syscall lab/alarm
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Frans Kaashoek
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labs/syscall.html
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labs/syscall.html
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<html>
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<head>
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<title>Lab: system calls</title>
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<link rel="stylesheet" href="homework.css" type="text/css" />
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</head>
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<body>
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<h1>Lab: system calls</h1>
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This lab makes you familiar with the implementation of system calls.
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In particular, you will implement a new system call: <tt>alarm</tt>.
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<h2>Warmup: system call tracing</h2>
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<p>In this exercise you will modify the xv6 kernel to print out a line
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for each system call invocation. It is enough to print the name of the
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system call and the return value; you don't need to print the system
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call arguments.
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<p>
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When you're done, you should see output like this when booting
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xv6:
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<pre>
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...
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fork -> 2
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exec -> 0
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open -> 3
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close -> 0
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$write -> 1
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write -> 1
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</pre>
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<p>
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That's init forking and execing sh, sh making sure only two file descriptors are
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open, and sh writing the $ prompt. (Note: the output of the shell and the
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system call trace are intermixed, because the shell uses the write syscall to
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print its output.)
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<p> Hint: modify the syscall() function in kernel/syscall.c.
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<p>Run the programs you wrote in the lab and inspect the system call
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trace. Are there many system calls? Which systems calls correspond
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to code in the applications you wrote above?
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<p>Optional: print the system call arguments.
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<h2>alarm</h2>
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<p>
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In this exercise you'll add a feature to xv6 that periodically alerts
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a process as it uses CPU time. This might be useful for compute-bound
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processes that want to limit how much CPU time they chew up, or for
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processes that want to compute but also want to take some periodic
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action. More generally, you'll be implementing a primitive form of
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user-level interrupt/fault handlers; you could use something similar
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to handle page faults in the application, for example.
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<p>
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You should add a new <tt>alarm(interval, handler)</tt> system call.
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If an application calls <tt>alarm(n, fn)</tt>, then after every
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<tt>n</tt> "ticks" of CPU time that the program consumes, the kernel
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will cause application function
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<tt>fn</tt> to be called. When <tt>fn</tt> returns, the application
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will resume where it left off. A tick is a fairly arbitrary unit of
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time in xv6, determined by how often a hardware timer generates
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interrupts.
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<p>
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You should put the following example program in <tt>user/alarmtest.c</tt>:
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<pre>
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#include "kernel/param.h"
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#include "kernel/types.h"
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#include "kernel/stat.h"
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#include "user/user.h"
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void periodic();
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int
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main(int argc, char *argv[])
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{
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int i;
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printf(1, "alarmtest starting\n");
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alarm(10, periodic);
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for(i = 0; i < 25*500000; i++){
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if((i % 250000) == 0)
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write(2, ".", 1);
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}
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exit();
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}
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void
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periodic()
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{
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printf(1, "alarm!\n");
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}
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</pre>
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The program calls <tt>alarm(10, periodic)</tt> to ask the kernel to
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force a call to <tt>periodic()</tt> every 10 ticks, and then spins for
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a while.
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After you have implemented the <tt>alarm()</tt> system call in the kernel,
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<tt>alarmtest</tt> should produce output like this:
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<pre>
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$ alarmtest
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alarmtest starting
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.....alarm!
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....alarm!
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.....alarm!
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......alarm!
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.....alarm!
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....alarm!
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....alarm!
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......alarm!
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.....alarm!
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...alarm!
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...$
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</pre>
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<p>
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<p>
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(If you only see one "alarm!", try increasing the number of iterations in
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<tt>alarmtest.c</tt> by 10x.)
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Here are some hints:
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<ul>
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<li>You'll need to modify the Makefile to cause <tt>alarmtest.c</tt>
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to be compiled as an xv6 user program.
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<li>The right declaration to put in <tt>user/user.h</tt> is:
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<pre>
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int alarm(int ticks, void (*handler)());
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</pre>
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<li>Update <tt>kernel/syscall.h</tt> and <tt>user/usys.S</tt> to
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allow <tt>alarmtest</tt> to invoke the alarm system call.
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<li>Your <tt>sys_alarm()</tt> should store the alarm interval and the
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pointer to the handler function in new fields in the <tt>proc</tt>
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structure; see <tt>kernel/proc.h</tt>.
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<li>
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You'll need to keep track of how many ticks have passed since
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the last call
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(or are left until the next call) to a process's alarm handler;
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you'll need a new field in <tt>struct proc</tt> for this too.
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You can initialize <tt>proc</tt> fields in <tt>allocproc()</tt>
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in <tt>proc.c</tt>.
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<li>
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Every tick, the hardware clock forces an interrupt, which
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is handled in <tt>usertrap()</tt>; you should add some code here.
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<li>
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You only want to manipulate a process's alarm ticks if there's a
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a timer interrupt; you want something like
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<pre>
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if(which_dev == 2) ..
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</pre>
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<p>
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In your usertrap, when a process's alarm interval expires,
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you'll want to cause it to execute its handler. How can you do that?
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<li>
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You need to arrange things so that, when the handler returns,
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the process resumes executing where it left off. How can you do that?
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<li>
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You can see the assembly code for the alarmtest program in alarmtest.asm.
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<li>
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It will be easier to look at traps with gdb if you tell qemu to use
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only one CPU, which you can do by running
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<pre>
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make CPUS=1 qemu
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</pre>
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<li>
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It's OK if your solution doesn't save the caller-saved user registers
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when calling the handler.
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<ul>
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<p>
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Optional challenges: 1) Save and restore the caller-saved user registers around the
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call to handler. 2) Prevent re-entrant calls to the handler -- if a handler
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hasn't returned yet, don't call it again. 3) Assuming your code doesn't
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check that <tt>tf->esp</tt> is valid, implement a security attack on the
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kernel that exploits your alarm handler calling code.
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