linux-kernel - RE: [RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and device memory management

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Message-ID: <EAF47CD23C76F840A9E7FCE10091EFAB02C61C60DD@dbde02.ent.ti.com>
Date:	Wed, 21 Jul 2010 13:15:14 +0530
From:	"Shilimkar, Santosh" <santosh.shilimkar@...com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	"stepanm@...eaurora.org" <stepanm@...eaurora.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"dwalker@...eaurora.org" <dwalker@...eaurora.org>,
	"mel@....ul.ie" <mel@....ul.ie>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	Zach Pfeffer <zpfeffer@...eaurora.org>,
	Michael Bohan <mbohan@...eaurora.org>,
	Tim HRM <zt.tmzt@...il.com>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"ebiederm@...ssion.com" <ebiederm@...ssion.com>
Subject: RE: [RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and device
	memory management

> -----Original Message-----
> From: Russell King - ARM Linux [mailto:linux@....linux.org.uk]
> Sent: Wednesday, July 21, 2010 12:59 PM
> To: Shilimkar, Santosh
> Cc: stepanm@...eaurora.org; linux-arch@...r.kernel.org;
> dwalker@...eaurora.org; mel@....ul.ie; linux-arm-msm@...r.kernel.org;
> linux-kernel@...r.kernel.org; FUJITA Tomonori; linux-mm@...ck.org;
> andi@...stfloor.org; Zach Pfeffer; Michael Bohan; Tim HRM; linux-
> omap@...r.kernel.org; linux-arm-kernel@...ts.infradead.org;
> ebiederm@...ssion.com
> Subject: Re: [RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and device
> memory management
> 
> On Wed, Jul 21, 2010 at 11:19:58AM +0530, Shilimkar, Santosh wrote:
> > > -----Original Message-----
> > > From: linux-arm-kernel-bounces@...ts.infradead.org [mailto:linux-arm-
> > > kernel-bounces@...ts.infradead.org] On Behalf Of Russell King - ARM
> Linux
> > > Sent: Wednesday, July 21, 2010 4:00 AM
> > > To: stepanm@...eaurora.org
> > > Cc: linux-arch@...r.kernel.org; dwalker@...eaurora.org; mel@....ul.ie;
> > > linux-arm-msm@...r.kernel.org; linux-kernel@...r.kernel.org; FUJITA
> > > Tomonori; linux-mm@...ck.org; andi@...stfloor.org; Zach Pfeffer;
> Michael
> > > Bohan; Tim HRM; linux-omap@...r.kernel.org; linux-arm-
> > > kernel@...ts.infradead.org; ebiederm@...ssion.com
> > > Subject: Re: [RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and
> device
> > > memory management
> 
> *************************************************************************
> > > This is difficult to achieve without remapping kernel memory using L2
> > > page tables, so we can unmap pages on 4K page granularity.  That's
> > > going to increase TLB overhead and result in lower system performance
> > > as there'll be a greater number of MMU misses.
> *************************************************************************
> 
> > > However, one obvious case would be to use highmem-only pages for
> > > remapping - but you then have to ensure that those pages are never
> > > kmapped in any way, because those mappings will fall into the same
> > > unpredictable category that we're already trying to avoid.  This
> > > may be possible, but you'll have to ensure that most of the system
> > > RAM is in highmem - which poses other problems (eg, if lowmem gets
> > > low.)
> >
> > Why can't we consider an option of removing the old mappings when
> > we need to create new ones with different attributes as suggested
> > by Catalin on similar thread previously. This will avoid the duplicate
> > mapping with different attributes issue on newer ARMs.
> 
> See the first paragraph which I've highlighted above.
>
Sorry about missing that para Russell.
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