linux-kernel - Re: [PATCH 0/5] Boot protocol changes

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Date:	Tue, 02 Oct 2007 17:43:44 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Vivek Goyal <vgoyal@...ibm.com>, lguest <lguest@...abs.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH 0/5] Boot protocol changes

Jeremy Fitzhardinge wrote:
> H. Peter Anvin wrote:
>>> This series looks like a good start for Xen, but we still need to work
>>> out where to stash the metadata which normally lives in ELF notes. 
>>> Using ELF is convenient for Xen because it lets a large chunk of domain
>>> builder code be reused; on the other hand, loading a plain bzImage is
>>> pretty simple, so maybe it isn't such a big deal.
>>>
>>> HPA, Eric: if we don't go the "embed ELF" path, where's a good
>>> backwards-compatible place to stash the note data?  If we do go with
>>> "embed ELF", how should we go about doing it?  Arrange to put the ELF
>>> headers before the 1M mark?
>>>
>> This sounds like another good reason to do the ELF image as the
>> postcompression image.  The interface to the embedded compression
>> routine is then unchanged, and we get the "full vmlinux" with any
>> notes that belongs there.
>>
>> I'll try to get an implementation of that done -- it really shouldn't
>> be very hard.
> 
> Please explain what you're proposing again, because my memory of your
> plan from last time wouldn't help in this case.  Are you proposing that
> the bzImage contains compressed data that its expecting the bootloader
> to decompress?  Won't that completely break backwards compatibility?  If
> we don't care about backwards compatibility with old bootloaders, then
> it doesn't matter what we do one way or the other.
> 

No, not at all.

I'm proposing that the existing bzImage format be retained, but that the 
payload of the decompressor (already a gzip file) simply be vmlinux.gz 
-- i.e. a gzip compressed ELF file, notes and all.  A pointer in the 
header will point to the offset of the payload (this is new, obviously.)

The decompression stub is adjusted to expect an ELF image, instead of a 
raw binary.

Existing bootloaders (16- or 32-bit) simply load the bzImage the way 
they do now; new bootloaders have the option of accessing the vmlinux.gz 
directly if they either want to load it themselves or want to examine 
the notes.

	-hpa

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