linux-kernel - Re: [Bug Fix]: Do 32-bit table calculations in pre-processor

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Date:	Fri, 03 Jul 2009 12:30:46 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	lkml@...eThan.org
CC:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org
Subject: Re: [Bug Fix]: Do 32-bit table calculations in pre-processor

On 07/03/09 12:13, Michael S. Zick wrote:
> On Fri July 3 2009, Jeremy Fitzhardinge wrote:
>   
>> On 07/03/09 11:38, Michael S. Zick wrote:
>>     
>>> I make no claims for it at the moment - too early in the test process.
>>> Just the general observation that it takes 0.5M to describe 0.5G of ram.
>>>   
>>>       
>> Only if you're using 4k pages.  With large pages, 1 pte can map 2M, so
>> 256 entries can map 512M, so you only need 1/2 a page of pagetable
>> (assuming PAE; if not a single entry can map 4M).
>>
>>     
>
> Ah, but you can't assume that - look at your VIA-C7M tech sheet - NO PAE.
>   

According to
http://www.via.com.tw/en/products/processors/c7-m/secure_by_design.jsp,
it supports NX, which means it must support PAE.

But even without PAE, it can still support PSE (large pages).

> Try ending the filename in ".S" and passing it to gcc, 
> like the build system does.
>   

It doesn't make any difference.  After going through cpp the expression is:

    # 62 "/home/jeremy/git/linux/arch/x86/kernel/head_32.S"
    MAPPING_BEYOND_END = (((((1<<32) - 0xC0000000) >> 12) / 512) + 4) << 12
      

which will be evaluated by the assembler.   The C preprocessor doesn't
evaluate expressions in the source; it only ever does substitutions and
leaves the results for the compiler/assembler to evaluate.  (It evals
expressions on cpp # lines, of course, but that's not relevant here.)

    J
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