linux-kernel - Re: reserve memory in low physical address

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Message-ID: <20061031151409.GB14272@srv.junsun.net>
Date:	Tue, 31 Oct 2006 07:14:09 -0800
From:	Jun Sun <jsun@...sun.net>
To:	Mark Hounschell <markh@...pro.net>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: reserve memory in low physical address - possible?

On Tue, Oct 31, 2006 at 07:45:57AM -0500, Mark Hounschell wrote:
> Jun Sun wrote:
> > This question is specific to i386 architecture.  While I am fairly 
> > comfortable with Linux kernel, I am not familiar with i386 arch. 
> > 
> > My objective is to reserve, or hide from kernel, some memory space in low
> > physical address range starting from 0.  The memory amount is in the order
> > of 100MB to 200MB.  The total memory is assumed to be around 512MB.
> > 
> > Is this possible?
> > 
> > I understand it is possible to reserve some memory at the end by
> > specifying "mem=xxxM" option in kernel command line.  I looked into
> > "memmap=xxxM" option but it appears not helpful for what I want.
> > 
> > While searching on the web I also found things like DMA zone and loaders
> > etc that all seem to assume the existence low-addressed physical
> > memory.  True?
> > 
> > I can certainly workaround the loader issue.  I can also re-code the real-mode
> > part of kernel code to migrate to higher addresses.  The DMA zone might be
> > a thorny one.  Any clues?  Are modern PCs still subject to
> > the 16MB DMA zone restriction?
> > 
> > Am I too far off from what I want to do?
> > 
> > Thanks.
> > 
> > Jun
> 
> Maybe the bigphysarea patch is what you want?
>

I took a look.  The patch will allocate (actually pre-allocate) a big chunk
at boot time, which is arguably more friendly to MM subsystem.  However,
you cannot specify the location of the memory chunk which is what I want.

Thanks.

Jun
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