Here's the description again:
Frontends are expected to write their protocol ABI to xenstore. Since
the protocol ABI defaults to the backend's native ABI, things work
fine without that as long as the frontend's native ABI is identical to
the backend's native ABI. This is not the case for xen-blkfront
running 32-on-64, because its ABI differs between 32 and 64 bit, and
thus needs this fix.
I can break the dependency by putting the same bug that is now in
xen-blkfront into xen-fbfront and xen-kbdfront.