[CentOS] Why is sha256-generic preferred over sha256-ssse3?

Tue Jun 9 17:37:23 UTC 2015
Steve Snyder <swsnyder at snydernet.net>

The newly-released kernel v2.6.32-504.23.4.el6 includes the back-ported SHA256-SSSE3 driver.

Why is the generic version of the SHA256 driver selected at runtime instead of the SSSE3 version on this x86_64 system?  

Yes, my CPU does support the SSSE3 instruction set, and the use of SHA256 is invoked by the LUKS "cipher=aes-cbc-essiv:sha256" option.

On the running system I see that the sha256-ssse3 driver is not loaded.  Further evidence that the generic driver is in use:

# grep sha256 /proc/crypto
name         : sha256
driver       : sha256-generic

So... what's the trick to using the sha256-ssse3 driver instead of the generic driver?

Thanks.