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

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Message-ID: <484AD50E.1080601@kernel.org>
Date:	Sat, 07 Jun 2008 11:35:58 -0700
From:	"H. Peter Anvin" <hpa@...nel.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Jan Beulich <jbeulich@...ell.com>, Ingo Molnar <mingo@...e.hu>,
	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 wrote:
>>
>> It should either be 52 bits or dynamic based on CPUID information.  
>> The latter is very expensive.
> 
> I'm more concerned that it might not be possible.  I'm trying to think 
> how many places have compile-time constants derived from this mask.  
> Maybe not too many.
> 
>> If there end up being additional control bits assigned in this space 
>> we won't use them  since we know the size of the address space (which 
>> won't include the control bits) and thus will leave them at zero.
> 
> You mean, if new bits appear we can just adjust the mask accordingly to 
> avoid them?  And if we don't use them, then they'll be zero?

Correct. Remember, the page table entries come from the kernel - not 
from some random areas.

>> It's largely theoretical, since I believe Linux on x86-64 relies on 
>> virtual >= physical+N, where I believe N is about 3 bits, and the page 
>> table format or page size need to change to support more than 48 bits 
>> of virtual address space.
> 
> I don't see any relationship between the physical and virtual size.  
> Certainly virtual is fixed at 48 bits (4*9+12), but I don't think 
> there's any deep reason why physical needs to be within 3 bits.
> 

Identity-mapping.  1 bit goes to kernel/user split, then the kernel area 
is split into multiple regions, one of which is identity-mapping.  It 
may be just 2.

	-hpa
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