linux-kernel - Re: [PATCH 1/1] x86: Exclude E820_RESERVED regions and memory holes above 4 GB from direct mapping.

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Date:	Thu, 20 Oct 2011 14:30:05 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Jacob Shin <jacob.shin@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Yinghai Lu <yinghai.lu@...cle.com>,
	Andreas Herrmann <andreas.herrmann3@....com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] x86: Exclude E820_RESERVED regions and memory holes
 above 4 GB from direct mapping.

On 10/20/2011 02:28 PM, Andi Kleen wrote:
> Jacob Shin <jacob.shin@....com> writes:
> 
>> On systems with very large memory (1 TB in our case), BIOS may report a
>> reserved region or a hole in the E820 map, even above the 4 GB range. Exclude
>> these from the direct mapping.
> 
> This doesn't make much sense.  Holes above 4GB are completely legal.
> 
> If you need to workaround a specific broken BIOS you would need a quirk
> only matching that system, with a suitable "BIOS is broken" message.
> 

The problem is that apparently right now we map those unconditionally
into the 1:1 map and mark them cacheable in PAT, which we *don't* for
the < 4 GiB map.

This thus makes the behavior match < 4 GiB, which is the correct
behavior; this should be made clear in the patch description.

	-hpa
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