The possible envisioned scenario is related to booting TF-A+UEFI and support for authenticated variables.

1. TF-A boots, including the SPM and SPs.
2. UEFI boots, registers RX/TX with SPM, performs a mem share operation for a comm. buffer with a SP (e.g. St MM like), unmaps RX/TX
3. OS boots, registers RX/TX with SPM, and can later invoke UEFI run-time services (e.g. query UEFI variable). UEFI uses the established comm. buffer but no longer needs RX/TX buffers.

Without FFA_RXTX_UNMAP it's not possible that both UEFI and OS invoke FFA_RXTX_MAP

FWIW, agree with Olivier's analysis.

Merged https://review.trustedfirmware.org/c/hafnium/hafnium/+/10759