Re: [patches] [PATCH] [21/22] x86_64: Extend bzImage protocol for relocatable bzImage

[Vivek Goyal <vgoyal@in.ibm.com>]

On Wed, May 02, 2007 at 02:59:11PM -0700, H. Peter Anvin wrote:
> Jeremy Fitzhardinge wrote:
> > 
> > So the bzImage structure is currently:
> > 
> >    1. old-style boot sector
> >    2. old-style boot info, followed by 0xaa55 at the end of the sector
> >    3. the HdrS boot param block
> >    4. setup.S boot code
> >    5. the self-decompressing kernel
> > 
> > If we make 5 actually an ELF file, containing properly formed Ehdr,
> > Phdrs (for all the mappings required), and the actual kernel
> > decompressor, relocator and compressed kernel data, then it would be
> > easy for the Xen domain builder to find that and use it as a basis for
> > loading.  I think it would just require the bzImage boot param block to
> > contain an offset of the start of the ELF file.  The contents of the ELF
> > file would be in a form where the normal boot code could just jump over
> > the ELF headers, directly into the segment data itself.
> > 
> > ie:
> > 
> >    1. old-style boot sector
> >    2. old-style boot info, followed by 0xaa55 at the end of the sector
> >    3. the HdrS boot param block
> >    4. setup.S boot code (jumps directly into 5.3)
> >    5. 32-bit self-decompressing kernel:
> >          1. Ehdr
> >          2. Phdrs for all necessary mappings
> >          3. decompressor/relocator .text
> >          4. compressed kernel data
> > 
> > Does that sound reasonable?
> > 
> 
> I don't know if that would break any programs that are currently
> bypassing the setup.

I think kexec bzImage loader will break. It bypasses the setup code and
directly jumps to the code present after setup sectors(decompressor).

> The existing setup protocol definitely allows
> invoking an entry point which isn't 0x100000 (rather, the 32-bit
> entrypoint is defined by code32_start); I'm not sure how Eric's
> relocatable kernel patches (2.05 protocol) affect that, mostly because I
> haven't seen any boot loaders which actually use it so I can't comment
> on what their code looks like.

With relocatable patches, if a boot loader decides to load protected mode
component at non-1MB address, then it shall have to modify code32_start to
reflect the new location of protected mode code.

Thanks
Vivek