Fence.i external handshake

CV32E40S includes an external handshake that will be exercised upon execution of the fence.i instruction. The handshake is composed of the signals fencei_flush_req_o and fencei_flush_ack_i and can for example be used to flush an externally connected cache.

The fencei_flush_req_o signal will go high upon executing a fence.i instruction ([RISC-V-UNPRIV]) once possible earlier store instructions have fully completed (including emptying of the the write buffer). The fencei_flush_req_o signal will go low again the cycle after sampling both fencei_flush_req_o and fencei_flush_ack_i high. Once fencei_flush_req_o has gone low again a branch will be taken to the instruction after the fence.i thereby flushing possibly prefetched instructions.

Fence instructions are not impacted by the distinction between main and I/O regions (defined in Physical Memory Attribution (PMA)) and execute as a conservative fence on all operations, ignoring the predecessor and successor fields.


If the fence.i external handshake is not used by the environment of CV32E40S, then it is recommended to tie the fencei_flush_ack_i to 1 in order to avoid stalling fence.i instructions indefinitely.