linux-kernel - Re: [PATCH RFC] x86: add brk allocation for very, very early allocations

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Message-ID: <49A8345F.8020800@zytor.com>
Date:	Fri, 27 Feb 2009 10:43:43 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] x86: add brk allocation for very, very early allocations

Jeremy Fitzhardinge wrote:
> I'd like to add a mechanism like this so I can dynamically allocate some
> Xen-related structures, rather than statically allocating them in the bss,
> both so that Xen has less overhead when it isn't being used, and so I can
> scale better to things like memory size.
> 
> I think this is more widely useful; it would supplant dmi_alloc_data[], for
> example, and I'm sure there's other cases.
> 
> This is fundimentally the same as head_32.S's extension of the bss to build
> the initial kernel mapping, but 64-bit doesn't currently do anything
> analogous
> to this.
> 
> Unfortunately when I use this code as-is I'm getting crashes when the slab
> allocator starts up.  I think this is all correct, but I'm wondering if
> there's something I'm overlooking which is broken in principle.

First of all, I like the concept.  As far as getting crashes, I suspect
what you're finding is some use of this type of extended memory space
that just isn't documented.  I would try this in a simulator, setting a
watchpoint on _end to see if you get any hits before the slab allocator
starts.

	-hpa
-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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