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

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Message-ID: <47CED7F8.1060205@zytor.com>
Date:	Wed, 05 Mar 2008 09:27:20 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Eduardo Habkost <ehabkost@...hat.com>,
	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>,
	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

Jeremy Fitzhardinge 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.
> 
> For the moment that's true, but we should be able to release those 
> pages.  On the other hand, there's been talk of making Xen hand out 
> memory in physically contigious 2M chunks, which means trying to unmap 
> 300k won't be possible or worthwhile.  I suspect real hardware will just 
> waste this memory too (it won't remap that 320k of ram to somewhere 
> else; it will just be shadowed) - which is nothing compared to chipsets 
> which will just throw away 1G to make space for PCI...
> 

Not that 384K is all that much to worry about, but you could just map 
memory from 2 MB upward, and set the kernel load address to 2 MB (which 
is good for performance anyway.)

There has been talk of making the default kernel load address 16 MB, to 
keep the DMA zone free.

	-hpa
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