linux-kernel - Re: Intel's response Linux/MTRR/8GB Memory Support / Why doesn't the kernel realize the BIOS has problems and re-map appropriately?

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Date:	Fri, 1 Jun 2007 15:19:12 -0400 (EDT)
From:	Justin Piszcz <jpiszcz@...idpixels.com>
To:	Jesse Barnes <jbarnes@...tuousgeek.org>
cc:	linux-kernel@...r.kernel.org
Subject: Re: Intel's response Linux/MTRR/8GB Memory Support / Why doesn't
 the kernel realize the BIOS has problems and re-map appropriately?

It works ok on
my machine (correctly detects the condition, adjusts end_pfn, and keeps
the machine fast), aside from the fact that X won't start.

But X won't start? :\

On Fri, 1 Jun 2007, Jesse Barnes wrote:

>> and the MTRRs (from /proc/mtrr, from private email):
>>
>> reg00: base=0x00000000 (   0MB), size=2048MB: write-back, count=1
>> reg01: base=0x80000000 (2048MB), size=1024MB: write-back, count=1
>> reg02: base=0xc0000000 (3072MB), size= 256MB: write-back, count=1
>> reg03: base=0xcf800000 (3320MB), size=   8MB: uncachable, count=1
>> reg04: base=0xcf700000 (3319MB), size=   1MB: uncachable, count=1
>> reg05: base=0x100000000 (4096MB), size=4096MB: write-back, count=1
>> reg06: base=0x200000000 (8192MB), size= 512MB: write-back, count=1
>> reg07: base=0x220000000 (8704MB), size= 128MB: write-back, count=1
>>
>> so the ranges mapped as cacheable are:
>>
>> 0-3319MB
>> 4096-8832MB
>>
>> leaving 64MB of memory at the top of RAM uncached. What do you want to
>> bet that something important (kernel code?) is getting loaded there..
>>
>> So essentially it's a BIOS problem, it's not setting up the MTRRs
>> properly in order to map all of RAM as cacheable. As Andi says, complain
>> to Intel.
>
> If it's just 64M you'll end up losing, you could try the "[RFC] trim memory
> not covered by MTRR WB type" patch I posted yesterday.  It won't reinit the
> MTRRs (maybe we should) but it will at least prevent your system from
> crawling if the BIOS doesn't set them up right.  That would at least let you
> use most of your memory until the BIOS guys acknowledge that they have a
> problem (or we get proper PAT support, which I think would make this problem
> go away as well).
>
> Jesse
>
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