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

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Message-ID: <4EA0937F.3040400@zytor.com>
Date:	Thu, 20 Oct 2011 14:32:47 -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 02:30 PM, H. Peter Anvin wrote:
> 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.
> 

Specifically, it's fine for them to be mapped; it's not fine for them to
be mapped cacheable.

	-hpa

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