Re: [PATCH 12/16] IB uverbs: add mthca user PD support

[Roland Dreier]

    Andrew> What is a userspace protection domain?

A protection domain is an abstraction enforced by IB hardware --
loosely put, every resource (work queue, memory region, etc) in put in
a PD when it is created, and different resources can only see each
other if they belong to the same PD.

As an example, PDs are needed because IB allows unprivileged processes
to directly post requests to work queues.  Work requests refer to
memory regions by memory keys (32 bit cookies).  Without PDs, a
process could get access to another process's memory region if it
could guess the 32-bit key -- with PDs, it can't because the other
process's memory region will be in a different PD from its work queue.

 - R.
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