Re: Kernel fast memory registration API proposal [RFC]

[Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>]

On Tue, Jul 14, 2015 at 11:39:24AM +0300, Sagi Grimberg wrote:
> This is exactly what I don't want to do. I don't think that implicit
> posting is a good idea for reasons that I mentioned earlier:
> 
> "This is where I have a problem. Providing an API that may or may not
> post a work request on my QP is confusing, and I don't understand its
> semantics at all. Do I need to reserve slots on my QP? should I ask for
> a completion? If we suppress the completion will I see an error
> completion? What should I expect to find in the wr_id?"
> 
> We're much better off with keeping the post interface in place but
> have it much simpler.

The ULP doesn't care if it needs to reserver the slot, and it generally
doesn't care about the notification either unless it needs to handle an
error.

Instead of the ib_device knows if a MR needs a post it can through
a helper set the right reservation.

The completions are another mad mightmare in the RDMA stack API.  Every
other subsystem would just allow submitter to attach a function pointer
that handles the completion to the posted item.  But no, the RDMA stack
instead require ID allocators and gigantic boilerplate code in the
consumer to untangle that gigantic mess again.

If we sort that out first the ULD doesn't have to care about FR
notifications.
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