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

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Message-Id: <200907031413.17556.lkml@morethan.org>
Date:	Fri, 3 Jul 2009 14:13:15 -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 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.

Mike

> > Also, 
> > the observation that (1<<32) drops the bit off the left end of a 32-bit value.
> > You can see the result in the portion of the post you snipped out.  ;)
> >   
> 
> Those computations aren't done as 32-bit.
>

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

And while your at it, thank A.K. for snipping off the before/after dmesg.

Mike
> 
> $ as << EOF
> .data
> .byte (1 << 100) >> 100
> EOF
> $ objdump -D a.out
> 
> a.out:     file format elf64-x86-64
> 
> Disassembly of section .data:
> 
> 0000000000000000 <.data>:
>    0:    01                       .byte 0x1
> 
> 
>     J
> 
> 

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