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

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Message-ID: <463909AF.9040205@zytor.com>
Date:	Wed, 02 May 2007 14:59:11 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Gerd Hoffmann <kraxel@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jeff Garzik <jeff@...zik.org>, patches@...-64.org,
	linux-kernel@...r.kernel.org, Vivek Goyal <vgoyal@...ibm.com>,
	virtualization <virtualization@...ts.linux-foundation.org>
Subject: Re: [patches] [PATCH] [21/22] x86_64: Extend bzImage protocol for
 relocatable bzImage

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.  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.

	-hpa
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