Messages in this thread | | | Subject | Re: iommu dma mapping alignment requirements | From | Benjamin Herrenschmidt <> | Date | Fri, 21 Dec 2007 08:26:57 +1100 |
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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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