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关于在bochs的gdb-stub中加入对虚拟内存访问的支持

最近操统课需要用到bochs这个模拟器，我用的2.3.5. 但是实际出现了以个问题，就是gdb-stub这个功能不大正常，运行gdb后可以正常下断点，但是单步进可以，单步过却和单步进相同，这个问题不大比较恶心的是无法显示局部变量的值，总是显示0xffffffff
　　 　　 　 　　 　　 　　　 　 　　　　
google了半天没看到一点有用的。。。。。唉，只好自己搞
　　 　　 　 　　 　　 　　　 　 　　　　
参阅了gdb protocol后我修改了bochs的gdbstub.cc，让其显示出与gdb交互的内容，大致了解了内存这部分的步骤，主要是gdb给server(即bochs)发一个内存读取的请求，格式为m addr,length, 然后server返回十六进制的数据
　　 　　 　 　　 　　 　　　 　 　　　　
在bochs的gdbstub.cc 558行为处理这个指令的，不难看懂，我修改了它，让它输出access_linear的返回值，然后再调试，查看局部变量，果然valid为0，进一步，到access_linear, 经过痛苦地多次调试，发现问题处在valid=BX_MEM(0)->dgb_fetch_mem...一行，最后进到或则个函数，在memory/misc_mem.cc 512行下看到了这部分，原来是bochs发现读取的地址越界，就用0xff填充，然后返回0， 我修改了0xff，然后用gdb调试时证实了这一点。
　　 　　 　 　　 　　 　　　 　 　　　　
最后往上看，发现是addr比内存的长度还要大， 加入调试输出后发现gdb读的地址是0xf010
9fe0左右，而BX_MEM_THIS len仅有2000000，这显然会越界。
　　 　　 　 　　 　　 　　　 　 　　　　
起初我考虑也许是那里关于内存大小的设置没有弄好，但没有找到具体位置，但后来发现其实那BX_MEM_THIS len 是在.bochsrc里有定义的,0x2000000=2^25=32k，而那个0xf0109fe0是内核做的虚拟内存，gdb只跟bochs打交道，而虚拟内存是内核弄的，虚拟机又管不着。因此好像没什么好办法。。。

之后我查了很多关于虚拟内存的资料，发现其实是cpu直接支持的，于是我看到了希望。仍是从bochs的源码开刀。

一开始我是把目光放在了gdt和ldt上，但是未果。最后又发现了它里面的CPU类有一个get_segment_base函数，几经尝试，找到了临时解决方法：

修改bochs的gdbstub.cc
将567行附近的
access_linear(addr,len,BX_READ,mem);
改成
access_linear((BX_CPU(0)->get_segment_base(BX_SEG_REG_CS))+addr,len,BX_READ,mem)
;
然后重新编译，记得是用gdb stub版本(./configure --enable-gdb-stub)

思路就是在gdb服务器端自动对虚拟内存作个映射。目前发现常用的关于断点，堆栈，数据，单步等相关命令都基本正常（不过好像有同学说w命令不好使，我没确认）。

不知道这个算不算bochs的一个bug, 跟它反映了一下，至今没有回应。

不过这还是第一次半系统地阅读了一个开源软件的代码，另外涉及到gdb协议，虚拟内存，cpu等知识，混在一起，最后还弄成功了，实在很过瘾。
　　
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