linux-kernel - Re: [PATCH]Fix broken VMI in 2.6.27-rc..

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Date:	Thu, 07 Aug 2008 14:52:03 -0700
From:	"H. Peter Anvin" <hpa@...nel.org>
To:	Zachary Amsden <zach@...are.com>
CC:	Alok Kataria <akataria@...are.com>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH]Fix broken VMI in 2.6.27-rc..

Zachary Amsden wrote:
>>>
>> Okay, you lost me about halfway through that... could you perhaps
>> describe the problem from the beginning, exactly what you're trying to do?
> 
> A kernel compiled with VMI enabled may run on a non-VMI platform.  If
> that is the case, the fixmap should not be relocated.  If however, a VMI
> ROM is found, we need to hijack up to 64-MB of linear address space from
> the top of memory down.  This means moving the fixmap down by the same
> amount.
> 

I take it there are no alternatives other than putting this at the end 
of memory?

> Right now the code is structured in such a way that it wants to know how
> much physical memory there is, so it can register a mapping table for
> mapping linear addresses in the lowmem area to physical addresses.  This
> causes the code to depend on max_low_pfn being initialized, which
> accounts for the current placement.
> 
> But it also must be called before anything that creates the fixmap,
> because the same code which registers the linear address mapping also
> reserves high memory above the fixmap.
> 
> My point is 1) these could be two separate calls, or 2) the lowmem
> mapping table need not depend on max_low_pfn at all, it is safe to
> create an extra large mapping which covers all possible lowmem instead
> of the physical ram that is actually available.

Realistically speaking, any (virtual) machine which does *not* have a 
full complement of lowmem (i.e. less than 896 MB in the common case) 
will not suffer significatly from losing a few megabytes of address space.

	-hpa

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