Re: [Qemu-devel] Support for multiboot images in elf64 (EM_X86_64) format

[Paul Brook <paul@codesourcery.com>]

> > starting your own toy kernel is a fun thing to do and there are many
> > tutorials out there on how to do it. Unfortunately when one wants to
> > write a kernel in 64bit it becomes much harder because one can't
> > compile 64bit code as elf32 image and converting a elf64 image to
> > elf32 format is a major hassle and looses debug information and symbols.

So just have two versions of your image:
- The elf64 image that has debug info, symbols, etc.  Point gdb at this.
- An elf32 image that you give to the bootloader (in this case kvm)

Generating the latter from the former is a trivial objcopy invocation.
The bootloader variant only needs enough information to get the loadable 
sections into memory.  We don't care about non-resident clutter like debug 
info or symbols.  Anything that cares about those will be using the full elf64 
image.

This is all standard practice.  I don't think I've never actually used a 
system where the image loaded by the target is the same file as the one that 
comes out of the linker and is used by the debugger.

> Yes, this is why I asked about support in other software.  If each
> bootloader implements a different custom method then it will be a pain
> to run your binary on real hardware in the future.  It's worth at
> least checking their source first - maybe there is already a similar
> code path that we can be compatible with, hence making life easier for
> developers who want to play with 64-bit payloads in multiboot
> executables.

The whole idea of entering a 64-bit image in 32-bit mode seems distinctly 
sketchy.  Surely it'd make more sense to define a 64-bit multiboot variant and 
do the job properly.

Paul