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

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Date:	Fri, 3 Jul 2009 15:03:09 -0500
From:	"Michael S. Zick" <lkml@...ethan.org>
To:	Jeremy Fitzhardinge <jeremy@...p.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 Fri July 3 2009, Jeremy Fitzhardinge wrote:
> 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.
>

Interesting - this *&^&^% technical datasheet contradicts itself (again).
It can't see to decide if it does or it doesn't - -

I guess I will have to ask the silicon - with my own code - I have a
hardware diagnostic here that claims it doesn't (but it is closed source...)
so I don't know if it is testing or guessing.

 
> 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.)
> 

That is probably what I had in my mind - the "# line" case, sorry, my bad.

Mike
>     J
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