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

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Wed, Jul 21, 2010 at 11:19:58AM +0530, Shilimkar, Santosh wrote:
> > -----Original Message-----
> > From: linux-arm-kernel-bounces@lists.infradead.org [mailto:linux-arm-
> > kernel-bounces@lists.infradead.org] On Behalf Of Russell King - ARM Linux
> > Sent: Wednesday, July 21, 2010 4:00 AM
> > To: stepanm@codeaurora.org
> > Cc: linux-arch@vger.kernel.org; dwalker@codeaurora.org; mel@csn.ul.ie;
> > linux-arm-msm@vger.kernel.org; linux-kernel@vger.kernel.org; FUJITA
> > Tomonori; linux-mm@kvack.org; andi@firstfloor.org; Zach Pfeffer; Michael
> > Bohan; Tim HRM; linux-omap@vger.kernel.org; linux-arm-
> > kernel@lists.infradead.org; ebiederm@xmission.com
> > Subject: Re: [RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and device
> > memory management

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> > 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.
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> > 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.