linux-kernel - Re: iommu dma mapping alignment requirements

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Message-Id: <1198186017.6779.28.camel@pasglop>
Date:	Fri, 21 Dec 2007 08:26:57 +1100
From:	Benjamin Herrenschmidt <benh@....ibm.com>
To:	Steve Wise <swise@...ngridcomputing.com>
Cc:	benh@...abs.au.ibm.com, Roland Dreier <rdreier@...co.com>,
	linux-kernel@...r.kernel.org,
	OpenFabrics General <general@...ts.openfabrics.org>,
	Benjamin Herrenschmidt <bherren@....ibm.com>,
	Wen Xiong <wenxiong@...ibm.com>,
	Olof Johansson <olof@...om.net>,
	Paul Mackerras <pmac@....ibm.com>
Subject: Re: iommu dma mapping alignment requirements


On Thu, 2007-12-20 at 15:02 -0600, Steve Wise wrote:
> Benjamin Herrenschmidt wrote:
> > Adding A few more people to the discussion. You may well be right and we
> > would have to provide the same alignment, though that sucks a bit as one
> > of the reason we switched to 4K for the IOMMU is that the iommu space
> > available on pSeries is very small and we were running out of it with
> > 64K pages and lots of networking activity.
> > 
> 
> But smarter NIC drivers can resolve this too, I think, but perhaps 
> carving up full pages of mapped buffers instead of just assuming mapping 
> is free...

True, but the problem still happenens today, if we switch back to 64K
iommu page size (which should be possible, I need to fix that), we
-will- run out of iommu space on typical workloads and that is not
acceptable.

So we need to find a compromise.

What I might do is something around the lines of: If size >= PAGE_SIZE,
and vaddr (page_address + offset) is PAGE_SIZE aligned, then I enforce
alignment of the resulting mapping.

That should fix your case. Anything requesting smaller than PAGE_SIZE
mappings would lose that alignment but I -think- it should be safe, and
you still always get 4K alignment anyway (+/- your offset) so at least
small alignment restrictions are still enforced (such as cache line
alignment etc...).

I'll send you a test patch later today.

Ben.


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