2025-10-27

veosat
一生一芯
2025-10-28
44热度
0评论

发现一个奇怪的现象，timer返回值发生在回调函数被调用之前，这样应用程序永远没法获得最新的时间。

原先nemu的timer回调函数如下

static void rtc_io_handler(uint32_t offset, int len, bool is_write) {
  assert(offset == 0 || offset == 4);
  if (!is_write && offset == 4) {
    uint64_t us = get_time();
    rtc_port_base[0] = (uint32_t)us;
    rtc_port_base[1] = us >> 32;
    printf("Timer:%u,%u\n",rtc_port_base[1],rtc_port_base[0]);
  }
}

<span class="line"><span style="color: #0000FF">static</span>
<span style="color: #000000"> </span>
<span style="color: #0000FF">void</span>
<span style="color: #000000"> </span>
<span style="color: #795E26">rtc_io_handler</span>
<span style="color: #000000">(</span>
<span style="color: #0000FF">uint32_t</span>
<span style="color: #000000"> </span>
<span style="color: #001080">offset</span>
<span style="color: #000000">, </span>
<span style="color: #0000FF">int</span>
<span style="color: #000000"> </span>
<span style="color: #001080">len</span>
<span style="color: #000000">, </span>
<span style="color: #0000FF">bool</span>
<span style="color: #000000"> </span>
<span style="color: #001080">is_write</span>
<span style="color: #000000">) {</span></span>
<span class="line"><span style="color: #000000">  </span>
<span style="color: #795E26">assert</span>
<span style="color: #000000">(offset == </span>
<span style="color: #098658">0</span>
<span style="color: #000000"> || offset == </span>
<span style="color: #098658">4</span>
<span style="color: #000000">);</span></span>
<span class="line"><span style="color: #000000">  </span>
<span style="color: #AF00DB">if</span>
<span style="color: #000000"> (!is_write && offset == </span>
<span style="color: #098658">4</span>
<span style="color: #000000">) {</span></span>
<span class="line"><span style="color: #000000">    </span>
<span style="color: #0000FF">uint64_t</span>
<span style="color: #000000"> us = </span>
<span style="color: #795E26">get_time</span>
<span style="color: #000000">();</span></span>
<span class="line"><span style="color: #000000">    </span>
<span style="color: #001080">rtc_port_base</span>
<span style="color: #000000">[</span>
<span style="color: #098658">0</span>
<span style="color: #000000">] = (</span>
<span style="color: #0000FF">uint32_t</span>
<span style="color: #000000">)us;</span></span>
<span class="line"><span style="color: #000000">    </span>
<span style="color: #001080">rtc_port_base</span>
<span style="color: #000000">[</span>
<span style="color: #098658">1</span>
<span style="color: #000000">] = us >> </span>
<span style="color: #098658">32</span>
<span style="color: #000000">;</span></span>
<span class="line"><span style="color: #000000">    </span>
<span style="color: #795E26">printf</span>
<span style="color: #000000">(</span>
<span style="color: #A31515">"Timer:</span>
<span style="color: #001080">%u</span>
<span style="color: #A31515">,</span>
<span style="color: #001080">%u</span>
<span style="color: #EE0000">\n</span>
<span style="color: #A31515">"</span>
<span style="color: #000000">,</span>
<span style="color: #001080">rtc_port_base</span>
<span style="color: #000000">[</span>
<span style="color: #098658">1</span>
<span style="color: #000000">],</span>
<span style="color: #001080">rtc_port_base</span>
<span style="color: #000000">[</span>
<span style="color: #098658">0</span>
<span style="color: #000000">]);</span></span>
<span class="line"><span style="color: #000000">  }</span></span>
<span class="line"><span style="color: #000000">}</span></span>

代码第3行表示读高地址时才更新时间，这样的问题是如果只读时间的低32位则时间就永远是初始时间，去掉地址的判断，把第3行改成下面的代码

...
if(!is_write)
...

<span class="line"><span style="color: #000000">...</span></span>
<span class="line"><span style="color: #AF00DB">if</span>
<span style="color: #000000">(!is_write)</span></span>
<span class="line"><span style="color: #000000">...</span></span>

系统就可以正常工作了。这里其实我早就注意到了，对timer工作方式也理解了，但是竟然一直没觉得这样是不对的。

总结一下am-kernels中的程序或者这些benchmark是怎么运行的。首先am-kernels中的程序可以称之为应用程序，这些程序是用C语言写的，用riscv工具链编译成可执行程序后需要一个“机器”来执行这些程序（当然因为没有操作系统，实际上执行的是bin文件）。NEMU就是执行这个程序的机器，NPC也是执行这个而程序的机器。一开始我觉得有点乱是因为NEMU和应用程序都是C语言，到底怎么执行起来的？NEMU是C语言编写的模拟器，编译成可执行文件后在Linux机器上执行，也就是在Ubuntu系统上执行。NEMU程序读取riscv工具编译出的应用程序bin文件然后执行bin文件里的指令。NEMU的好处是这个机器内部对我们完全是透明的，如果我们要debug应用程序，可以让NEMU进入单步执行模式，通过前面实现的各种基础设施可以获取当前NEMU的状态。如果我们要debug NEMU，就要用Linux系统上的gdb等工具。

NEMU里的代码是用来模拟硬件的，包括timer。因为不可能一直更新时间，所以Timer更新时间采用了回调函数的方式：如果用户程序读取timer的寄存器，则这个读取动作会调用timer里的更新函数更新timer的时间寄存器然后返回。这部分代码如下

nemu/src/device/io/map.c

word_t map_read(paddr_t addr, int len, IOMap *map) {
  assert(len >= 1 && len low;
  invoke_callback(map->callback, offset, len, false); // prepare data to read
  word_t ret = host_read(map->space + offset, len);
  return ret;
}

<span class="line"><span style="color: #267F99">word_t</span>
<span style="color: #000000"> </span>
<span style="color: #795E26">map_read</span>
<span style="color: #000000">(</span>
<span style="color: #267F99">paddr_t</span>
<span style="color: #000000"> </span>
<span style="color: #001080">addr</span>
<span style="color: #000000">, </span>
<span style="color: #0000FF">int</span>
<span style="color: #000000"> </span>
<span style="color: #001080">len</span>
<span style="color: #000000">, IOMap *</span>
<span style="color: #001080">map</span>
<span style="color: #000000">) {</span></span>
<span class="line"><span style="color: #000000">  </span>
<span style="color: #795E26">assert</span>
<span style="color: #000000">(len >= </span>
<span style="color: #098658">1</span>
<span style="color: #000000"> && len <= </span>
<span style="color: #098658">8</span>
<span style="color: #000000">);</span></span>
<span class="line"><span style="color: #000000">  </span>
<span style="color: #795E26">check_bound</span>
<span style="color: #000000">(map, addr);</span></span>
<span class="line"><span style="color: #000000">  </span>
<span style="color: #267F99">paddr_t</span>
<span style="color: #000000"> offset = addr - </span>
<span style="color: #001080">map</span>
<span style="color: #000000">-></span>
<span style="color: #001080">low</span>
<span style="color: #000000">;</span></span>
<span class="line"><span style="color: #000000">  </span>
<span style="color: #795E26">invoke_callback</span>
<span style="color: #000000">(</span>
<span style="color: #001080">map</span>
<span style="color: #000000">-></span>
<span style="color: #001080">callback</span>
<span style="color: #000000">, offset, len, </span>
<span style="color: #0000FF">false</span>
<span style="color: #000000">);</span>
<span style="color: #008000"> // prepare data to read</span></span>
<span class="line"><span style="color: #000000">  </span>
<span style="color: #267F99">word_t</span>
<span style="color: #000000"> ret = </span>
<span style="color: #795E26">host_read</span>
<span style="color: #000000">(</span>
<span style="color: #001080">map</span>
<span style="color: #000000">-></span>
<span style="color: #001080">space</span>
<span style="color: #000000"> + offset, len);</span></span>
<span class="line"><span style="color: #000000">  </span>
<span style="color: #AF00DB">return</span>
<span style="color: #000000"> ret;</span></span>
<span class="line"><span style="color: #000000">}</span></span>

NEMU将用户程序译码之后，如果涉及内存操作则会调用vaddr_read和vaddr_write，而这两个函数会分别调用paddr_read和paddr_write，如果生成NEMU时开启了添加Device选项，则paddr_read和paddr_write会调用mmio_read和mmio_write，mmio_read和mmio_write最终调用的就是map_read和map_write。

尝试运行demo的程序，出现了取指令错误的问题