Ideas to Improve GDB Stub in Qemu for i8086

[Davidson Francis <davidsondfgl@gmail.com>]

Hi,
Some time ago, I wrote a Gist [1] outlining what I believe to be the 
ideal environment for debugging 16-bit code in real mode on Qemu. Based 
on the feedback I've received, I decided to share it here to gather more 
opinions.

It is commonly known that Qemu does not handle real/16-bit mode well with 
GDB. To work around this, there are various hacks available, most of 
which involve GDB scripts to make debugging less painful.

Basically, my idea involves two small and specific changes to the GDB 
stub:
1) Returning the correct CPU mode as "i8086" or "i386" depending on the 
current CPU mode (on x86_gdb_arch_name), instead of always returning 
"i386".

2) Translating segmented memory to linear if in real mode, i.e., 
returning CS*0x10+EIP instead of just EIP, and similarly for ESP, etc.

Originally, I considered submitting a patch directly, but I wanted to get 
your thoughts first. I understand that Bochs and similar tools have 
sufficiently good debuggers, but it would be interesting if Qemu had some 
improvements in this area.

I also acknowledge that my solution might not help in all scenarios, such 
as CPU mode switches, so its effectiveness would be limited.

Ref:
[1]: https://gist.github.com/Theldus/4e1efc07ec13fb84fa10c2f3d054dccd

Kind regards,
Davidson Francis.