linux-kernel - Re: [PATCH 1/2] x86: ioremap: fix wrong physical address handling

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Date:	Thu, 17 Jun 2010 06:46:29 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Kenji Kaneshige <kaneshige.kenji@...fujitsu.com>,
	Matthew Wilcox <matthew@....cx>, tglx@...utronix.de,
	mingo@...hat.com, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, macro@...ux-mips.org,
	kamezawa.hiroyu@...fujitsu.com, eike-kernel@...tec.de
Subject: Re: [PATCH 1/2] x86: ioremap: fix wrong physical address handling

On 06/17/2010 02:35 AM, Jeremy Fitzhardinge wrote:
>>>
>>> By the way, is there linux kernel limit regarding above 44-bits physical
>>> address in x86_32 PAE? For example, pfn above 32-bits is not supported?
> 
> That's an awkward situation.  I would tend to suggest that you not
> support this type of machine with a 32-bit kernel.  Is it a sparse
> memory system, or is there a device mapped in that range?
> 
> I guess it would be possible to special-case ioremap to allow the
> creation of such mappings, but I don't know what kind of system-wide
> fallout would happen as a result.  The consequences of something trying
> to extract a pfn from one of those ptes would be
> 
>> There are probably places at which PFNs are held in 32-bit numbers,
>> although it would be good to track them down if it isn't too expensive
>> to fix them (i.e. doesn't affect generic code.)
>>   
> 
> There are many places which hold pfns in 32 bit variables on 32 bit
> systems; the standard type for pfns is "unsigned long", pretty much
> everywhere in the kernel.  It might be worth defining a pfn_t and
> converting usage over to that, but it would be a pervasive change.
> 

I think you're right, and just making 2^44 work correctly would be good
enough.  Doing special forwarding of all 52 bits of the real physical
address in the paravirt case (where it is self-contained and doesn't
spill into the rest of the kernel) would probably be a good thing, though.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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