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

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Date:	Tue, 8 Aug 2006 16:58:52 +0900
From:	Horms <horms@...ge.net.au>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, vgoyal@...ibm.com,
	fastboot@...l.org, linux-kernel@...r.kernel.org,
	Jan Kratochvil <lace@...kratochvil.net>,
	Magnus Damm <magnus.damm@...il.com>,
	Linda Wang <lwang@...hat.com>
Subject: Re: [RFC] ELF Relocatable x86 and x86_64 bzImages

On Tue, Aug 08, 2006 at 01:23:15AM -0600, Eric W. Biederman wrote:
> Horms <horms@...ge.net.au> writes:
> 
> > On Mon, Aug 07, 2006 at 09:32:23PM -0700, H. Peter Anvin wrote:
> >> Horms wrote:
> >> >
> >> >I also agree that it is non-intitive. But I wonder if a cleaner
> >> >fix would be to remove CONFIG_PHYSICAL_START all together. Isn't
> >> >it just a work around for the kernel not being relocatable, or
> >> >are there uses for it that relocation can't replace?
> >> >
> >> 
> >> Yes, booting with the 2^n existing bootloaders.
> >
> > Ok, I must be confused then. I though CONFIG_PHYSICAL_START was
> > introduced in order to allow an alternative address to be provided for
> > kdump, and that previously it was hard-coded to some
> > architecture-specific value.
> >
> > What I was really getting as is if it needs to be configurable at
> > compile time or not. Obviously there needs to be some sane default
> > regardless.
> 
> CONFIG_PHYSICAL_START has had 2 uses.
> 1) To allow a kernel to run a completely different address for use
>    with kexec on panic.
> 2) To allow the kernel to be better aligned for better performance.

Thanks for making that clear

> For maintenance reasons I propose we introduce CONFIG_PHYSICAL_ALIGN.
> Which will round our load address up to the nearest aligned address
> and run the kernel there.  That is roughly what I am doing on x86_64
> at this point.
> 
> s/CONFIG_PHYSICAL_START/CONFIG_PHYSICAL_ALIGN/ gives me well defined
> behavior and allows the alignment optimization without getting into 
> weird semantics.
> 
> Before CONFIG_PHYSICAL_START we _always_ ran the arch/i386 kernel
> where it was loaded and I assumed we always would.  Since people have
> realized better aligned kernels can run better this assumption became
> false.  Going to CONFIG_PHYSICAL_ALIGN allows us to return to the
> simple assumption of always running the kernel where it is loaded
> modulo a little extra alignment.

That sounds reasonable to me. Though it is a little less flexible,
do you think that could be a problem? Perhaps we could have both,
though that would probably be quite confusing.

-- 
Horms
  H: http://www.vergenet.net/~horms/
  W: http://www.valinux.co.jp/en/

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