job is to complete `uthread_switch.S`, so that you see output similar to this (make sure to run with CPUS=1):

    ~/classes/6828/xv6$ make CPUS=1 qemu
    ...
    $ uthread
    my thread running
    my thread 0x0000000000002A30
    my thread running
    my thread 0x0000000000004A40
    my thread 0x0000000000002A30
    my thread 0x0000000000004A40
    my thread 0x0000000000002A30
    my thread 0x0000000000004A40
    my thread 0x0000000000002A30
    my thread 0x0000000000004A40
    my thread 0x0000000000002A30
    ...
    my thread 0x0000000000002A88
    my thread 0x0000000000004A98
    my thread: exit
    my thread: exit
    thread_schedule: no runnable threads
    $

`uthread` creates two threads and switches back and forth between them. Each thread prints "my thread ..." and then yields to give the other thread a chance to run.

To observe the above output, you need to complete `uthread_switch.S`, but before jumping into `uthread_switch.S`, first understand how `uthread.c` uses `uthread_switch`. `uthread.c` has two global variables `current_thread` and `next_thread`. Each is a pointer to a `thread` structure. The thread structure has a stack for a thread and a saved stack pointer (`sp`, which points into the thread's stack). The job of `uthread_switch` is to save the current thread state into the structure pointed to by `current_thread`, restore `next_thread`'s state, and make `current_thread` point to where `next_thread` was pointing to, so that when `uthread_switch` returns `next_thread` is running and is the `current_thread`.

You should study `thread_create`, which sets up the initial stack for a new thread. It provides hints about what `uthread_switch` should do. Note that `thread_create` simulates saving all callee-save registers on a new thread's stack.

To write the assembly in `thread_switch`, you need to know how the C compiler lays out `struct thread` in memory, which is as follows:

        --------------------
        | 4 bytes for state|
        --------------------
        | stack size bytes |
        | for stack        |
        --------------------
        | 8 bytes for sp   |
        --------------------  <--- current_thread
             ......

             ......
        --------------------
        | 4 bytes for state|
        --------------------
        | stack size bytes |
        | for stack        |
        --------------------
        | 8 bytes for sp   |
        --------------------  <--- next_thread

The variables `&next_thread` and `¤t_thread` each contain the address of a pointer to `struct thread`, and are passed to `thread_switch`. The following fragment of assembly will be useful:

       ld t0, 0(a0)
       sd sp, 0(t0)

This saves `sp` in `current_thread->sp`. This works because `sp` is at offset 0 in the struct. You can study the assembly the compiler generates for `uthread.c` by looking at `uthread.asm`.

To test your code it might be helpful to single step through your `uthread_switch` using `riscv64-linux-gnu-gdb`. You can get started in this way:

    (gdb) file user/_uthread
    Reading symbols from user/_uthread...
    (gdb) b *0x230

0x230 is the address of uthread_switch (see uthread.asm). When you compile it may be at a different address, so check uthread_asm. You may also be able to type "b uthread_switch". XXX This doesn't work for me; why?

The breakpoint may (or may not) be triggered before you even run `uthread`. How could that happen?

Once your xv6 shell runs, type "uthread", and gdb will break at `thread_switch`. Now you can type commands like the following to inspect the state of `uthread`:

      (gdb) p/x *next_thread
      $1 = {sp = 0x4a28, stack = {0x0 (repeats 8088 times),
          0x68, 0x1, 0x0 <repeats 102 times>}, state = 0x1}

What address is `0x168`, which sits on the bottom of the stack of `next_thread`? With "x", you can examine the content of a memory location

      (gdb) x/x next_thread->sp
      0x4a28 <all_thread+16304>:      0x00000168

Why does that print `0x168`?

### Optional challenges

The user-level thread package interacts badly with the operating system in several ways. For example, if one user-level thread blocks in a system call, another user-level thread won't run, because the user-level threads scheduler doesn't know that one of its threads has been descheduled by the xv6 scheduler. As another example, two user-level threads will not run concurrently on different cores, because the xv6 scheduler isn't aware that there are multiple threads that could run in parallel. Note that if two user-level threads were to run truly in parallel, this implementation won't work because of several races (e.g., two threads on different processors could call `thread_schedule` concurrently, select the same runnable thread, and both run it on different processors.)

There are several ways of addressing these problems. One is using scheduler activations and another is to use one kernel thread per user-level thread (as Linux kernels do). Implement one of these ways in xv6. This is not easy to get right; for example, you will need to implement TLB shootdown when updating a page table for a multithreaded user process.

Add locks, condition variables, barriers, etc. to your thread package.