Messages in this thread | | | Date | Thu, 20 Dec 2007 16:12:25 -0600 | From | Steve Wise <> | Subject | Re: iommu dma mapping alignment requirements |
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Benjamin Herrenschmidt wrote: > 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. >
Sounds good. Thanks!
Note, that these smaller sub-host-page-sized mappings might pollute the address space causing full aligned host-page-size maps to become scarce... Maybe there's a clever way to keep those in their own segment of the address space?
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