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

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Date:	Fri, 06 Jun 2008 09:15:47 +0100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Jan Beulich <jbeulich@...ell.com>
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

Jan Beulich wrote:
>>>> 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).

Right, I see.  And suddenly 64-bits seems... constrained. ;)

    J

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