linux-kernel - Re: 2.6.21-rc5-mm3 - no boot, "address not 2M aligned"

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Message-ID: <m1648eg01d.fsf@ebiederm.dsl.xmission.com>
Date:	Mon, 02 Apr 2007 11:26:38 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	vgoyal@...ibm.com
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jurriaan <thunder7@...all.nl>,
	Helge Hafting <helgehaf@...el.hist.no>,
	linux-kernel@...r.kernel.org
Subject: Re: 2.6.21-rc5-mm3 - no boot, "address not 2M aligned"

Vivek Goyal <vgoyal@...ibm.com> writes:

> Only advantage of CONFIG_PHYSICAL_START seems to be that one has got
> capability to run the kernel from other addresses without modifying the
> boot-loader. One can argue that now people should use a relocatable kernel
> for such a feature. But for using relocatable kenrel, one needs to modify
> grub, lilo and I am not sure if somebody is going to do that. Secondly, how
> would one specify an address to a boot-loader to load image at?

I thought this was important for vmlinux and Xen?

I guess at this point the easy case is that we modify /sbin/kexec to support
it.  And the other bootloaders can come be upgraded if the feature is
interesting enough.

> On i386, somebody already found an interesting usage of CONFIG_PHYSICAL_START
> where he was running his kernel above 16MB so that he can maximize on
> DMA ZONE. Can't think of any usage for x86_64 at the moment but I think
> down the line people might come up with such usages.

Agreed.  We do have CONFIG_PHYSICAL_ALIGN that can handle that case,
although I admit that is a bit of a hack.

> To me, retaining CONFIG_PHYSICAL_START gives added flexibility to the user,
> at the expense of reduced simplicity. We should definitely change the type
> of vmlinux to ET_DYN but at the same time it might still be worth to retain
> CONFIG_PHYSICAL_START option.

I think something like CONFIG_PHYSICAL_START currently gives us very
little gain, and is hard to use correctly, and there are alternative
solutions.  So if we can get rid of it, by only inconveniencing users
who want load their kernels at a weird address it is worth it.

>> I think I can switch the vmlinux header type in about 100 lines or so
>> of code.  Assuming I can ever get 30 minutes with the appropriate
>> kernel.
>> 
>
> That would be awesome. Then vmlinux will be relocatable too. (Officially).

Yes.  For x86_64 I can do this.  i386 is more difficult.  (Although with
a little cleverness we can move the code that processes relocations into
vmlinux).  

Eric
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