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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Message-ID: <20070602010539.GA13512@linux-os.sc.intel.com>
Date:	Fri, 1 Jun 2007 18:05:39 -0700
From:	Venki Pallipadi <venkatesh.pallipadi@...el.com>
To:	Jesse Barnes <jbarnes@...tuousgeek.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Justin Piszcz <jpiszcz@...idpixels.com>,
	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?

On Fri, Jun 01, 2007 at 02:41:57PM -0700, Jesse Barnes wrote:
> On Friday, June 1, 2007 2:19:43 Andi Kleen wrote:
> > And normally the MTRRs win, don't they (if I remember the table correctly)
> > So if the MTRR says UC and PAT disagrees it might not actually help
> 
> I just checked, yes the MTRRs win for UC types.  But it sounds like the cases 
> we're talking about are actually situations where there's no MTRR coverage, 
> so the default type is used.  The manual doesn't specifically call out how 
> memory using the default type interacts with PAT, but it may well be that it 
> stays uncached if the default type is uncached.  Again that argues for fixing 
> the MTRR mapping problem in some way.
> 

I feel, having a silent/transparent workaround is not a good idea. With that
chances are BIOS bug will go unnoticed (having an error message in dmesg may not
get noticed either). Probably we should just panic at boot with a
detailed message about the e820 mtrr discrepancy (which can be logged as
a BUG to BIOS provider) and suggest a temporary workaround of "mem=___".

Thanks,
Venki
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