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<li class="toctree-l1"><a class="reference internal" href="../appendix-d/index.html">附录 D：RISC-V相关信息</a></li>
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</div>
<div class="section" id="id2">
<h2>将应用程序链接到内核<a class="headerlink" href="#id2" title="永久链接至标题"></a></h2>
<p>在本章中，我们要把应用程序的二进制镜像文件作为数据段链接到内核里，
内核需要知道应用程序的数量和它们的位置。</p>
<p>在 <code class="docutils literal notranslate"><span class="pre">os/src/main.rs</span></code> 中能够找到这样一行：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="n">core</span>::<span class="n">arch</span>::<span class="fm">global_asm!</span><span class="p">(</span><span class="fm">include_str!</span><span class="p">(</span><span class="s">"link_app.S"</span><span class="p">));</span><span class="w"></span>
</pre></div>
</div>
<p>这里我们引入了一段汇编代码 <code class="docutils literal notranslate"><span class="pre">link_app.S</span></code> ，它是在 <code class="docutils literal notranslate"><span class="pre">make</span> <span class="pre">run</span></code> 构建操作系统时自动生成的，里面的内容大致如下：</p>
<div class="highlight-asm notranslate"><div class="highlight"><pre><span></span><span class="linenos"> 1</span><span class="c1"># os/src/link_app.S</span>
<span class="linenos"> 2</span>
<span class="linenos"> 3</span> <span class="na">.align</span> <span class="mi">3</span>
<span class="linenos"> 4</span> <span class="na">.section</span> <span class="no">.data</span>
<span class="linenos"> 5</span> <span class="na">.global</span> <span class="no">_num_app</span>
<span class="linenos"> 6</span><span class="nl">_num_app:</span>
<span class="linenos"> 7</span> <span class="na">.quad</span> <span class="mi">3</span>
<span class="linenos"> 8</span> <span class="na">.quad</span> <span class="no">app_0_start</span>
<span class="linenos"> 9</span> <span class="na">.quad</span> <span class="no">app_1_start</span>
<span class="linenos">10</span> <span class="na">.quad</span> <span class="no">app_2_start</span>
<span class="linenos">11</span> <span class="na">.quad</span> <span class="no">app_2_end</span>
<span class="linenos">12</span>
<span class="linenos">13</span> <span class="na">.section</span> <span class="no">.data</span>
<span class="linenos">14</span> <span class="na">.global</span> <span class="no">app_0_start</span>
<span class="linenos">15</span> <span class="na">.global</span> <span class="no">app_0_end</span>
<span class="linenos">16</span><span class="nl">app_0_start:</span>
<span class="linenos">17</span> <span class="na">.incbin</span> <span class="s">"../user/target/riscv64gc-unknown-none-elf/release/hello_world.bin"</span>
<span class="linenos">18</span><span class="nl">app_0_end:</span>
<span class="linenos">19</span>
<span class="linenos">20</span> <span class="na">.section</span> <span class="no">.data</span>
<span class="linenos">21</span> <span class="na">.global</span> <span class="no">app_1_start</span>
<span class="linenos">22</span> <span class="na">.global</span> <span class="no">app_1_end</span>
<span class="linenos">23</span><span class="nl">app_1_start:</span>
<span class="linenos">24</span> <span class="na">.incbin</span> <span class="s">"../user/target/riscv64gc-unknown-none-elf/release/bad_address.bin"</span>
<span class="linenos">25</span><span class="nl">app_1_end:</span>
<span class="linenos">26</span>
<span class="linenos">27</span> <span class="na">.section</span> <span class="no">.data</span>
<span class="linenos">28</span> <span class="na">.global</span> <span class="no">app_2_start</span>
<span class="linenos">29</span> <span class="na">.global</span> <span class="no">app_2_end</span>
<span class="linenos">30</span><span class="nl">app_2_start:</span>
<span class="linenos">31</span> <span class="na">.incbin</span> <span class="s">"../user/target/riscv64gc-unknown-none-elf/release/power.bin"</span>
<span class="linenos">32</span><span class="nl">app_2_end:</span>
</pre></div>
</div>
<p>第 13 行开始的三个数据段分别插入了三个应用程序的二进制镜像，
并且各自有一对全局符号 <code class="docutils literal notranslate"><span class="pre">app_*_start,</span> <span class="pre">app_*_end</span></code> 指示它们的开始和结束位置。
而第 3 行开始的另一个数据段相当于一个 64 位整数数组。
数组中的第一个元素表示应用程序的数量，后面则按照顺序放置每个应用程序的起始地址，
最后一个元素放置最后一个应用程序的结束位置。这样数组中相邻两个元素记录了每个应用程序的始末位置，
这个数组所在的位置由全局符号 <code class="docutils literal notranslate"><span class="pre">_num_app</span></code> 所指示。</p>
<p>这个文件是在 <code class="docutils literal notranslate"><span class="pre">cargo</span> <span class="pre">build</span></code> 时，由脚本 <code class="docutils literal notranslate"><span class="pre">os/build.rs</span></code> 控制生成的。</p>
</div>
<div class="section" id="id3">
<h2>找到并加载应用程序二进制码<a class="headerlink" href="#id3" title="永久链接至标题"></a></h2>
<p>我们在 <code class="docutils literal notranslate"><span class="pre">os</span></code> 的 <code class="docutils literal notranslate"><span class="pre">batch</span></code> 子模块中实现一个应用管理器 <code class="docutils literal notranslate"><span class="pre">AppManager</span></code> ，结构体定义如下：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="k">struct</span> <span class="nc">AppManager</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">num_app</span>: <span class="kt">usize</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">current_app</span>: <span class="kt">usize</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">app_start</span>: <span class="p">[</span><span class="kt">usize</span><span class="p">;</span><span class="w"> </span><span class="n">MAX_APP_NUM</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>
</pre></div>
</div>
<p>初始化 <code class="docutils literal notranslate"><span class="pre">AppManager</span></code> 的全局实例：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="n">lazy_static</span><span class="o">!</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">static</span><span class="w"> </span><span class="k">ref</span><span class="w"> </span><span class="n">APP_MANAGER</span>: <span class="nc">UPSafeCell</span><span class="o">&lt;</span><span class="n">AppManager</span><span class="o">&gt;</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">unsafe</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">UPSafeCell</span>::<span class="n">new</span><span class="p">({</span><span class="w"></span>
<span class="w"> </span><span class="k">extern</span><span class="w"> </span><span class="s">"C"</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">fn</span> <span class="nf">_num_app</span><span class="p">();</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">num_app_ptr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">_num_app</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="kt">usize</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="o">*</span><span class="k">const</span><span class="w"> </span><span class="kt">usize</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="n">num_app</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">num_app_ptr</span><span class="p">.</span><span class="n">read_volatile</span><span class="p">();</span><span class="w"></span>
<span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="k">mut</span><span class="w"> </span><span class="n">app_start</span>: <span class="p">[</span><span class="kt">usize</span><span class="p">;</span><span class="w"> </span><span class="n">MAX_APP_NUM</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="p">[</span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">MAX_APP_NUM</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="kd">let</span><span class="w"> </span><span class="n">app_start_raw</span>: <span class="kp">&amp;</span><span class="p">[</span><span class="kt">usize</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"></span>
<span class="w"> </span><span class="n">core</span>::<span class="n">slice</span>::<span class="n">from_raw_parts</span><span class="p">(</span><span class="n">num_app_ptr</span><span class="p">.</span><span class="n">add</span><span class="p">(</span><span class="mi">1</span><span class="p">),</span><span class="w"> </span><span class="n">num_app</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="n">app_start</span><span class="p">[</span><span class="o">..=</span><span class="n">num_app</span><span class="p">].</span><span class="n">copy_from_slice</span><span class="p">(</span><span class="n">app_start_raw</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="n">AppManager</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">num_app</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">current_app</span>: <span class="mi">0</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">app_start</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </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>初始化的逻辑很简单，就是找到 <code class="docutils literal notranslate"><span class="pre">link_app.S</span></code> 中提供的符号 <code class="docutils literal notranslate"><span class="pre">_num_app</span></code> ，并从这里开始解析出应用数量以及各个应用的开头地址。
用容器 <code class="docutils literal notranslate"><span class="pre">UPSafeCell</span></code> 包裹 <code class="docutils literal notranslate"><span class="pre">AppManager</span></code> 是为了防止全局对象 <code class="docutils literal notranslate"><span class="pre">APP_MANAGER</span></code> 被重复获取。</p>
<div class="admonition note">
<p class="admonition-title">注解</p>
<p><code class="docutils literal notranslate"><span class="pre">UPSafeCell</span></code> 实现在 <code class="docutils literal notranslate"><span class="pre">sync</span></code> 模块中，调用 <code class="docutils literal notranslate"><span class="pre">exclusive_access</span></code> 方法能获取其内部对象的可变引用，
如果程序运行中同时存在多个这样的引用，会触发 <code class="docutils literal notranslate"><span class="pre">already</span> <span class="pre">borrowed:</span> <span class="pre">BorrowMutError</span></code>。</p>
<p><code class="docutils literal notranslate"><span class="pre">UPSafeCell</span></code> 既提供了内部可变性，又在单核情境下防止了内部对象被重复借用，我们将在后文中多次见到它。</p>
</div>
<p>这里使用了外部库 <code class="docutils literal notranslate"><span class="pre">lazy_static</span></code> 提供的 <code class="docutils literal notranslate"><span class="pre">lazy_static!</span></code> 宏。</p>
<p><code class="docutils literal notranslate"><span class="pre">lazy_static!</span></code> 宏提供了全局变量的运行时初始化功能。一般情况下，全局变量必须在编译期设置初始值，
但是有些全局变量的初始化依赖于运行期间才能得到的数据。
如这里我们借助 <code class="docutils literal notranslate"><span class="pre">lazy_static!</span></code> 声明了一个 <code class="docutils literal notranslate"><span class="pre">AppManager</span></code> 结构的名为 <code class="docutils literal notranslate"><span class="pre">APP_MANAGER</span></code> 的全局实例，
只有在它第一次被使用到的时候才会进行实际的初始化工作。</p>
<p><code class="docutils literal notranslate"><span class="pre">AppManager</span></code> 的方法中， <code class="docutils literal notranslate"><span class="pre">print_app_info/get_current_app/move_to_next_app</span></code> 都相当简单直接，需要说明的是 <code class="docutils literal notranslate"><span class="pre">load_app</span></code>：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos"> 1</span><span class="k">unsafe</span><span class="w"> </span><span class="k">fn</span> <span class="nf">load_app</span><span class="p">(</span><span class="o">&amp;</span><span class="bp">self</span><span class="p">,</span><span class="w"> </span><span class="n">app_id</span>: <span class="kt">usize</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos"> 2</span><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="n">app_id</span><span class="w"> </span><span class="o">&gt;=</span><span class="w"> </span><span class="bp">self</span><span class="p">.</span><span class="n">num_app</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="linenos"> 3</span><span class="w"> </span><span class="fm">panic!</span><span class="p">(</span><span class="s">"All applications completed!"</span><span class="p">);</span><span class="w"></span>
<span class="linenos"> 4</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="linenos"> 5</span><span class="w"> </span><span class="n">info</span><span class="o">!</span><span class="p">(</span><span class="s">"[kernel] Loading app_{}"</span><span class="p">,</span><span class="w"> </span><span class="n">app_id</span><span class="p">);</span><span class="w"></span>
<span class="linenos"> 6</span><span class="w"> </span><span class="c1">// clear icache</span>
<span class="linenos"> 7</span><span class="w"> </span><span class="n">core</span>::<span class="n">arch</span>::<span class="fm">asm!</span><span class="p">(</span><span class="s">"fence.i"</span><span class="p">);</span><span class="w"></span>
<span class="linenos"> 8</span><span class="w"> </span><span class="c1">// clear app area</span>
<span class="linenos"> 9</span><span class="w"> </span><span class="n">core</span>::<span class="n">slice</span>::<span class="n">from_raw_parts_mut</span><span class="p">(</span><span class="n">APP_BASE_ADDRESS</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="o">*</span><span class="k">mut</span><span class="w"> </span><span class="kt">u8</span><span class="p">,</span><span class="w"> </span><span class="n">APP_SIZE_LIMIT</span><span class="p">).</span><span class="n">fill</span><span class="p">(</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="kd">let</span><span class="w"> </span><span class="n">app_src</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">core</span>::<span class="n">slice</span>::<span class="n">from_raw_parts</span><span class="p">(</span><span class="w"></span>
<span class="linenos">11</span><span class="w"> </span><span class="bp">self</span><span class="p">.</span><span class="n">app_start</span><span class="p">[</span><span class="n">app_id</span><span class="p">]</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="o">*</span><span class="k">const</span><span class="w"> </span><span class="kt">u8</span><span class="p">,</span><span class="w"></span>
<span class="linenos">12</span><span class="w"> </span><span class="bp">self</span><span class="p">.</span><span class="n">app_start</span><span class="p">[</span><span class="n">app_id</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="bp">self</span><span class="p">.</span><span class="n">app_start</span><span class="p">[</span><span class="n">app_id</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="kd">let</span><span class="w"> </span><span class="n">app_dst</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">core</span>::<span class="n">slice</span>::<span class="n">from_raw_parts_mut</span><span class="p">(</span><span class="n">APP_BASE_ADDRESS</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="o">*</span><span class="k">mut</span><span class="w"> </span><span class="kt">u8</span><span class="p">,</span><span class="w"> </span><span class="n">app_src</span><span class="p">.</span><span class="n">len</span><span class="p">());</span><span class="w"></span>
<span class="linenos">15</span><span class="w"> </span><span class="n">app_dst</span><span class="p">.</span><span class="n">copy_from_slice</span><span class="p">(</span><span class="n">app_src</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>这个方法负责将参数 <code class="docutils literal notranslate"><span class="pre">app_id</span></code> 对应的应用程序的二进制镜像加载到物理内存以 <code class="docutils literal notranslate"><span class="pre">0x80400000</span></code> 起始的位置，
这个位置是批处理操作系统和应用程序之间约定的常数地址。
我们将从这里开始的一块内存清空，然后找到待加载应用二进制镜像的位置，并将它复制到正确的位置。</p>
<p>清空内存前，我们插入了一条奇怪的汇编指令 <code class="docutils literal notranslate"><span class="pre">fence.i</span></code> ，它是用来清理 i-cache 的。
我们知道， 缓存又分成 <strong>数据缓存</strong> (d-cache) 和 <strong>指令缓存</strong> (i-cache) 两部分，分别在 CPU 访存和取指的时候使用。
通常情况下， CPU 会认为程序的代码段不会发生变化，因此 i-cache 是一种只读缓存。
但在这里，我们会修改会被 CPU 取指的内存区域，使得 i-cache 中含有与内存不一致的内容，
必须使用 <code class="docutils literal notranslate"><span class="pre">fence.i</span></code> 指令手动清空 i-cache ，让里面所有的内容全部失效，
才能够保证程序执行正确性。</p>
<div class="admonition warning">
<p class="admonition-title">警告</p>
<p><strong>模拟器与真机的不同之处</strong></p>
<p>在 Qemu 模拟器上，即使不加刷新 i-cache 的指令，大概率也能正常运行，但在物理计算机上不是这样。</p>
</div>
<p><code class="docutils literal notranslate"><span class="pre">batch</span></code> 子模块对外暴露出如下接口：</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">init</span></code> ：调用 <code class="docutils literal notranslate"><span class="pre">print_app_info</span></code> 的时第一次用到了全局变量 <code class="docutils literal notranslate"><span class="pre">APP_MANAGER</span></code> ，它在这时完成初始化；</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">run_next_app</span></code> ：批处理操作系统的核心操作，即加载并运行下一个应用程序。
批处理操作系统完成初始化，或者应用程序运行结束/出错后会调用该函数。下节再介绍其具体实现。</p></li>
</ul>
</div>
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