linux-kernel - Re: [PATCH] x86: set PAE PHYSICAL_MASK

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Date:	Fri, 06 Jun 2008 09:14:04 +0100
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"Ingo Molnar" <mingo@...e.hu>, "Andi Kleen" <andi@...stfloor.org>,
	"Stable Kernel" <stable@...nel.org>, <x86@...nel.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: set PAE PHYSICAL_MASK_SHIFT to match 64-bit

>>> Jeremy Fitzhardinge <jeremy@...p.org> 06.06.08 09:59 >>>
>Andi Kleen wrote:
>> Jeremy Fitzhardinge <jeremy@...p.org> writes:
>>   
>>> The 46-bit mask used in 64-bit seems pretty arbitrary. 
>>>     
>>
>> The rationale for the 46 bits is that the kernel needs roughly 4x as 
>> much virtual space as physical space and the virtual space is limited
>> to 48bits.
>>
>> To be exact 47 bits is always user space and the 47 bits remaining
>> for the kernel are split into half, with one half for the direct mapping
>> and the other half for random mappings.  With some pushing you could
>> extend it to 46.5 bits or so, but beyond that you'll be in trouble.
>>   
>
>Why's that?  Is the issue the amount of memory needed for pagetables and 
>page structures if you did have more than 2^48 bytes of physical memory?

No, it's the fact that the 1:1 mapping needs as much virtual space as
the physical range covered (including all holes).

Jan

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