linux-kernel - Re: [PATCH] x86: simplify mtrr_bp

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Date:	Wed, 25 Jul 2012 09:57:07 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	Yinghai Lu <yinghai@...nel.org>, mingo@...e.hu, tglx@...utronix.de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: simplify mtrr_bp_init()

On 07/25/2012 12:59 AM, Jan Beulich wrote:
>>
>> should drop all phys_addr assignment in this function.
>>
>> x86_phys_bits should have all correct value?
>
> Is it certain that all special cases (setting phys_addr to 32) are
> covered by those CPUs not having PAE/PSE36? One would
> think that this is valid to imply, but getting cpu_info's phys_bits
> wrong isn't fatal as long as no addresses beyond 4G would ever
> be encountered anywhere, whereas using too large an address
> width here would result in the MTRR writes causing #GP. So
> when I did this adjustment (a couple of years ago already - this
> isn't the first submission), I decided to remain on the safe side.
>
> Does any of the maintainers have an opinion either way?
>

There are definitely CPUs which have PAE but only has a 32-bit address 
bus.  On the other hand there are tons of chipsets which arbitrary 
address caps that almost nothing in the system knows about, so I don't 
think this matters.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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