Split QEMU run, automate GDB debugging

README.md

@@ -1,9 +1,10 @@
 # Linux Kernel Module Cheat
 
-Run one command, get into QEMU Buildroot BusyBox with several minimal Linux kernel 4.9 module example tutorials. Tested in Ubuntu 14.04.
+Run one command, get into QEMU Buildroot BusyBox with several minimal Linux kernel 4.9 module example tutorials. Tested in Ubuntu 14.04 host.
 
 Usage:
 
+    sudo apt-get install qemu
     ./run
 
 First build will take a while (GCC, Linux kernel).
@@ -22,19 +23,72 @@ Each module comes from a C file under `kernel_module/`. For module usage do:
 
     grep BR2_LINUX_KERNEL_VERSION buildroot/.config
 
+After the first build, you can also run just:
+
+    ./runqemu
+
+to save a few seconds. `./run` wouldn't rebuild everything, but checking timestamps takes a few moments.
+
 We use `printk` a lot, and it shows on the QEMU terminal by default. If that annoys you (e.g. you want to see stdout separately), do:
 
     dmesg -n 1
 
 See also: <https://superuser.com/questions/351387/how-to-stop-kernel-messages-from-flooding-my-console>
 
-When your kernel starts crashing, get the full trace with:
+## Debugging
+
+GDB the linux kernel:
+
+    ./runqemu -d
+
+If you want to break immediately at a symbol, e.g. `start_kernel` of the boot sequence, open another terminal and run for example:
+
+    ./rungdb start_kernel
+
+Now QEMU will stop there, and you can use the normal GDB commands:
+
+    l
+    n
+    c
+
+To skip the boot, run just:
+
+    ./rungdb
+
+and when you want to break, do `Ctrl + C` from GDB.
+
+To have some fun, you can first run inside QEMU:
+
+    /count.sh
+
+which counts to infinity to stdout, and then:
+
+    Ctrl + C
+    break sys_write
+
+And now you can control the counting from GDB:
+
+    continue
+    continue
+    continue
+
+## Text mode
+
+Show serial output of QEMU directly on the current terminal, without opening a QEMU window:
 
     ./run -n
 
+To exit, just do a regular:
+
+    poweroff
+
+This is particularly useful to get full panic traces when you start making the kernel crashing :-)
+
+In case of a panic, you want your terminal back with `Ctrl + C, A` and type `quit`. See also: <http://stackoverflow.com/questions/14165158/how-to-switch-to-qemu-monitor-console-when-running-with-curses>
+
 See also: <https://unix.stackexchange.com/questions/208260/how-to-scroll-up-after-a-kernel-panic>
 
-Then if you want your terminal back, hit `Ctrl + C, A` and type `quit`. See also: <http://stackoverflow.com/questions/14165158/how-to-switch-to-qemu-monitor-console-when-running-with-curses>
+## Table of contents
 
 1.  [Introduction](introduction.md)
 1.  [Build](build.md)