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

[Goswin von Brederlow <goswin-v-b@web.de>]

Paul Brook <paul@codesourcery.com> writes:

>> > 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.

Is it? I tried for a while and couldn't figure it out. I checked how
linux does it and it does quite a dance to achieve it.

> 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

That will happen in multiboot2 format and is a much larger patch,
including a complete new boot rom actualy. I do have patches for that
too but there aren't complete yet. I only support the bare minimum of
the multiboot2 specs so far.

MfG
        Goswin