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* Add xv6 debug
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<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Xv6 introduction « Stop Talking, Start Doing - 停止空想，开始行动</title>
<meta name="description" content="I hate xv6, a stupid, useless education-oriented system. In this article, I will generally talk about how to implement system call to this operating system.">
<title>Xv6 introduction « Stop Talking, Start Doing</title>
<meta name="description" content="In this post, you will learn a few basic concepts of xv6. Learning path will be closed coupled to first project assignment I gave when I assisted in teaching...">
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@@ -38,13 +38,13 @@
<header class="header">
<div class="wrapper">
<a class="site-title" href="/">Stop Talking, Start Doing - 停止空想，开始行动</a>
<a class="site-title" href="/">Stop Talking, Start Doing</a>
<nav class="site-nav">
<a class="page-link" href="/about/">About Me</a>
<a class="page-link" href="/about/">About</a>
@@ -74,9 +74,12 @@
</header>
<article class="post-content">
<p>I hate xv6, a stupid, useless education-oriented system. In this article, I will generally talk about how to implement system call to this operating system.</p>
<p>In this post, you will learn a few basic concepts of xv6. Learning path will be closed coupled to first project assignment I gave when I assisted in teaching OS classes.
Understand system call and know how to implement a simple one will be coved as the first half.
In the second half of this post, I will discuss a little bit more on how to debug xv6 using gdb.</p>
<h2 id="xv6-systemcall">Xv6 Systemcall</h2>
<p>To invoke a system call, we have to first define a user mode function to be the interface of the kernel instruction in file <em>user.h</em>.</p>
<div class="language-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kt">void</span> <span class="nf">function</span> <span class="p">(</span><span class="kt">void</span><span class="p">);</span>
@@ -84,15 +87,69 @@
<p>This interface-like function will then pass the function name, in this case function, to <em>usys.S</em>. When using user mode function in programs, <em>usys.S</em> will generate a reference to SYS_function and push system call number of this function into %eax. After that, system can know from <em>syscall.c</em> and determining whether this system call is available. We must define same name system function and add it into <em>syscall.h</em> and <em>syscall.c</em>.</p>
<div class="language-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp">#define SYS_function ## // ## is the system call number
</span><span class="p">[</span><span class="n">SYS_function</span><span class="p">]</span> <span class="n">sys_function</span> <span class="c1">// real system function name</span>
<span class="k">extern</span> <span class="kt">int</span> <span class="nf">sys_function</span><span class="p">(</span><span class="kt">void</span><span class="p">);</span> <span class="c1">// real system function declaration</span>
<div class="language-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp">#define SYS_function ## // ## is the system call number
</span><span class="p">[</span><span class="n">SYS_function</span><span class="p">]</span> <span class="n">sys_function</span> <span class="c1">// real system function name</span>
<span class="k">extern</span> <span class="kt">int</span> <span class="nf">sys_function</span><span class="p">(</span><span class="kt">void</span><span class="p">);</span> <span class="c1">// real system function declaration</span>
</code></pre></div></div>
<p>After adding these sentences to syscall files, we can implement real function in specific place where you want to make the function works well.</p>
<p>Sometimes, we need to pass variables among system calls. In this case, variables’ values are not necessary and even can’t be pass directly into system_function. When invoke a system call function, all variables of this system call will be pushed into current process’ stack. In file <em>syscall.c</em>, multiple functions are provided to get these variables from the process. I won’t waste time on explaining how to use these functions especially when elegant and detailed comments were written in source codes. However, I will explain concepts and how process organized and works in xv6 in future articles.</p>
<h2 id="debug-xv6-with-gdb">Debug xv6 with gdb</h2>
<p>Please make sure that you have used gdb before.
If you never used gdb, you may write a simple 50-100 lines c code and practice how to use gdb first.</p>
<ul>
<li><a href="https://sourceware.org/gdb/current/onlinedocs/gdb/">GDB Manual</a></li>
<li><a href="https://darkdust.net/files/GDB%20Cheat%20Sheet.pdf">GDB cheatsheet (pdf)</a></li>
</ul>
<p>To make sure xv6 gdb enabled, please check if <em>.gdbinit.tmpl</em> file exist.
This file is used for generate <em>.gdbinit</em> file which you can late consider it as a configuration for gdb.</p>
<p>Before running the xv6 instance in QEMU, one more thing you need to know is that using gdb to debug xv6 must be attached remotely.
This is because xv6 was running within QEMU, and emulator is virtually gapped from the host device.
Later when you start debugging, QEMU will open a gdb server to let gdb client connect to.</p>
<p>Once you want to start, using following command to compile and run xv6</p>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nv">$ </span>make qemu-nox-gdb
<span class="k">***</span> Now run <span class="s1">'gdb'</span><span class="nb">.</span>
qemu-system-i386 <span class="nt">-nographic</span> <span class="nt">-drive</span> <span class="nv">file</span><span class="o">=</span>fs.img,index<span class="o">=</span>1,media<span class="o">=</span>disk,format<span class="o">=</span>raw <span class="nt">-drive</span> <span class="nv">file</span><span class="o">=</span>xv6.img,index<span class="o">=</span>0,media<span class="o">=</span>disk,format<span class="o">=</span>raw <span class="nt">-smp</span> 2 7
</code></pre></div></div>
<p>At this moment, it feels xv6 was stuck, this is because QEMU is ready to be connected by the gdb client.
You may use the <em>.gdbinit</em> to automatically finish this remote connection by simple typein following command in another terminal.</p>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nv">$ </span>gdb <span class="nt">-x</span> .gdbinit
GNU gdb <span class="o">(</span>Debian 8.2.1-2+b3<span class="o">)</span> 8.2.1
...
The target architecture is assumed to be i8086
<span class="o">[</span>f000:fff0] 0xffff0: ljmp <span class="nv">$0x3630</span>,<span class="nv">$0xf000e05b</span>
0x0000fff0 <span class="k">in</span> ?? <span class="o">()</span>
+ symbol-file kernel
warning: A handler <span class="k">for </span>the OS ABI <span class="s2">"GNU/Linux"</span> is not built into this configuration
of GDB. Attempting to <span class="k">continue </span>with the default i8086 settings.
<span class="o">(</span>gdb<span class="o">)</span>
</code></pre></div></div>
<p>Now within this gdb client shell, type ‘c’ to continue the xv6, and you will see xv6 start execution in the first terminal.</p>
<p>At this moment, you may add breakpoints to your code to see if your code is correctly implemented or not.</p>
<p><strong>One more thing</strong>, if you open <em>.gdbinit</em> file, you’ll find that it by default connect to a localhost target.
If you are working on some other environment that target and client were not placed in the same device, change the localhost to ip address correspondingly.
Using ssh may connect to different physical devices under same domain name, this is because load balancer were used. To check ip address, search command <em>ip</em>.</p>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code>target remote localhost:28467
<span class="c"># target remote [ip-addr]:28467</span>
</code></pre></div></div>
</article>
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</div>
<div class="col-box post-toc hide">
<div class="col-box-title">TOC</div>
<div class="col-box-title">Indexes</div>
</div>
</div>
</div>