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

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Date:	Thu, 07 Aug 2008 14:55:54 -0700
From:	Zachary Amsden <zach@...are.com>
To:	"H. Peter Anvin" <hpa@...nel.org>
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..

On Thu, 2008-08-07 at 14:52 -0700, H. Peter Anvin wrote:
> 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?

Nope, it must be in an area allowing for segmentation protection, while
keeping the kernel on zero-based segments; that means only the end of
linear address space is sufficient.


> 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.

Yes, the reason to make the fixmap moveable is to allow as much address
space as possible for big memory (physical) machines.

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