On Tue, Aug 21, 2018 at 09:40:34PM -0400, Martin K. Petersen wrote:

When crc-t10dif is initialized, the crypto infrastructure will pick the
algorithm with the highest priority currently registered. Both block and
SCSI will cause crc-t10dif to be compiled as a built-in so this
selection happens very early.

Ouch.  This might actually happen in a lot of other users of the crypto
functionality as well.

However, it seems like a bit of a deficiency in crypto that there is no
way to upgrade existing transformations if higher priority algorithms
become available. btrfs and a few others work around this issue by not
using the generic lib/ CRC functions (which defeats the purpose of
having these in the first place). Instead they are registering their own
transformation at a later time where any accelerator modules are more
likely to be loaded.

If we can't fix this in crypto (which doesn't seem that easy), we
should at least clearly document the issue somewhere, and fix this in
the t10pi code by initializing crct10dif_tfm in a lazy fashion only
once the fist block device starts using it.