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<html> <html>
<head> <head>
<title>Lab: system calls</title> <title>Lab: Alarm and uthread</title>
<link rel="stylesheet" href="homework.css" type="text/css" /> <link rel="stylesheet" href="homework.css" type="text/css" />
</head> </head>
<body> <body>
<h1>Lab: system calls</h1> <h1>Lab: Alarm and uthread</h1>
This lab makes you familiar with the implementation of system calls. This lab makes you familiar with the implementation of system calls
In particular, you will implement a new system and switching between threads of execution. In particular, you will
calls: <tt>sigalarm</tt> and <tt>sigreturn</tt>. implement new system calls (<tt>sigalarm</tt> and <tt>sigreturn</tt>)
and switching between threads of a user-level thread package.
<b>Note: before this lab, it would be good to have recitation section on gdb and understanding assembly</b>
<h2>Warmup: system call tracing</h2>
<p>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.
<p>
When you're done, you should see output like this when booting
xv6:
<pre>
...
fork -> 2
exec -> 0
open -> 3
close -> 0
$write -> 1
write -> 1
</pre>
<p>
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.)
<p> Hint: modify the syscall() function in kernel/syscall.c.
<p>Run the programs you wrote in the lab and inspect the system call
trace. Are there many system calls? Which systems calls correspond
to code in the applications you wrote above?
<p>Optional: print the system call arguments.
<h2>RISC-V assembly</h2> <h2>RISC-V assembly</h2>
<p>For the alarm system call it will be important to understand RISC-V <p>For this lab it will be important to understand RISC-V assembly.
assembly. Since in later labs you will also read and write assembly,
it is important that you familiarize yourself with RISC_V assembly.
<p>Add a file user/call.c with the following content, modify the <p>Add a file user/call.c with the following content, modify the
Makefile to add the program to the user programs, and compile (make Makefile to add the program to the user programs, and compile (make
@ -96,8 +58,43 @@ void main(void) {
to <tt>printf</tt> in <tt>main</tt>? to <tt>printf</tt> in <tt>main</tt>?
</ul> </ul>
<h2>Warmup: system call tracing</h2>
<h2>alarm</h2> <p>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.
<p>
When you're done, you should see output like this when booting
xv6:
<pre>
...
fork -> 2
exec -> 0
open -> 3
close -> 0
$write -> 1
write -> 1
</pre>
<p>
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.)
<p> Hint: modify the syscall() function in kernel/syscall.c.
<p>Run the programs you wrote in the lab and inspect the system call
trace. Are there many system calls? Which systems calls correspond
to code in the applications you wrote above?
<p>Optional: print the system call arguments.
<h2>Alarm</h2>
<p> <p>
In this exercise you'll add a feature to xv6 that periodically alerts In this exercise you'll add a feature to xv6 that periodically alerts
@ -227,7 +224,7 @@ alarmtest starting
code for the alarmtest program in alarmtest.asm, which will be handy code for the alarmtest program in alarmtest.asm, which will be handy
for debugging. for debugging.
<h2>Test0: invoke handler</h2> <h3>Test0: invoke handler</h3>
<p>To get started, the best strategy is to first pass test0, which <p>To get started, the best strategy is to first pass test0, which
will force you to handle the main challenge above. Here are some will force you to handle the main challenge above. Here are some
@ -279,7 +276,7 @@ use only one CPU, which you can do by running
</ul> </ul>
<h2>test1(): resume interrupted code</h2> <h3>test1(): resume interrupted code</h3>
<p>Test0 doesn't tests whether the handler returns correctly to <p>Test0 doesn't tests whether the handler returns correctly to
interrupted instruction in test0. If you didn't get this right, it interrupted instruction in test0. If you didn't get this right, it
@ -311,16 +308,182 @@ use only one CPU, which you can do by running
<li>Prevent re-entrant calls to the handler----if a handler hasn't <li>Prevent re-entrant calls to the handler----if a handler hasn't
returned yet, don't call it again. returned yet, don't call it again.
<ul> </ul>
<p>Once you pass <tt>test0</tt> and <tt>test1</tt>, run usertests to <p>Once you pass <tt>test0</tt> and <tt>test1</tt>, run usertests to
make sure you didn't break any other parts of the kernel. make sure you didn't break any other parts of the kernel.
<h2>Uthread: switching between threads</h2>
<p>Download <a href="uthread.c">uthread.c</a> and <a
href="uthread_switch.S">uthread_switch.S</a> into your xv6 directory.
Make sure <tt>uthread_switch.S</tt> ends with <tt>.S</tt>, not
<tt>.s</tt>. Add the
following rule to the xv6 Makefile after the _forktest rule:
<pre>
$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 > $U/uthread.asm
</pre>
Make sure that the blank space at the start of each line is a tab,
not spaces.
<p>
Add <tt>_uthread</tt> in the Makefile to the list of user programs defined by UPROGS.
<p>Run xv6, then run <tt>uthread</tt> from the xv6 shell. The xv6 kernel will print an error message about <tt>uthread</tt> encountering a page fault.
<p>Your job is to complete <tt>uthread_switch.S</tt>, so that you see output similar to
this (make sure to run with CPUS=1):
<pre>
~/classes/6828/xv6$ 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
$
</pre>
<p><tt>uthread</tt> 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.
<p>To observe the above output, you need to complete <tt>uthread_switch.S</tt>, but before
jumping into <tt>uthread_switch.S</tt>, first understand how <tt>uthread.c</tt>
uses <tt>uthread_switch</tt>. <tt>uthread.c</tt> has two global variables
<tt>current_thread</tt> and <tt>next_thread</tt>. Each is a pointer to a
<tt>thread</tt> structure. The thread structure has a stack for a thread and a
saved stack pointer (<tt>sp</tt>, which points into the thread's stack). The
job of <tt>uthread_switch</tt> is to save the current thread state into the
structure pointed to by <tt>current_thread</tt>, restore <tt>next_thread</tt>'s
state, and make <tt>current_thread</tt> point to where <tt>next_thread</tt> was
pointing to, so that when <tt>uthread_switch</tt> returns <tt>next_thread</tt>
is running and is the <tt>current_thread</tt>.
<p>You should study <tt>thread_create</tt>, which sets up the initial stack for
a new thread. It provides hints about what <tt>uthread_switch</tt> should do.
Note that <tt>thread_create</tt> simulates saving all callee-save registers
on a new thread's stack.
<p>To write the assembly in <tt>thread_switch</tt>, you need to know how the C
compiler lays out <tt>struct thread</tt> in memory, which is as
follows:
<pre>
--------------------
| 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
</pre>
The variables <tt>&next_thread</tt> and <tt>&current_thread</tt> each
contain the address of a pointer to <tt>struct thread</tt>, and are
passed to <tt>thread_switch</tt>. The following fragment of assembly
will be useful:
<pre>
ld t0, 0(a0)
sd sp, 0(t0)
</pre>
This saves <tt>sp</tt> in <tt>current_thread->sp</tt>. This works because
<tt>sp</tt> is at
offset 0 in the struct.
You can study the assembly the compiler generates for
<tt>uthread.c</tt> by looking at <tt>uthread.asm</tt>.
<p>To test your code it might be helpful to single step through your
<tt>uthread_switch</tt> using <tt>riscv64-linux-gnu-gdb</tt>. You can get started in this way:
<pre>
(gdb) file user/_uthread
Reading symbols from user/_uthread...
(gdb) b *0x230
</pre>
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". <b>XXX This doesn't work
for me; why?</b>
<p>The breakpoint may (or may not) be triggered before you even run
<tt>uthread</tt>. How could that happen?
<p>Once your xv6 shell runs, type "uthread", and gdb will break at
<tt>thread_switch</tt>. Now you can type commands like the following to inspect
the state of <tt>uthread</tt>:
<pre>
(gdb) p/x *next_thread
$1 = {sp = 0x4a28, stack = {0x0 (repeats 8088 times),
0x68, 0x1, 0x0 <repeats 102 times>}, state = 0x1}
</pre>
What address is <tt>0x168</tt>, which sits on the bottom of the stack
of <tt>next_thread</tt>?
With "x", you can examine the content of a memory location
<pre>
(gdb) x/x next_thread->sp
0x4a28 <all_thread+16304>: 0x00000168
</pre>
Why does that print <tt>0x168</tt>?
<h3>Optional challenges</h3>
<p>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 <tt>thread_schedule</tt>
concurrently, select the same runnable thread, and both run it on different
processors.)
<p>There are several ways of addressing these problems. One is
using <a href="http://en.wikipedia.org/wiki/Scheduler_activations">scheduler
activations</a> 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.
<p>Add locks, condition variables, barriers,
etc. to your thread package.
</body> </body>
</html> </html>