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

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Message-ID: <484AD5DD.9040403@kernel.org>
Date:	Sat, 07 Jun 2008 11:39:25 -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>,
	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 wrote:
>>
>> 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. ;)
> 

Not really.  The vendors are aware of this constraint -- it's hardly 
unique to Linux.  The reason for canonical addresses and all that jazz 
is to keep people from doing stupid things like store stuff in the upper 
16 bits of a pointer (happened a lot on the 68000, where the first 
implementation had only 24 address bits.)  Thus, all changes needed to 
go to a larger virtual address space are all internal to the kernel.

	-hpa
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