BTL UCT causing problems with UCX memory hooks

[jsquyres]

From https://www.mail-archive.com/devel@lists.open-mpi.org/msg20895.html and https://www.mail-archive.com/devel@lists.open-mpi.org/msg20904.html:

The issue below is caused by btl_uct – it mistakenly calls ucm_set_external_event() without checking opal_mem_hooks_support_level().
This leads UCX to believe that memory hooks would be provided by OMPI, but in fact they are not, so pinned physical pages become out-of-sync with process virtual address.

btl_uct wrong call: https://github.com/open-mpi/ompi/blob/master/opal/mca/btl/uct/btl_uct_component.c#L132
Correct way: https://github.com/open-mpi/ompi/blob/master/opal/mca/common/ucx/common_ucx.c#L104

FYI @yosefe

v4.0.x RMs: this feels like a blocker for v4.0.x. Thoughts?

[yosefe]

related to openucx/ucx#3581

[hppritcha]

@jsquyres yes I agree.

[hppritcha]

I really don't want to bother fixing this. @gpaulsen we should discuss on Friday. I'm in the mood to by default not build the UCT BTL at all. That would also solve the API breakage issue.

[hjelmn]

@yosefe Could have just tagged me and the fix would have been in within an hour or so. Good grief.

[jsquyres]

My bad -- I shouldn't have set #6668 to auto-close this issue. Leaving it open until the fix is applied to v4.0.x.