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

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Date:	Fri, 06 Jun 2008 08:59:24 +0100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Ingo Molnar <mingo@...e.hu>, x86@...nel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jan Beulich <jbeulich@...ell.com>,
	Stable Kernel <stable@...nel.org>
Subject: Re: [PATCH] x86: set PAE PHYSICAL_MASK_SHIFT to match 64-bit

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?

> It's not arbitrary at all.
I didn't say it was.  That was the introduction to my explanation of why 
I didn't think it was arbitrary.  Of course, if there had been a comment 
there explaining the rationale, I wouldn't have had to make one up...

    J
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