FFA_RXTX_UNMAP_32 is the mandatory interface and must be implemented by the SPM.
The possible envisioned scenario is related to booting TF-A+UEFI and support for authenticated variables.
- TF-A boots, including the SPM and SPs.
- 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
- 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