[openib-general] SA cache design

[Sean Hefty]

Sean Hefty wrote:
> - The MAD interface will result in additional data copies and userspace 
> to kernel transitions for clients residing on the local system.
> - Clients require a mechanism to locate the sa_cache, or need to make 
> assumptions about its location.

Based on some comments from people, I believe that we can handle the latter 
problem when the sa_cache/sa_replica/sa_whateveryouwanttocallit registers with 
the MAD layer.  Ib_mad can record an sa_lid and sa_sl as part of a device's port 
attributes.  These would initially be set the same as sm_lid and sm_sl.  When a 
client registers to receive unsolicited SA MADs, the attributes would be updated 
accordingly.  ib_sa and other clients sending MADs to the SA would use these 
values in place of the SM values.

I'm not fond of the idea of pushing an SA switch into the MAD layer, since this 
makes it more difficult for the actual cache to query the SA directly.

Another approach that may work better long term is treating the cache as a 
redirected SA request.  Something along the lines of:

http://openib.org/pipermail/openib-general/2005-September/011349.html

(but with a restricted implementation for now) might also work.

- Sean