Date: Wed, 6 Jun 2007 10:37:23 -0400
To: Bodo Eggert <7eggert@gmx.de>
Cc: Andi Kleen <andi@firstfloor.org>, tmoore@spatial.ca,
       linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCH] Re: 4Gb ram not showing up
Message-ID: <20070606143723.GF10008@csclub.uwaterloo.ca>
References: <Pine.LNX.4.58.0706061356310.2657@be1.lrz>
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From: lsorense@csclub.uwaterloo.ca (Lennart Sorensen)
Sender: linux-kernel-owner@vger.kernel.org
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On Wed, Jun 06, 2007 at 02:12:22PM +0200, Bodo Eggert wrote:
> Change the description of CONFIG_*HIGHMEM* to reflect "lost" memory due to 
> PCI space and the existence of the NX flag.
> 
> Signed-Off-By: Bodo Eggert <7eggert@gmx.de>
> ---
> I made this quick patch using the information from LKML as I remembered 
> it. Please verify.
> 
> --- 2.6.21/arch/i386/Kconfig.ori	2007-06-06 13:41:09.000000000 +0200
> +++ 2.6.21/arch/i386/Kconfig	2007-06-06 14:07:40.000000000 +0200
> @@ -495,8 +495,8 @@ config NOHIGHMEM
>  	bool "off"
>  	depends on !X86_NUMAQ
>  	---help---
> -	  Linux can use up to 64 Gigabytes of physical memory on x86 systems.
> -	  However, the address space of 32-bit x86 processors is only 4
> +	  Linux can use up to 64 Gigabytes of physical address space on x86
> +	  systems. However, the address space of 32-bit x86 processors is only 4
>  	  Gigabytes large. That means that, if you have a large amount of
>  	  physical memory, not all of it can be "permanently mapped" by the
>  	  kernel. The physical memory that's not permanently mapped is called
> @@ -510,8 +510,15 @@ config NOHIGHMEM
>  	  by the kernel to permanently map as much physical memory as
>  	  possible.
>  
> -	  If the machine has between 1 and 4 Gigabytes physical RAM, then
> +
> +	  If the machine has between 1 and 3.5 Gigabytes physical RAM, then
>  	  answer "4GB" here.
> +	  
> +	  The PCI address space will usurally take 512 MB or 1 GB of address
                                     usually

> +	  space. This address space is unavailable to RAM, but depending on the
> +	  chipset (and BIOS settings), memory overlapping the PCI address space
> +	  may be mapped beyond the 4 GB limit and be available using "64GB".
> +
>  
>  	  If more than 4 Gigabytes is used then answer "64GB" here. This
>  	  selection turns Intel PAE (Physical Address Extension) mode on.
> @@ -520,6 +527,10 @@ config NOHIGHMEM
>  	  processors (Pentium Pro and better). NOTE: If you say "64GB" here,
>  	  then the kernel will not boot on CPUs that don't support PAE!
>  
> +	  An additional benefit of the 64GB-Mode is the availability of the
> +	  no-execute-pageflag, which can be used to prevent some attacks from
> +	  injecting malicious code into applications.
> +
>  	  The actual amount of total physical memory will either be
>  	  auto detected or can be forced by using a kernel command line option
>  	  such as "mem=256M". (Try "man bootparam" or see the documentation of
> @@ -532,14 +543,14 @@ config HIGHMEM4G
>  	bool "4GB"
>  	depends on !X86_NUMAQ
>  	help
> -	  Select this if you have a 32-bit processor and between 1 and 4
> +	  Select this if you have a 32-bit processor and between 1 and 3.5
>  	  gigabytes of physical RAM.
>  
>  config HIGHMEM64G
> -	bool "64GB"
> +	bool "64GB (enables no-execute memory protection if available)"
>  	depends on X86_CMPXCHG64
>  	help
> -	  Select this if you have a 32-bit processor and more than 4
> +	  Select this if you have a 32-bit processor and more than 3.5
>  	  gigabytes of physical RAM.
>  
>  endchoice

Seems like an improvement to me.  To fully explain how it could be 3 or
3.5 or 3.25 or who knows how many GB you can actually use without PAE
would probably require writing a small novel.  Certainly talking about
address space instead of amounts of physical memory is more correct.

--
Len Sorensen
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