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<article itemscope itemtype="http://schema.org/Article"><div class="article__header"><header><h1>Xv6 introduction</h1></header></div><meta itemprop="headline" content="Xv6 introduction"><div class="article__info clearfix"><ul class="left-col menu"><li>
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<a class="button button--secondary button--pill button--sm"
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href="/archive.html?tag=Xv6">Xv6</a>
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</li><li>
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<a class="button button--secondary button--pill button--sm"
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href="/archive.html?tag=Teaching">Teaching</a>
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<a class="button button--secondary button--pill button--sm"
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href="/archive.html?tag=Operating+system">Operating system</a>
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</li></ul><ul class="right-col menu"><li><i class="fas fa-user"></i> <span>Pengzhan Hao</span></li><li><i class="far fa-calendar-alt"></i> <span>Jul 28, 2017</span>
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</li></ul></div><meta itemprop="author" content="Pengzhan Hao"/><meta itemprop="datePublished" content="2017-07-28T14:56:55-04:00">
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<meta itemprop="keywords" content="Xv6,Teaching,Operating system"><div class="js-article-content"><div class="layout--article"><!-- start custom article top snippet -->
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<div class="article__content" itemprop="articleBody"><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.<br />
|
||
<!--more--></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>
|
||
</code></pre></div></div>
|
||
|
||
<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>
|
||
</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>
|
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</svg>
|
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|
||
</a><li title="Follow me on Linkedin.">
|
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|
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<path d="M260.096 155.648c0 27.307008-9.899008 50.516992-29.696 69.632-19.796992 19.115008-45.396992 28.672-76.8 28.672-30.036992 0-54.612992-9.556992-73.728-28.672-19.115008-19.115008-28.672-42.324992-28.672-69.632 0-28.672 9.556992-52.224 28.672-70.656 19.115008-18.432 44.372992-27.648 75.776-27.648 31.403008 0 56.32 9.216 74.752 27.648 18.432 18.432 28.331008 41.984 29.696 70.656 0 0 0 0 0 0m-202.752 808.96c0 0 0-632.832 0-632.832 0 0 196.608 0 196.608 0 0 0 0 632.832 0 632.832 0 0-196.608 0-196.608 0 0 0 0 0 0 0m313.344-430.08c0-58.708992-1.364992-126.292992-4.096-202.752 0 0 169.984 0 169.984 0 0 0 10.24 88.064 10.24 88.064 0 0 4.096 0 4.096 0 40.96-68.267008 105.812992-102.4 194.56-102.4 68.267008 0 123.220992 22.868992 164.864 68.608 41.643008 45.739008 62.464 113.664 62.464 203.776 0 0 0 374.784 0 374.784 0 0-196.608 0-196.608 0 0 0 0-350.208 0-350.208 0-91.476992-33.451008-137.216-100.352-137.216-47.787008 0-81.236992 24.576-100.352 73.728-4.096 8.192-6.144 24.576-6.144 49.152 0 0 0 364.544 0 364.544 0 0-198.656 0-198.656 0 0 0 0-430.08 0-430.08 0 0 0 0 0 0" />
|
||
</svg>
|
||
</div>
|
||
</a>
|
||
</li><li title="Follow me on Github.">
|
||
<a class="button button--circle github-button" itemprop="sameAs" href="https://github.com/codersherlock" target="_blank">
|
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<div class="icon"><svg fill="#000000" width="24px" height="24px" viewBox="0 0 1024 1024" version="1.1" xmlns="http://www.w3.org/2000/svg">
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<path class="svgpath" data-index="path_0" fill="#272636" d="M0 525.2c0 223.6 143.3 413.7 343 483.5 26.9 6.8 22.8-12.4 22.8-25.4l0-88.7c-155.3 18.2-161.5-84.6-172-101.7-21.1-36-70.8-45.2-56-62.3 35.4-18.2 71.4 4.6 113.1 66.3 30.2 44.7 89.1 37.2 119 29.7 6.5-26.9 20.5-50.9 39.7-69.6C248.8 728.2 181.7 630 181.7 513.2c0-56.6 18.7-108.7 55.3-150.7-23.3-69.3 2.2-128.5 5.6-137.3 66.5-6 135.5 47.6 140.9 51.8 37.8-10.2 80.9-15.6 129.1-15.6 48.5 0 91.8 5.6 129.8 15.9 12.9-9.8 77-55.8 138.8-50.2 3.3 8.8 28.2 66.7 6.3 135 37.1 42.1 56 94.6 56 151.4 0 117-67.5 215.3-228.8 243.7 26.9 26.6 43.6 63.4 43.6 104.2l0 128.8c0.9 10.3 0 20.5 17.2 20.5C878.1 942.4 1024 750.9 1024 525.3c0-282.9-229.3-512-512-512C229.1 13.2 0 242.3 0 525.2L0 525.2z" />
|
||
</svg>
|
||
</div>
|
||
</a>
|
||
</li></ul>
|
||
</div>
|
||
</div>
|
||
</div><div class="site-info mt-2">
|
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<div>© STSD 2021,
|
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Powered by <a title="Jekyll is a simple, blog-aware, static site generator." href="http://jekyllrb.com/">Jekyll</a> & <a
|
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title="TeXt is a super customizable Jekyll theme." href="https://github.com/kitian616/jekyll-TeXt-theme">TeXt Theme</a>.
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$searchToggle.on('click', function() {
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<div class="search__header">Search</div>
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<div class="search-box js-search-box">
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<div class="search-box__icon-search"><i class="fas fa-search"></i></div>
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<a><i class="fas fa-times"></i></a>
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</div>
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Cancel</button>
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<div class="search-result js-search-result"></div>
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</div></div>
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||
|
||
function setOptions(options) {
|
||
var _options = options || {};
|
||
_options.selectors && (selectors = _options.selectors);
|
||
_options.container && (container = _options.container);
|
||
_options.scrollTarget && (scrollTarget = _options.scrollTarget);
|
||
_options.scroller && (scroller = _options.scroller);
|
||
_options.disabled !== undefined && (disabled = _options.disabled);
|
||
$headings = $(container).find(selectors).filter('[id]');
|
||
$scrollTarget = $(scrollTarget);
|
||
$scroller = $(scroller);
|
||
}
|
||
function calc() {
|
||
headingsPos = [];
|
||
$headings.each(function() {
|
||
headingsPos.push(Math.floor($(this).position().top));
|
||
});
|
||
}
|
||
function setState(element, disabled) {
|
||
var scrollTop = $scrollTarget.scrollTop(), i;
|
||
if (disabled || !headingsPos || headingsPos.length < 1) { return; }
|
||
if (element) {
|
||
$activeCur = element;
|
||
} else {
|
||
for (i = 0; i < headingsPos.length; i++) {
|
||
if (scrollTop >= headingsPos[i]) {
|
||
$activeCur = $tocLi.eq(i);
|
||
} else {
|
||
$activeCur || ($activeCur = $tocLi.eq(i));
|
||
break;
|
||
}
|
||
}
|
||
}
|
||
$activeLast && $activeLast.removeClass('active');
|
||
($activeLast = $activeCur).addClass('active');
|
||
}
|
||
function render() {
|
||
if(!hasRendered) {
|
||
$root.append($tocUl);
|
||
$headings.each(function() {
|
||
var $this = $(this);
|
||
$tocUl.append($('<li></li>').addClass('toc-' + $this.prop('tagName').toLowerCase())
|
||
.append($('<a></a>').text($this.text()).attr('href', '#' + $this.prop('id'))));
|
||
});
|
||
$tocLi = $tocUl.children('li');
|
||
$tocUl.on('click', 'a', function(e) {
|
||
e.preventDefault();
|
||
var $this = $(this);
|
||
scrolling = true;
|
||
setState($this.parent());
|
||
$scroller.scrollToAnchor($this.attr('href'), 400, function() {
|
||
scrolling = false;
|
||
});
|
||
});
|
||
}
|
||
hasRendered = true;
|
||
}
|
||
function init() {
|
||
var interval, timeout;
|
||
if(!hasInit) {
|
||
render(); calc(); setState(null, scrolling);
|
||
// run calc every 100 millisecond
|
||
interval = setInterval(function() {
|
||
calc();
|
||
}, 100);
|
||
timeout = setTimeout(function() {
|
||
clearInterval(interval);
|
||
}, 45000);
|
||
window.pageLoad.then(function() {
|
||
setTimeout(function() {
|
||
clearInterval(interval);
|
||
clearTimeout(timeout);
|
||
}, 3000);
|
||
});
|
||
$scrollTarget.on('scroll', function() {
|
||
disabled || setState(null, scrolling);
|
||
});
|
||
$window.on('resize', window.throttle(function() {
|
||
if (!disabled) {
|
||
render(); calc(); setState(null, scrolling);
|
||
}
|
||
}, 100));
|
||
}
|
||
hasInit = true;
|
||
}
|
||
|
||
setOptions(options);
|
||
if (!disabled) {
|
||
init();
|
||
}
|
||
$window.on('resize', window.throttle(function() {
|
||
init();
|
||
}, 200));
|
||
return {
|
||
setOptions: setOptions
|
||
};
|
||
}
|
||
$.fn.toc = toc;
|
||
});
|
||
})();
|
||
/*(function () {
|
||
|
||
})();*/
|
||
</script><script>
|
||
/* toc must before affix, since affix need to konw toc' height. */(function() {
|
||
var SOURCES = window.TEXT_VARIABLES.sources;
|
||
var TOC_SELECTOR = window.TEXT_VARIABLES.site.toc.selectors;
|
||
window.Lazyload.js(SOURCES.jquery, function() {
|
||
var $window = $(window);
|
||
var $articleContent = $('.js-article-content');
|
||
var $tocRoot = $('.js-toc-root'), $col2 = $('.js-col-aside');
|
||
var toc;
|
||
var tocDisabled = false;
|
||
var hasSidebar = $('.js-page-root').hasClass('layout--page--sidebar');
|
||
var hasToc = $articleContent.find(TOC_SELECTOR).length > 0;
|
||
|
||
function disabled() {
|
||
return $col2.css('display') === 'none' || !hasToc;
|
||
}
|
||
|
||
tocDisabled = disabled();
|
||
|
||
toc = $tocRoot.toc({
|
||
selectors: TOC_SELECTOR,
|
||
container: $articleContent,
|
||
scrollTarget: hasSidebar ? '.js-page-main' : null,
|
||
scroller: hasSidebar ? '.js-page-main' : null,
|
||
disabled: tocDisabled
|
||
});
|
||
|
||
$window.on('resize', window.throttle(function() {
|
||
tocDisabled = disabled();
|
||
toc && toc.setOptions({
|
||
disabled: tocDisabled
|
||
});
|
||
}, 100));
|
||
|
||
});
|
||
})();
|
||
(function() {
|
||
var SOURCES = window.TEXT_VARIABLES.sources;
|
||
window.Lazyload.js(SOURCES.jquery, function() {
|
||
var $window = $(window), $pageFooter = $('.js-page-footer');
|
||
var $pageAside = $('.js-page-aside');
|
||
var affix;
|
||
var tocDisabled = false;
|
||
var hasSidebar = $('.js-page-root').hasClass('layout--page--sidebar');
|
||
|
||
affix = $pageAside.affix({
|
||
offsetBottom: $pageFooter.outerHeight(),
|
||
scrollTarget: hasSidebar ? '.js-page-main' : null,
|
||
scroller: hasSidebar ? '.js-page-main' : null,
|
||
scroll: hasSidebar ? $('.js-page-main').children() : null,
|
||
disabled: tocDisabled
|
||
});
|
||
|
||
$window.on('resize', window.throttle(function() {
|
||
affix && affix.setOptions({
|
||
disabled: tocDisabled
|
||
});
|
||
}, 100));
|
||
|
||
window.pageAsideAffix = affix;
|
||
});
|
||
})();
|
||
</script>
|
||
</div>
|
||
<script>(function () {
|
||
var $root = document.getElementsByClassName('root')[0];
|
||
if (window.hasEvent('touchstart')) {
|
||
$root.dataset.isTouch = true;
|
||
document.addEventListener('touchstart', function(){}, false);
|
||
}
|
||
})();
|
||
</script>
|
||
</body>
|
||
</html>
|
||
|