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

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Date:	Wed, 05 Mar 2008 08:08:01 -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:
>>
>> diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
>> index 3bad477..2341492 100644
>> --- a/arch/x86/xen/setup.c
>> +++ b/arch/x86/xen/setup.c
>> @@ -38,7 +38,8 @@ char * __init xen_memory_setup(void)
>>  	unsigned long max_pfn = xen_start_info->nr_pages;
>>  
>>  	e820.nr_map = 0;
>> -	add_memory_region(0, PFN_PHYS(max_pfn), E820_RAM);
>> +	add_memory_region(0, LOWMEMSIZE(), E820_RAM);
>> +	add_memory_region(HIGH_MEMORY, PFN_PHYS(max_pfn)-HIGH_MEMORY, E820_RAM);
> 
> Won't this waste 300+ KB of Perfectly Good RAM? Or I understood it
> incorrectly?
> 
> 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.

	-hpa
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