linux-kernel - Re: [PATCH 1/4] x86, gart: Don't enforce GART aperture lower-bound by alignment

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Message-ID: <4DAC5E16.1020408@zytor.com>
Date:	Mon, 18 Apr 2011 08:51:50 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Roedel, Joerg" <Joerg.Roedel@....com>
CC:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Yinghai Lu <yinghai@...nel.org>
Subject: Re: [PATCH 1/4] x86, gart: Don't enforce GART aperture lower-bound
 by alignment

On 04/18/2011 07:56 AM, Roedel, Joerg wrote:
> On Mon, Apr 18, 2011 at 10:45:19AM -0400, H. Peter Anvin wrote:
>> On 04/18/2011 06:45 AM, Joerg Roedel wrote:
>>> This patch changes the allocation of the GART aperture to
>>> enforce only natural alignment instead of aligning it on
>>> 512MB. This big alignment was used to force the GART
>>> aperture to be over 512MB. This is enforced by using 512MB
>>> as the lower-bound address in the allocation range.
>>>
>>> Cc: Yinghai Lu <yinghai@...nel.org>
>>> Signed-off-by: Joerg Roedel <joerg.roedel@....com>
>>
>> Better implementation of the existing bounds, yes, but I think the
>> algorithm is still wrong.  Specifically, 512 MiB seems to have been the
>> maximum address of the kernel at some point, but that is historic at
>> this point, at least on 64 bits.
> 
> I am fine with a smaller lower-bound, but I am not sure what a better
> choice is. The comment about kexec seems to be valid. It shouldn't matter
> for kdump because in this case the memory is allocated independently and
> the kdump kernel will only use this part, but for other kexec uses it is
> a bit harder. Probably any number we choose as a lower bound is an
> arbitrary choice at some point. But I am open for
> suggestions/corrections to this.
> 

The right thing to do for in-place kexec it to turn it off, not rely on
any specific magic addresses.  We have had this problem with a number of
drivers in the context of kexec.

	-hpa

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

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