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

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Message-ID: <20110418173655.GA8155@8bytes.org>
Date:	Mon, 18 Apr 2011 19:36:55 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Roedel, Joerg" <Joerg.Roedel@....com>,
	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 Mon, Apr 18, 2011 at 08:51:50AM -0700, H. Peter Anvin wrote:
> 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.

Ok, so changing the start-address to zero seems the best thing to do.
This way the GART will working again on machines with less than 512MB of
RAM (and a BIOS that does not initialize the GART as we want it).

Regards,

	Joerg

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