IB/mthca: Avoid changing userspace ABI to handle DMA write barrier attribute
Commit cb9fbc5c ("IB: expand ib_umem_get() prototype") changed the mthca userspace ABI to provide a way for userspace to indicate which memory regions need the DMA write barrier attribute. However, it is possible to handle this without breaking existing userspace, by having the mthca kernel driver recognize whether it is talking to old or new userspace, depending on the size of the register MR structure passed in. The only potential drawback of this is that is allows old userspace (which has a bug with DMA ordering on large SGI Altix systems) to continue to run on new kernels, but the advantage of allowing old userspace to continue to work on unaffected systems seems to outweigh this, and we can print a warning to push people to upgrade their userspace. Signed-off-by: Roland Dreier <rolandd@cisco.com>
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