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<li class="toctree-l1 has-children"><a class="reference internal" href="../chapter2/index.html">第二章：批处理系统</a><input class="toctree-checkbox" id="toctree-checkbox-7" name="toctree-checkbox-7" role="switch" type="checkbox"/><label for="toctree-checkbox-7"><div class="visually-hidden">Toggle child pages in navigation</div><i class="icon"><svg><use href="#svg-arrow-right"></use></svg></i></label><ul>
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<li class="toctree-l2"><a class="reference internal" href="../chapter3/2task-switching.html">任务切换</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter3/3multiprogramming.html">管理多道程序</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter3/5exercise.html">chapter3练习</a></li>
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<li class="toctree-l2 current current-page"><a class="current reference internal" href="#">实现 SV39 多级页表机制（上）</a></li>
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<li class="toctree-l2"><a class="reference internal" href="4sv39-implementation-2.html">实现 SV39 多级页表机制（下）</a></li>
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<li class="toctree-l2"><a class="reference internal" href="5kernel-app-spaces.html">内核与应用的地址空间</a></li>
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<li class="toctree-l2"><a class="reference internal" href="7exercise.html">chapter4练习</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter5/0intro.html">引言</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter5/1process.html">与进程有关的重要系统调用</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter5/2core-data-structures.html">进程管理的核心数据结构</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter5/3implement-process-mechanism.html">进程管理机制的设计实现</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter5/4exercise.html">chapter5练习</a></li>
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<li class="toctree-l1 has-children"><a class="reference internal" href="../chapter6/index.html">第六章：文件系统与I/O重定向</a><input class="toctree-checkbox" id="toctree-checkbox-14" name="toctree-checkbox-14" role="switch" type="checkbox"/><label for="toctree-checkbox-14"><div class="visually-hidden">Toggle child pages in navigation</div><i class="icon"><svg><use href="#svg-arrow-right"></use></svg></i></label><ul>
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<li class="toctree-l2"><a class="reference internal" href="../chapter6/1file-descriptor.html">文件与文件描述符</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter6/1fs-interface.html">文件系统接口</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter6/2fs-implementation-1.html">简易文件系统 easy-fs (上)</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter6/2fs-implementation-2.html">简易文件系统 easy-fs (下)</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter6/3using-easy-fs-in-kernel.html">在内核中使用 easy-fs</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter6/4exercise.html">chapter6练习</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter7/1pipe.html">管道</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter8/1thread-kernel.html">内核态的线程管理</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter8/4condition-variable.html">条件变量机制</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter8/5exercise.html">chapter8 练习</a></li>
|
||
</ul>
|
||
</li>
|
||
</ul>
|
||
<p class="caption" role="heading"><span class="caption-text">附录</span></p>
|
||
<ul>
|
||
<li class="toctree-l1 has-children"><a class="reference internal" href="../appendix-a/index.html">附录 A：Rust 系统编程资料</a><input class="toctree-checkbox" id="toctree-checkbox-17" name="toctree-checkbox-17" role="switch" type="checkbox"/><label for="toctree-checkbox-17"><div class="visually-hidden">Toggle child pages in navigation</div><i class="icon"><svg><use href="#svg-arrow-right"></use></svg></i></label><ul class="simple">
|
||
</ul>
|
||
</li>
|
||
<li class="toctree-l1 has-children"><a class="reference internal" href="../appendix-b/index.html">附录 B：常见工具的使用方法</a><input class="toctree-checkbox" id="toctree-checkbox-18" name="toctree-checkbox-18" role="switch" type="checkbox"/><label for="toctree-checkbox-18"><div class="visually-hidden">Toggle child pages in navigation</div><i class="icon"><svg><use href="#svg-arrow-right"></use></svg></i></label><ul class="simple">
|
||
</ul>
|
||
</li>
|
||
<li class="toctree-l1 has-children"><a class="reference internal" href="../appendix-c/index.html">附录 C：深入机器模式：RustSBI</a><input class="toctree-checkbox" id="toctree-checkbox-19" name="toctree-checkbox-19" role="switch" type="checkbox"/><label for="toctree-checkbox-19"><div class="visually-hidden">Toggle child pages in navigation</div><i class="icon"><svg><use href="#svg-arrow-right"></use></svg></i></label><ul class="simple">
|
||
</ul>
|
||
</li>
|
||
<li class="toctree-l1"><a class="reference internal" href="../appendix-d/index.html">附录 D：RISC-V相关信息</a></li>
|
||
</ul>
|
||
<p class="caption" role="heading"><span class="caption-text">开发注记</span></p>
|
||
<ul>
|
||
<li class="toctree-l1 has-children"><a class="reference internal" href="../setup-sphinx.html">修改和构建本项目</a><input class="toctree-checkbox" id="toctree-checkbox-20" name="toctree-checkbox-20" role="switch" type="checkbox"/><label for="toctree-checkbox-20"><div class="visually-hidden">Toggle child pages in navigation</div><i class="icon"><svg><use href="#svg-arrow-right"></use></svg></i></label><ul class="simple">
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<li class="toctree-l1 has-children"><a class="reference internal" href="../rest-example.html">reStructuredText 基本语法</a><input class="toctree-checkbox" id="toctree-checkbox-21" name="toctree-checkbox-21" role="switch" type="checkbox"/><label for="toctree-checkbox-21"><div class="visually-hidden">Toggle child pages in navigation</div><i class="icon"><svg><use href="#svg-arrow-right"></use></svg></i></label><ul class="simple">
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||
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||
|
||
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||
</aside>
|
||
<div class="main">
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||
<div class="content">
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||
<article role="main">
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<div class="theme-toggle-container theme-toggle-content">
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<button class="theme-toggle">
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<div class="visually-hidden">Toggle Light / Dark / Auto color theme</div>
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<svg class="theme-icon-when-auto"><use href="#svg-sun-half"></use></svg>
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<svg class="theme-icon-when-dark"><use href="#svg-moon"></use></svg>
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</button>
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<label class="toc-overlay-icon toc-content-icon" for="__toc">
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<div class="visually-hidden">Toggle table of contents sidebar</div>
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<i class="icon"><svg><use href="#svg-toc"></use></svg></i>
|
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</label>
|
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</div>
|
||
<div class="section" id="sv39">
|
||
<h1>实现 SV39 多级页表机制（上）<a class="headerlink" href="#sv39" title="永久链接至标题">¶</a></h1>
|
||
<div class="admonition note">
|
||
<p class="admonition-title">注解</p>
|
||
<p>背景知识： <a class="reference external" href="https://rcore-os.github.io/rCore-Tutorial-Book-v3/chapter4/2address-space.html">地址空间</a></p>
|
||
<p>背景知识： <a class="reference external" href="https://rcore-os.github.io/rCore-Tutorial-Book-v3/chapter4/3sv39-implementation-1.html#id6">SV39 多级页表原理</a></p>
|
||
</div>
|
||
<p>我们将在内核实现 RV64 架构 SV39 分页机制。由于内容过多，分成两个小节。</p>
|
||
<div class="section" id="id3">
|
||
<h2>虚拟地址和物理地址<a class="headerlink" href="#id3" title="永久链接至标题">¶</a></h2>
|
||
<div class="section" id="csr">
|
||
<h3>内存控制相关的CSR寄存器<a class="headerlink" href="#csr" title="永久链接至标题">¶</a></h3>
|
||
<p>默认情况下 MMU 未被使能，此时无论 CPU 处于哪个特权级，访存的地址都将直接被视作物理地址。
|
||
可以通过修改 S 特权级的 <code class="docutils literal notranslate"><span class="pre">satp</span></code> CSR 来启用分页模式，此后 S 和 U 特权级的访存地址会被视为虚拟地址，经过 MMU 的地址转换获得对应物理地址，再通过它来访问物理内存。</p>
|
||
<img alt="../_images/satp.png" id="satp-layout" src="../_images/satp.png"/>
|
||
<p>上图是 RV64 架构下 <code class="docutils literal notranslate"><span class="pre">satp</span></code> 的字段分布。当 <code class="docutils literal notranslate"><span class="pre">MODE</span></code> 设置为 0 的时候，所有访存都被视为物理地址；而设置为 8
|
||
时，SV39 分页机制被启用，所有 S/U 特权级的访存被视为一个 39 位的虚拟地址，MMU 会将其转换成 56 位的物理地址；如果转换失败，则会触发异常。</p>
|
||
</div>
|
||
<div class="section" id="id4">
|
||
<h3>地址格式与组成<a class="headerlink" href="#id4" title="永久链接至标题">¶</a></h3>
|
||
<img alt="../_images/sv39-va-pa.png" src="../_images/sv39-va-pa.png"/>
|
||
<p>我们采用分页管理，单个页面的大小设置为 <span class="math notranslate nohighlight">\(4\text{KiB}\)</span> ，每个虚拟页面和物理页帧都按 4 KB 对齐。
|
||
<span class="math notranslate nohighlight">\(4\text{KiB}\)</span> 需要用 12 位字节地址来表示，因此虚拟地址和物理地址都被分成两部分：
|
||
它们的低 12 位被称为 <strong>页内偏移</strong> (Page Offset) 。虚拟地址的高 27 位，即 <span class="math notranslate nohighlight">\([38:12]\)</span> 为它的虚拟页号 VPN；
|
||
物理地址的高 44 位，即 <span class="math notranslate nohighlight">\([55:12]\)</span> 为它的物理页号 PPN。页号可以用来定位一个虚拟/物理地址属于哪一个虚拟页面/物理页帧。</p>
|
||
<p>地址转换是以页为单位进行的，转换前后地址页内偏移部分不变。MMU 只是从虚拟地址中取出 27 位虚拟页号，
|
||
在页表中查到其对应的物理页号，如果找到，就将得到的 44 位的物理页号与 12 位页内偏移拼接到一起，形成 56 位物理地址。</p>
|
||
<div class="admonition note">
|
||
<p class="admonition-title">注解</p>
|
||
<p><strong>RV64 架构中虚拟地址为何只有 39 位？</strong></p>
|
||
<p>虚拟地址长度确实应该和位宽一致为 64 位，但是在启用 SV39 分页模式下，只有低 39 位是真正有意义的。
|
||
SV39 分页模式规定 64 位虚拟地址的 <span class="math notranslate nohighlight">\([63:39]\)</span> 这 25 位必须和第 38 位相同，否则 MMU 会直接认定它是一个
|
||
不合法的虚拟地址。。</p>
|
||
<p>也就是说，所有 <span class="math notranslate nohighlight">\(2^{64}\)</span> 个虚拟地址中，只有最低的 <span class="math notranslate nohighlight">\(256\text{GiB}\)</span> （当第 38 位为 0 时）
|
||
以及最高的 <span class="math notranslate nohighlight">\(256\text{GiB}\)</span> （当第 38 位为 1 时）是可能通过 MMU 检查的。</p>
|
||
</div>
|
||
</div>
|
||
<div class="section" id="id5">
|
||
<h3>地址相关的数据结构抽象与类型定义<a class="headerlink" href="#id5" title="永久链接至标题">¶</a></h3>
|
||
<p>实现页表之前，先将地址和页号的概念抽象为 Rust 中的类型。</p>
|
||
<p>首先是这些类型的定义：</p>
|
||
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// os/src/mm/address.rs</span>
|
||
|
||
<span class="cp">#[derive(Copy, Clone, Ord, PartialOrd, Eq, PartialEq)]</span><span class="w"></span>
|
||
<span class="k">pub</span><span class="w"> </span><span class="k">struct</span> <span class="nc">PhysAddr</span><span class="p">(</span><span class="k">pub</span><span class="w"> </span><span class="kt">usize</span><span class="p">);</span><span class="w"></span>
|
||
|
||
<span class="cp">#[derive(Copy, Clone, Ord, PartialOrd, Eq, PartialEq)]</span><span class="w"></span>
|
||
<span class="k">pub</span><span class="w"> </span><span class="k">struct</span> <span class="nc">VirtAddr</span><span class="p">(</span><span class="k">pub</span><span class="w"> </span><span class="kt">usize</span><span class="p">);</span><span class="w"></span>
|
||
|
||
<span class="cp">#[derive(Copy, Clone, Ord, PartialOrd, Eq, PartialEq)]</span><span class="w"></span>
|
||
<span class="k">pub</span><span class="w"> </span><span class="k">struct</span> <span class="nc">PhysPageNum</span><span class="p">(</span><span class="k">pub</span><span class="w"> </span><span class="kt">usize</span><span class="p">);</span><span class="w"></span>
|
||
|
||
<span class="cp">#[derive(Copy, Clone, Ord, PartialOrd, Eq, PartialEq)]</span><span class="w"></span>
|
||
<span class="k">pub</span><span class="w"> </span><span class="k">struct</span> <span class="nc">VirtPageNum</span><span class="p">(</span><span class="k">pub</span><span class="w"> </span><span class="kt">usize</span><span class="p">);</span><span class="w"></span>
|
||
</pre></div>
|
||
</div>
|
||
<p id="term-type-convertion">上面分别给出了物理地址、虚拟地址、物理页号、虚拟页号的 Rust 类型声明，它们都是 usize 的一种简单包装。
|
||
将它们各自抽象出来而不是直接使用 usize，是为了在 Rust 编译器的帮助下进行多种方便且安全的 <strong>类型转换</strong> (Type Convertion) 。</p>
|
||
<p>这些类型本身可以和 usize 之间互相转换，地址和页号之间也可以相互转换。以物理地址和物理页号之间的转换为例：</p>
|
||
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos"> 1</span><span class="c1">// os/src/mm/address.rs</span>
|
||
<span class="linenos"> 2</span>
|
||
<span class="linenos"> 3</span><span class="k">impl</span><span class="w"> </span><span class="n">PhysAddr</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos"> 4</span><span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">page_offset</span><span class="p">(</span><span class="o">&</span><span class="bp">self</span><span class="p">)</span><span class="w"> </span>-> <span class="kt">usize</span> <span class="p">{</span><span class="w"> </span><span class="bp">self</span><span class="p">.</span><span class="mi">0</span><span class="w"> </span><span class="o">&</span><span class="w"> </span><span class="p">(</span><span class="n">PAGE_SIZE</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="mi">1</span><span class="p">)</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos"> 5</span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos"> 6</span>
|
||
<span class="linenos"> 7</span><span class="k">impl</span><span class="w"> </span><span class="nb">From</span><span class="o"><</span><span class="n">PhysAddr</span><span class="o">></span><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="n">PhysPageNum</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos"> 8</span><span class="w"> </span><span class="k">fn</span> <span class="nf">from</span><span class="p">(</span><span class="n">v</span>: <span class="nc">PhysAddr</span><span class="p">)</span><span class="w"> </span>-> <span class="nc">Self</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos"> 9</span><span class="w"> </span><span class="fm">assert_eq!</span><span class="p">(</span><span class="n">v</span><span class="p">.</span><span class="n">page_offset</span><span class="p">(),</span><span class="w"> </span><span class="mi">0</span><span class="p">);</span><span class="w"></span>
|
||
<span class="linenos">10</span><span class="w"> </span><span class="n">v</span><span class="p">.</span><span class="n">floor</span><span class="p">()</span><span class="w"></span>
|
||
<span class="linenos">11</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">12</span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">13</span>
|
||
<span class="linenos">14</span><span class="k">impl</span><span class="w"> </span><span class="nb">From</span><span class="o"><</span><span class="n">PhysPageNum</span><span class="o">></span><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="n">PhysAddr</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">15</span><span class="w"> </span><span class="k">fn</span> <span class="nf">from</span><span class="p">(</span><span class="n">v</span>: <span class="nc">PhysPageNum</span><span class="p">)</span><span class="w"> </span>-> <span class="nc">Self</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="bp">Self</span><span class="p">(</span><span class="n">v</span><span class="p">.</span><span class="mi">0</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">PAGE_SIZE_BITS</span><span class="p">)</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">16</span><span class="p">}</span><span class="w"></span>
|
||
</pre></div>
|
||
</div>
|
||
<p>其中 <code class="docutils literal notranslate"><span class="pre">PAGE_SIZE</span></code> 为 <span class="math notranslate nohighlight">\(4096\)</span> ， <code class="docutils literal notranslate"><span class="pre">PAGE_SIZE_BITS</span></code> 为 <span class="math notranslate nohighlight">\(12\)</span> ，它们均定义在 <code class="docutils literal notranslate"><span class="pre">config</span></code> 子模块
|
||
中，分别表示每个页面的大小和页内偏移的位宽。从物理页号到物理地址的转换只需左移 <span class="math notranslate nohighlight">\(12\)</span> 位即可，但是物理地址需要
|
||
保证它与页面大小对齐才能通过右移转换为物理页号。</p>
|
||
<p>对于不对齐的情况，物理地址不能通过 <code class="docutils literal notranslate"><span class="pre">From/Into</span></code> 转换为物理页号，而是需要通过它自己的 <code class="docutils literal notranslate"><span class="pre">floor</span></code> 或 <code class="docutils literal notranslate"><span class="pre">ceil</span></code> 方法来
|
||
进行下取整或上取整的转换。</p>
|
||
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// os/src/mm/address.rs</span>
|
||
|
||
<span class="k">impl</span><span class="w"> </span><span class="n">PhysAddr</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">floor</span><span class="p">(</span><span class="o">&</span><span class="bp">self</span><span class="p">)</span><span class="w"> </span>-> <span class="nc">PhysPageNum</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">PhysPageNum</span><span class="p">(</span><span class="bp">self</span><span class="p">.</span><span class="mi">0</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">PAGE_SIZE</span><span class="p">)</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">ceil</span><span class="p">(</span><span class="o">&</span><span class="bp">self</span><span class="p">)</span><span class="w"> </span>-> <span class="nc">PhysPageNum</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">PhysPageNum</span><span class="p">((</span><span class="bp">self</span><span class="p">.</span><span class="mi">0</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">PAGE_SIZE</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="mi">1</span><span class="p">)</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">PAGE_SIZE</span><span class="p">)</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="p">}</span><span class="w"></span>
|
||
</pre></div>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<div class="section" id="id6">
|
||
<h2>页表项的数据结构抽象与类型定义<a class="headerlink" href="#id6" title="永久链接至标题">¶</a></h2>
|
||
<img alt="../_images/sv39-pte.png" src="../_images/sv39-pte.png"/>
|
||
<p>上图为 SV39 分页模式下的页表项，其中 <span class="math notranslate nohighlight">\([53:10]\)</span> 这 <span class="math notranslate nohighlight">\(44\)</span> 位是物理页号，最低的 <span class="math notranslate nohighlight">\(8\)</span> 位
|
||
<span class="math notranslate nohighlight">\([7:0]\)</span> 则是标志位，它们的含义如下：</p>
|
||
<ul class="simple">
|
||
<li><p>仅当 V(Valid) 位为 1 时，页表项才是合法的；</p></li>
|
||
<li><p>R/W/X 分别控制索引到这个页表项的对应虚拟页面是否允许读/写/取指；</p></li>
|
||
<li><p>U 控制索引到这个页表项的对应虚拟页面是否在 CPU 处于 U 特权级的情况下是否被允许访问；</p></li>
|
||
<li><p>G 我们不理会；</p></li>
|
||
<li><p>A(Accessed) 记录自从页表项上的这一位被清零之后，页表项的对应虚拟页面是否被访问过；</p></li>
|
||
<li><p>D(Dirty) 则记录自从页表项上的这一位被清零之后，页表项的对应虚拟页表是否被修改过。</p></li>
|
||
</ul>
|
||
<p>先来实现页表项中的标志位 <code class="docutils literal notranslate"><span class="pre">PTEFlags</span></code> ：</p>
|
||
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// os/src/main.rs</span>
|
||
|
||
<span class="cp">#[macro_use]</span><span class="w"></span>
|
||
<span class="k">extern</span><span class="w"> </span><span class="k">crate</span><span class="w"> </span><span class="n">bitflags</span><span class="p">;</span><span class="w"></span>
|
||
|
||
<span class="c1">// os/src/mm/page_table.rs</span>
|
||
|
||
<span class="k">use</span><span class="w"> </span><span class="n">bitflags</span>::<span class="o">*</span><span class="p">;</span><span class="w"></span>
|
||
|
||
<span class="n">bitflags</span><span class="o">!</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="k">struct</span> <span class="nc">PTEFlags</span>: <span class="kt">u8</span> <span class="p">{</span><span class="w"></span>
|
||
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">V</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"></span>
|
||
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">R</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span><span class="w"></span>
|
||
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">W</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">2</span><span class="p">;</span><span class="w"></span>
|
||
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">X</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">3</span><span class="p">;</span><span class="w"></span>
|
||
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">U</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">4</span><span class="p">;</span><span class="w"></span>
|
||
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">G</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">5</span><span class="p">;</span><span class="w"></span>
|
||
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">A</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">6</span><span class="p">;</span><span class="w"></span>
|
||
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">D</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">7</span><span class="p">;</span><span class="w"></span>
|
||
<span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="p">}</span><span class="w"></span>
|
||
</pre></div>
|
||
</div>
|
||
<p><a class="reference external" href="https://docs.rs/bitflags/1.2.1/bitflags/">bitflags</a> 是一个 Rust 中常用来比特标志位的 crate 。它提供了
|
||
一个 <code class="docutils literal notranslate"><span class="pre">bitflags!</span></code> 宏，如上面的代码段所展示的那样，可以将一个 <code class="docutils literal notranslate"><span class="pre">u8</span></code> 封装成一个标志位的集合类型，支持一些常见的集合
|
||
运算。</p>
|
||
<p>接下来我们实现页表项 <code class="docutils literal notranslate"><span class="pre">PageTableEntry</span></code> ：</p>
|
||
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos"> 1</span><span class="c1">// os/src/mm/page_table.rs</span>
|
||
<span class="linenos"> 2</span>
|
||
<span class="linenos"> 3</span><span class="cp">#[derive(Copy, Clone)]</span><span class="w"></span>
|
||
<span class="linenos"> 4</span><span class="cp">#[repr(C)]</span><span class="w"></span>
|
||
<span class="linenos"> 5</span><span class="k">pub</span><span class="w"> </span><span class="k">struct</span> <span class="nc">PageTableEntry</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos"> 6</span><span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="n">bits</span>: <span class="kt">usize</span><span class="p">,</span><span class="w"></span>
|
||
<span class="linenos"> 7</span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos"> 8</span>
|
||
<span class="linenos"> 9</span><span class="k">impl</span><span class="w"> </span><span class="n">PageTableEntry</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">10</span><span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">new</span><span class="p">(</span><span class="n">ppn</span>: <span class="nc">PhysPageNum</span><span class="p">,</span><span class="w"> </span><span class="n">flags</span>: <span class="nc">PTEFlags</span><span class="p">)</span><span class="w"> </span>-> <span class="nc">Self</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">11</span><span class="w"> </span><span class="n">PageTableEntry</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">12</span><span class="w"> </span><span class="n">bits</span>: <span class="nc">ppn</span><span class="p">.</span><span class="mi">0</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">10</span><span class="w"> </span><span class="o">|</span><span class="w"> </span><span class="n">flags</span><span class="p">.</span><span class="n">bits</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="kt">usize</span><span class="p">,</span><span class="w"></span>
|
||
<span class="linenos">13</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">14</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">15</span><span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">empty</span><span class="p">()</span><span class="w"> </span>-> <span class="nc">Self</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">16</span><span class="w"> </span><span class="n">PageTableEntry</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">17</span><span class="w"> </span><span class="n">bits</span>: <span class="mi">0</span><span class="p">,</span><span class="w"></span>
|
||
<span class="linenos">18</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">19</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">20</span><span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">ppn</span><span class="p">(</span><span class="o">&</span><span class="bp">self</span><span class="p">)</span><span class="w"> </span>-> <span class="nc">PhysPageNum</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">21</span><span class="w"> </span><span class="p">(</span><span class="bp">self</span><span class="p">.</span><span class="n">bits</span><span class="w"> </span><span class="o">>></span><span class="w"> </span><span class="mi">10</span><span class="w"> </span><span class="o">&</span><span class="w"> </span><span class="p">((</span><span class="mi">1</span><span class="k">usize</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">44</span><span class="p">)</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="mi">1</span><span class="p">)).</span><span class="n">into</span><span class="p">()</span><span class="w"></span>
|
||
<span class="linenos">22</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">23</span><span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">flags</span><span class="p">(</span><span class="o">&</span><span class="bp">self</span><span class="p">)</span><span class="w"> </span>-> <span class="nc">PTEFlags</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">24</span><span class="w"> </span><span class="n">PTEFlags</span>::<span class="n">from_bits</span><span class="p">(</span><span class="bp">self</span><span class="p">.</span><span class="n">bits</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="kt">u8</span><span class="p">).</span><span class="n">unwrap</span><span class="p">()</span><span class="w"></span>
|
||
<span class="linenos">25</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">26</span><span class="p">}</span><span class="w"></span>
|
||
</pre></div>
|
||
</div>
|
||
<ul class="simple">
|
||
<li><p>第 3 行我们让编译器自动为 <code class="docutils literal notranslate"><span class="pre">PageTableEntry</span></code> 实现 <code class="docutils literal notranslate"><span class="pre">Copy/Clone</span></code> Trait，来让这个类型以值语义赋值/传参的时候
|
||
不会发生所有权转移，而是拷贝一份新的副本。</p></li>
|
||
<li><p>第 10 行使得我们可以从一个物理页号 <code class="docutils literal notranslate"><span class="pre">PhysPageNum</span></code> 和一个页表项标志位 <code class="docutils literal notranslate"><span class="pre">PTEFlags</span></code> 生成一个页表项
|
||
<code class="docutils literal notranslate"><span class="pre">PageTableEntry</span></code> 实例；而第 20 行和第 23 行则分别可以从一个页表项将它们两个取出。</p></li>
|
||
<li><p>第 15 行中，我们也可以通过 <code class="docutils literal notranslate"><span class="pre">empty</span></code> 方法生成一个全零的页表项，注意这隐含着该页表项的 V 标志位为 0 ，
|
||
因此它是不合法的。</p></li>
|
||
</ul>
|
||
<p>后面我们还为 <code class="docutils literal notranslate"><span class="pre">PageTableEntry</span></code> 实现了一些辅助函数(Helper Function)，可以快速判断一个页表项的 V/R/W/X 标志位是否为 1，以 V
|
||
标志位的判断为例：</p>
|
||
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// os/src/mm/page_table.rs</span>
|
||
|
||
<span class="k">impl</span><span class="w"> </span><span class="n">PageTableEntry</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">is_valid</span><span class="p">(</span><span class="o">&</span><span class="bp">self</span><span class="p">)</span><span class="w"> </span>-> <span class="kt">bool</span> <span class="p">{</span><span class="w"></span>
|
||
<span class="w"> </span><span class="p">(</span><span class="bp">self</span><span class="p">.</span><span class="n">flags</span><span class="p">()</span><span class="w"> </span><span class="o">&</span><span class="w"> </span><span class="n">PTEFlags</span>::<span class="n">V</span><span class="p">)</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">PTEFlags</span>::<span class="n">empty</span><span class="p">()</span><span class="w"></span>
|
||
<span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="p">}</span><span class="w"></span>
|
||
</pre></div>
|
||
</div>
|
||
<p>这里相当于判断两个集合的交集是否为空。</p>
|
||
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|
||
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|
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