Re: [Etherboot-developers] NetBSD ELF/a.out support

[<ebi...@ln...>]

Wojciech Puchar <wo...@te...> writes:

> anyone working on it?
> 
> how can i help?
> 
> all i found is that kernel should be loaded beginning from
> LMA(VMA)-c0000000 (which is 1MB in all kernels i looked at from version
> 1.3.3 to 1.6)
> 
> Idx Name          Size      VMA       LMA       File off  Algn
>   0 .text         0014df54  c0100000  c0100000  00001000  2**2
>                   CONTENTS, ALLOC, LOAD, READONLY, CODE
>   1 .rodata       000287a1  c024df60  c024df60  0014ef60  2**5
>                   CONTENTS, ALLOC, LOAD, READONLY, DATA
>   2 .data         0001acf0  c0277720  c0277720  00177720  2**5
>                   CONTENTS, ALLOC, LOAD, DATA
>   3 .bss          0003e1fc  c0292420  c0292420  00192420  2**5
> 
> 
> kernel should be started in flat mode with paging off, given parameters on
> stack with C standard:
> 
> (howto, [bootdev], bootinfo, esym, basemem, extmem)
> 
> howto is (2 if single user)|(1 if ask for boot device)|(4 if start
> debugger)
> 
> it could be always 0 for etherboot
> 
> bootdev should be 0 for new kernels - not used.
> 
> esym is used for debugger if kernel symbols are loaded to memory. it's
> probably offset from kernel code to beginning of symbols, 0 if no symbols.
> 
> bootinfo is a pointer to bootinfo structure. first 4 bytes says how many
> elements it has, then there are elements (like boot console info, bios
> disk geometry etc.) - all well defined in NetBSD's bootinfo.h
> 
> 
> how can i help to support it (my assembly isn't best right now)?
> 
> tried using FreeBSD support in etherboot - kernel is loaded fine then
> hangs.
> 
> 
> NetBSD already has ROM files but for only few cards and needs 32K ROMs,
> and
> pxeboot.bin that works for any PXE capable cards.
> 
> anyway having this in etherboot will give user a flexibility to run any OS
> without reprogramming EPROM

We have that:

There are 2 ways to go.  
Either small modifications to NetBSD, or write a mkelf-BSD.

- Alternative 1 (tweaking BSD).

Modify the linker script so the LMA is 0x100000.  Once the physical
addresses are correct etherboot will load your ELF executable into
memory just fine.

The ELF loader in 5.1.x with no special options passes a pointer
to a structure using the standard C calling conventions.  After
the header the structure contains an array of tagged records, allowing
etherboot or whomever use the format to pass whatever values make
sense.

The structure has a magic number at the start, and a checksum so
you can test to see if you were actually passed the appropriate
structure.  It is fairly well defined in etherboot's elf_boot.h
The memory size information along with the dhcp options is present
there.  And this same format (with different tagged values is used on
multiple architectures).

So with a little extra code to parse the etherboot's elf boot
structure, and a small tweak to your linker script you can have
native support of etherboot.

- Alternative 2 (writing a converter).

You can write a little stub piece of code that reads etherboot's
elf boot structure and converts it into a form that NetBSD can
use today.

In addition this requires a stub program to glue the converter
code and the NetBSD kernel together into one image.  Examples
are out there for several other formats.  See mknbi/mkelf and
mkelfImage.

I hope this helps,

And my apologies for not sending this off earlier I got distracted
just before I hit send.

Eric