How to solve qemu gdb debug error: Remote 'g' packet reply is too long?

The reason for all the issues is that you are compiling 64-bit code and running it in 32-bit protected mode. 64-bit code will not run in that environment properly. Oddly enough it often manifests itself at first when trying to write to the video display. Often things will print but not quite the way you want. Improperly decoded instructions will cause the debugger to work erratically as you observed.

One way to fix your problem is to compile and link the kernel as a 32-bit executable. You are using a 64-bit compiler so you'll need to add -m32 CFLAGS (or your GCC command line). if using LD to link -melf_i386 will be needed. Assembling with NASM should be -felf32 rather than -felf64.

Alternatively you would have to place the processor into 64-bit long mode in the bootloader. You can read more about that process on the OSDev wiki.

If debugging 32-bit code you will probably want to use qemu-system-i386 . You will have fewer hassles.

Connect and disconnect

I got it working as detailed at: How to debug the Linux kernel with GDB and QEMU?

The key thing was connect and disconnect on GDB as:

gdb \
    -ex "add-auto-load-safe-path $(pwd)" \
    -ex "file vmlinux" \
    -ex 'set arch i386:x86-64:intel' \
    -ex 'target remote localhost:1234' \
    -ex 'break start_kernel' \
    -ex 'continue' \
    -ex 'disconnect' \
    -ex 'set arch i386:x86-64' \
    -ex 'target remote localhost:1234'

Related: Remote 'g' packet reply is too long