With the new HHVM release 3.1.0 they've improved the stuff to make it 'easy' to port extensions to HHVM; https://github.com/facebook/hhvm/tree/master/hphp/runtime/ext_zend_compat
But they still have a list of "Bugs and Caveats" that to me sound like it's not really usable yet...
Unfortunatly I have zero experience with PHP extensions, but maybe it's time to invest some time into this.
Bofore I even begin, what do you think, the bugs they mention are show stoppers?
or is the best way forward to make a PHP implementation of a streaming parser for the Redis protocol that implements the same interface as the phpiredis classes as you mentioned in the HHVM ticket in async-predis?