linux-kernel - Re: [RFC] use realmode code to reserve end-of-conventional-memory to 1MB

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Date:	Wed, 05 Mar 2008 09:28:51 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Eduardo Habkost <ehabkost@...hat.com>
CC:	Ian Campbell <ijc@...lion.org.uk>,
	Alexander van Heukelum <heukelum@...lshack.com>,
	Ingo Molnar <mingo@...e.hu>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	Andi Kleen <ak@...e.de>, Thomas Gleixner <tglx@...utronix.de>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Mark McLoughlin <markmc@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] use realmode code to reserve end-of-conventional-memory
 to 1MB

Eduardo Habkost wrote:
>>>
>>> I am aware that it would take more work to tell all kernel code that it
>>> shouldn't look for BIOS data on this region when running as a domU guest,
>>> but it seems that it would be a better solution.
>>>
>> No, the right thing is for Xen to not try to map RAM in this area.
> 
> Why? If the Xen host is telling us there is valid RAM in this area,
> why can't we use it?
> 
> Existing Xen hosts use those physical address ranges for RAM. We can't
> fix this on the e820 map on the guest side without making otherwise-valid
> RAM unused.
> 

x86 has a number of historical assumptions, both in kernel and in 
userspace (consider dmidecode!)  Trying to go against them is a losing 
proposition, *AND* a headache for the maintainers, which will have to 
consider "oh yes, and Xen does this dumb thing which goes against what 
all the hardware does."

	-hpa
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