linux-kernel - Re: [RFC] ELF Relocatable x86 and x86

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Message-ID: <aec7e5c30608020134h2d0f9955p34a0cd76d8836acd@mail.gmail.com>
Date:	Wed, 2 Aug 2006 17:34:44 +0900
From:	"Magnus Damm" <magnus.damm@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	fastboot@...l.org, linux-kernel@...r.kernel.org,
	Horms <horms@...ge.net.au>,
	"Jan Kratochvil" <lace@...kratochvil.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Vivek Goyal" <vgoyal@...ibm.com>, "Linda Wang" <lwang@...hat.com>
Subject: Re: [RFC] ELF Relocatable x86 and x86_64 bzImages

On 8/2/06, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> "Magnus Damm" <magnus.damm@...il.com> writes:
> > Eric, could you please list the advantages of your run-time relocation
> > code over my incomplete relocate-in-userspace prototype posted to
> > fastboot a few weeks ago?
>
> If you watch an architecture evolve one thing you will notice is that
> the kinds of relocations keep growing.  An ever growing list of things
> to for the bootloader to do is a pain.  Especially when bootloaders
> generally need to be as simple and as fixed as possible because bootloaders
> are not something you generally want to update.

I agree that updating bootloaders is something you want to avoid. I'm
not however sure that I would call kexec-tools a bootloader...

> Beyond that if you look at head.S the code to process the relocations
> (after I have finished post processing them at build time) is 9 instructions.
> Which is absolutely trivial, at least for now.

Yeah, but the 33 patches are touching more than 9 instructions. =)

> By keeping the bzImage processing the relocations we have kept the
> bootloader/kernel interface simple.

Agreed. I think your patch makes sense.

> > One thing I know for sure is that your implementation supports bzImage
> > while my only supports relocation of vmlinux files. Are there any
> > other uses for relocatable bzImage except kdump?
>
> I can't think of any volume users.  A hypervisor that would actually report
> the real physical addresses would be a candidate.  It's a general purpose
> facility so if it is interesting users will show up.  Static
> relocation has already found another use on x86_64.
>
> There are definitely users of an ELF bzImage beyond the kdump case.
> Anything that doesn't have a traditional 16bit BIOS on it.  LinuxBIOS,
> and Xen, and some others.
>
> Not having to keep track of anything but your bzImage to boot is also
> a serious advantage.  It's the one binary to rule them all. :)

One binary to rule them all... If that is true, is there any simple
way then to extract vmlinux from the bzImage?

Thanks!

/ magnus
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