docs/develop: Add doc on debugging bootloaders with gef

Change-Id: Ib2d161adf75904f592286451279c77fb61ea52c2

docs/develop/kernel/boot/Debugging_Bootloaders_GEF.md

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+# Bootloader debugging with GEF
+
+When Haiku's early boot process is experiencing unknown crashes or faults, it can be extremely
+difficult to troubleshoot (especially when serial, video, or other i/o devices are non-functional)
+
+It **is** possible to step through the boot of any architecture of Haiku in a debugger if the system
+boots and the issue can be reproduced in qemu.
+
+> This works for any architecture and is _extremely_ helpful to trouble early platforms. Linux or Mac OS
+> are requirements. You need a full POSIX environment.
+
+## Building Haiku
+
+On most non-x86 platforms, you will need a "kernel" (haiku_loader) and an "initrd" (haiku_floppyboot).
+
+For arm/arm64: ```jam -q @minimum-mmc```
+
+## Launching Haiku in QEMU
+
+In the example below, we will prepare Haiku arm in QEMU for debugging.
+
+```
+qemu-system-arm -M raspi2 -kernel haiku_loader.u-boot -initrd haiku-floppyboot.tgz.u-boot -serial stdio -m 2G -dtb rpi2.dtb -s -S
+```
+
+**Key Flags:**
+
+  * **-s**
+    * Shorthand for -gdb tcp::1234, i.e. open a gdbserver on TCP port 1234.
+  * **-S**
+    * Do not start CPU at startup (you must type 'c' in the monitor).
+
+These simple flags will make qemu listen for a debugger connection on localhost:1234 and have the VM not start until you tell it to.
+
+> In the example above, we are Emulating a Raspberry Pi 2, and using our Raspberry Pi 2 dtb. If you don't have a dtb for the machine
+> you're emulating, you can dump qemu's internal dtb by adding ```-M dumpdtb=myboard.dtb``` to the end of your qemu command.
+
+## Attaching GEF
+
+[GEF](https://github.com/hugsy/gef) is an enhanced debugger which works extremely well for debugging code running in virtual machines.
+It piggy-backs on gdb and offers a lot of valueable insight at a glance without requiring to know every gdb command.
+
+Once GEF is installed, we can step through the process to attach gdb to qemu.
+
+### Open gdb with our symbols.
+
+First we run gdb pointed at our boot loader.  We use the native ELF binary as that seems to give gdb/gef the most accurate knowledge
+of our symbols. (the haiku_loader.u-boot is wrapped by u-boot's mkimage, your milage may vary based on platform)
+
+```gdb objects/haiku/arm/release/system/boot/u-boot/boot_loader_u-boot```
+
+### Set the architecture
+
+This may not be required, but re-enforces to gef/gdb that we're working on arm.
+
+```set architecture arm```
+
+### Connect to QEMU
+
+Now we tell gdb/gef about out running (but paused) QEMU instance.
+
+```gef-remote -q localhost:1234```
+
+A successful connection should occur.
+
+### Step into debugging
+
+Before you begin execution, it's handy to set a *breakpoint*. A *breakpoint* tells gdb/gef where it should pause execution to begin
+the debugging process. All of our bootloaders start in a ```start_gen``` function, so this is a good place to start.
+
+```breakpoint start_gen```
+
+Now that a breakpoint is defined, lets run the virtual machine.
+
+In gef, type ```continue```.
+
+If everything is working as expected, you should now be "paused" at the ```start_gen``` function (hopefully showing the C/C++ code).
+
+Now, you have a few commands to leverage:
+
+  * **step**
+    * Take a single step forward and execute the code listed.
+    * Does **not** step "into" functions, just over them getting the return from the code.
+    * Alias: s
+  * **stepi**
+    * step forward "into" the next code.
+    * If you're on a function it will enter the function and show the code executed.
+  * **break**
+    * add additional "breakpoints" where you can step through the code execution.
+  * **continue**
+    * Resume execution.
+    * If you have no additional breakpoints the code will "go do what it's supposed to"
+    * Alias: c