linux-kernel - Re: [PATCH 07/12] i386: Add missing !X86

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Message-ID: <m1slah3jfl.fsf@ebiederm.dsl.xmission.com>
Date:	Mon, 30 Apr 2007 10:39:58 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andi Kleen <ak@...e.de>,
	virtualization <virtualization@...ts.linux-foundation.org>,
	linux-kernel@...r.kernel.org, Dave Jones <davej@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [PATCH 07/12] i386: Add missing !X86_PAE dependincy to the 2G/2G split.

"H. Peter Anvin" <hpa@...or.com> writes:

> Eric W. Biederman wrote:
>> When in PAE mode we require that the user kernel divide to be
>> on a 1G boundary.  The 2G/2G split does not have that property
>> so require !X86_PAE
>
> ?????
>
> 	-hpa

>From arch/i386/Kconfig:
> 
> choice
>         depends on EXPERIMENTAL
>         prompt "Memory split" if EMBEDDED
>         default VMSPLIT_3G
>         help
>           Select the desired split between kernel and user memory.
> 
>           If the address range available to the kernel is less than the
>           physical memory installed, the remaining memory will be available
>           as "high memory". Accessing high memory is a little more costly
>           than low memory, as it needs to be mapped into the kernel first.
>           Note that increasing the kernel address space limits the range
>           available to user programs, making the address space there
>           tighter.  Selecting anything other than the default 3G/1G split
>           will also likely make your kernel incompatible with binary-only
>           kernel modules.
> 
>           If you are not absolutely sure what you are doing, leave this
>           option alone!
> 
>         config VMSPLIT_3G
>                 bool "3G/1G user/kernel split"
>         config VMSPLIT_3G_OPT
>                 depends on !HIGHMEM
>                 bool "3G/1G user/kernel split (for full 1G low memory)"
>         config VMSPLIT_2G
>                 depends on !X86_PAE
>                 bool "2G/2G user/kernel split"
>         config VMSPLIT_1G
>                 bool "1G/3G user/kernel split"
> endchoice
> 
> config PAGE_OFFSET
>         hex
>         default 0xB0000000 if VMSPLIT_3G_OPT
>         default 0x78000000 if VMSPLIT_2G
>         default 0x40000000 if VMSPLIT_1G
>         default 0xC0000000

The default PAGE_OFFSET is at 0x7800000 for the 2G/2G split.
All of these options were originally !X86_PAE, I assume
the intention was to be able to have 2G of RAM without
needing high memory.

I don't really care I just saw the problem and decided to prevent
people trying a combination that simply doesn't work.

Eric
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