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<h1 class="post-title">Xv6 introduction</h1>
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<p class="post-meta">Jul 28, 2017</p>
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<article class="post-content">
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<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.
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Understand system call and know how to implement a simple one will be coved as the first half.
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In the second half of this post, I will discuss a little bit more on how to debug xv6 using gdb.</p>
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<h2 id="xv6-systemcall">Xv6 Systemcall</h2>
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<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>
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<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>
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</code></pre></div></div>
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<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>
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<div class="language-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="cp">#define SYS_function ## // ## is the system call number
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</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>
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<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>
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</code></pre></div></div>
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<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>
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<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>
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<h2 id="debug-xv6-with-gdb">Debug xv6 with gdb</h2>
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<p>Please make sure that you have used gdb before.
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If you never used gdb, you may write a simple 50-100 lines c code and practice how to use gdb first.</p>
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<ul>
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<li><a href="https://sourceware.org/gdb/current/onlinedocs/gdb/">GDB Manual</a></li>
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<li><a href="https://darkdust.net/files/GDB%20Cheat%20Sheet.pdf">GDB cheatsheet (pdf)</a></li>
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</ul>
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<p>To make sure xv6 gdb enabled, please check if <em>.gdbinit.tmpl</em> file exist.
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This file is used for generate <em>.gdbinit</em> file which you can late consider it as a configuration for gdb.</p>
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<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.
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This is because xv6 was running within QEMU, and emulator is virtually gapped from the host device.
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Later when you start debugging, QEMU will open a gdb server to let gdb client connect to.</p>
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<p>Once you want to start, using following command to compile and run xv6</p>
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<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nv">$ </span>make qemu-nox-gdb
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<span class="k">***</span> Now run <span class="s1">'gdb'</span><span class="nb">.</span>
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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
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</code></pre></div></div>
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<p>At this moment, it feels xv6 was stuck, this is because QEMU is ready to be connected by the gdb client.
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You may use the <em>.gdbinit</em> to automatically finish this remote connection by simple typein following command in another terminal.</p>
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<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nv">$ </span>gdb <span class="nt">-x</span> .gdbinit
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GNU gdb <span class="o">(</span>Debian 8.2.1-2+b3<span class="o">)</span> 8.2.1
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...
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The target architecture is assumed to be i8086
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<span class="o">[</span>f000:fff0] 0xffff0: ljmp <span class="nv">$0x3630</span>,<span class="nv">$0xf000e05b</span>
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0x0000fff0 <span class="k">in</span> ?? <span class="o">()</span>
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+ symbol-file kernel
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warning: A handler <span class="k">for </span>the OS ABI <span class="s2">"GNU/Linux"</span> is not built into this configuration
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of GDB. Attempting to <span class="k">continue </span>with the default i8086 settings.
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<span class="o">(</span>gdb<span class="o">)</span>
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</code></pre></div></div>
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<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>
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<p>At this moment, you may add breakpoints to your code to see if your code is correctly implemented or not.</p>
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<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.
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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.
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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>
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<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code>target remote localhost:28467
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<span class="c"># target remote [ip-addr]:28467</span>
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</code></pre></div></div>
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</article>
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<div class="col-second">
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<div class="col-box col-box-author">
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<img class="avatar" src="/static/avatar.jpg" alt="Pengzhan Hao">
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<div class="col-box-title name">Pengzhan Hao</div>
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<a href="https://github.com/codersherlock">GitHub</a>
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<a href="mailto:haopengzhan@gmail.com">Email</a>
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<div class="col-box">
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<div class="col-box-title">Newest Posts</div>
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<ul class="post-list">
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<li><a class="post-link" href="/archivers/generate-word-cloud-with-chinese-fenci">Generate Word Cloud Figures with Chinese-Tokenization and WordCloud python libraries</a></li>
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<li><a class="post-link" href="/archivers/intro-xv6">Xv6 introduction</a></li>
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<li><a class="post-link" href="/archivers/some-of-my-previews-exper-work">Some of my previews experiment works: 2016</a></li>
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<li><a class="post-link" href="/archivers/charles-is-not-a-good-tool">Using charles proxy to monitor mobile SSL traffics</a></li>
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<li><a class="post-link" href="/archivers/hello">Stop Talking is the worst title of one blog</a></li>
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<div class="col-box-title">Indexes</div>
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