[MIT-6.S081/Fall2020] 实验笔记零 Lab0: Preparation

写在前面的话

为了更好地理解RISC-V指令集，也为了熟悉QEMU，所以选择刷一遍MIT-6.S081的实验。希望不仅仅以做Lab为目的，同时也精读几篇OS领域经典文章。

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可能好多人都卡在环境准备上，但后来我发现：趁早动手做实验，比准备环境更重要~

环境搭建

参考 6.S081 官方的配置教程：Tools

本人用的环境是基于Qemu(KVM)上的Ubuntu 20.04。

首先安装RISC-V工具链、GDB-Multiarch、QEMU：

sudo apt-get install git build-essential gdb-multiarch qemu-system-misc gcc-riscv64-linux-gnu  binutils-riscv64-linux-gnu gcc-riscv64-unknown-elf

# fix qemu
sudo apt-get remove qemu-system-misc
sudo apt-get install qemu-system-misc=1:4.2-3ubuntu6

检查一遍自己的环境

$ riscv64-linux-gnu-gcc --version
riscv64-linux-gnu-gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ qemu-system-riscv64 --version
QEMU emulator version 4.2.0 (Debian 1:4.2-3ubuntu6)
Copyright (c) 2003-2019 Fabrice Bellard and the QEMU Project developers

$ riscv64-unknown-elf-gcc --version
riscv64-unknown-elf-gcc () 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

测试环境

能在QEMU中启动xv6，环境就算准备完了，可以动手做实验了~

$ git clone git://g.csail.mit.edu/xv6-labs-2020
Cloning into 'xv6-labs-2020'...
...
$ cd xv6-labs-2020
$ git checkout util
Branch 'util' set up to track remote branch 'util' from 'origin'.
Switched to a new branch 'util'

$ make qemu
···
xv6 kernel is booting

hart 2 starting
hart 1 starting
init: starting sh
$ ls
.              1 1 1024
..             1 1 1024
README         2 2 2059
xargstest.sh   2 3 93
cat            2 4 24256
echo           2 5 23080
(...other files)

退出 QEMU 快捷键是 Ctrl-a + x

好了，我们现在可以动手做实验了。

如何在 QEMU 中使用 gdb

1. 首先用 tmux 开两个终端窗口。

2. 第一个窗口键入make qemu-gdb.

   $ make qemu-gdb
   *** Now run 'gdb' in another window. qemu-system-riscv64 -machine virt -bios none -kernel kernel/kernel -m 128M -smp 3 -nographic -drive file=fs.img,if=none,format=raw,id=x0 -device virtio-blk-device,drive=x0,bus=virtio-mmio-bus.0 -S -gdb tcp::26000

3. 在第二个窗口中:
   3.1. 修改自己 home 目录下的 .gdbinit 文件，允许 gdb 在xv6-labs-2020这个目录启动的时候，加载该文件夹下的 .gdbinit 文件。

   echo "add-auto-load-safe-path $(pwd)/.gdbinit " >> ~/.gdbinit

   3.2. 启动 gdb-multiarch

   $ gdb-multiarch      
   GNU gdb (Ubuntu 9.2-0ubuntu1~20.04) 9.2
   Copyright (C) 2020 Free Software Foundation, Inc.
   License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.ht
   ml>
   This is free software: you are free to change and redistribute it.
   There is NO WARRANTY, to the extent permitted by law.
   Type "show copying" and "show warranty" for details.
   This GDB was configured as "x86_64-linux-gnu".
   Type "show configuration" for configuration details.
   For bug reporting instructions, please see:
   <http://www.gnu.org/software/gdb/bugs/>.
   Find the GDB manual and other documentation resources online at:
       <http://www.gnu.org/software/gdb/documentation/>.
   
   For help, type "help".
   Type "apropos word" to search for commands related to "word".
   The target architecture is assumed to be riscv:rv64
   warning: No executable has been specified and target does not support
   determining executable automatically.  Try using the "file" command.
   0x0000000000001000 in ?? ()
   (gdb)

   3.3 给代码打断点

   • file user/_ls
   • b main
   • c
   • 之后就是正常的GDB 使用流程了

   第二个窗口：

   (gdb) file user/_ls
   Reading symbols from user/_ls...
   (gdb) b main
   Breakpoint 1 at 0x27a: file user/ls.c, line 75.
   (gdb) c
   Continuing.
   [Switching to Thread 1.3]
   
   Thread 3 hit Breakpoint 1, main (argc=0, argv=0x6c <fmtname+108>)
       at user/ls.c:75
   75      {
   (gdb)

   第一个窗口

   (......)
   xv6 kernel is booting                                        
                                                                   
   hart 1 starting                                                  
   hart 2 starting                                                  
   init: starting sh                                                
   $ ls
   (等待 gdb 发送 continue 信号)

   

4. 之后我们就可以正常地通过 GDB 调试程序了~