Re: iommu dma mapping alignment requirements

[Steve Wise <swise@opengridcomputing.com>]

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?