[rtl] Implement FENCE.I

Fixes #391

azure-pipelines.yml

@@ -114,7 +114,7 @@ jobs:
       export RISCV_TARGET=ibex
       export RISCV_DEVICE=rv32imc
       fail=0
-      for isa in rv32i rv32im rv32imc rv32Zicsr; do
+      for isa in rv32i rv32im rv32imc rv32Zicsr rv32Zifencei; do
         make -C build/riscv-compliance RISCV_ISA=$isa 2>&1 | tee run.log
         if [ ${PIPESTATUS[0]} != 0 ]; then
           echo -n "##vso[task.logissue type=error]"

doc/introduction.rst

@@ -50,6 +50,10 @@ In addition, the following instruction set extensions are available.
      - 2.0
      - always enabled
 
+   * - **Zifencei**: Instruction-Fetch Fence
+     - 2.0
+     - always enabled
+
 Most content of the RISC-V privileged specification is optional.
 Ibex currently supports the following features according to the RISC-V Privileged Specification, version 1.11.
 

doc/pipeline_details.rst

@@ -75,3 +75,8 @@ Read the description for more information.
 |                       |                       | instruction-side memory interface takes to receive data the |
 |                       |                       | longer the branch will stall.                               |
 +-----------------------+-----------------------+-------------------------------------------------------------+
+| Instruction Fence     | 1 - N                 | The FENCE.I instruction as defined in 'Zifencei' of the     |
+|                       |                       | RISC-V specification. Internally it is implemented as a     |
+|                       |                       | jump (which does the required flushing) so it has the same  |
+|                       |                       | stall characteristics (see above).                          |
++-----------------------+-----------------------+-------------------------------------------------------------+

rtl/ibex_decoder.sv

@@ -490,20 +490,36 @@ module ibex_decoder #(
       /////////////
 
       OPCODE_MISC_MEM: begin
-        // For now, treat the fence (funct3 == 000) instruction as a nop.
-        // This may not be correct in a system with caches and should be
-        // revisited.
-        // fence.i (funct3 == 001) was moved to a separate Zifencei extension
-        // in the RISC-V ISA spec proposed for ratification, so we treat it as
-        // an illegal instruction.
-        if (instr[14:12] == 3'b000) begin
-          alu_operator_o     = ALU_ADD; // nop
-          alu_op_a_mux_sel_o = OP_A_REG_A;
-          alu_op_b_mux_sel_o = OP_B_IMM;
-          regfile_we         = 1'b0;
-        end else begin
-          illegal_insn       = 1'b1;
-        end
+        // For now, treat the FENCE (funct3 == 000) instruction as a NOP.  This may not be correct
+        // in a system with caches and should be revisited.
+        // FENCE.I will flush the IF stage and prefetch buffer but nothing else.
+        unique case (instr[14:12])
+          3'b000: begin
+            alu_operator_o     = ALU_ADD; // nop
+            alu_op_a_mux_sel_o = OP_A_REG_A;
+            alu_op_b_mux_sel_o = OP_B_IMM;
+            regfile_we         = 1'b0;
+          end
+          3'b001: begin
+            // FENCE.I is implemented as a jump to the next PC, this gives the required flushing
+            // behaviour (iside prefetch buffer flushed and response to any outstanding iside
+            // requests will be ignored).
+            jump_in_dec_o      = 1'b1;
+
+            alu_op_a_mux_sel_o = OP_A_CURRPC;
+            alu_op_b_mux_sel_o = OP_B_IMM;
+            imm_b_mux_sel_o    = IMM_B_INCR_PC;
+            alu_operator_o     = ALU_ADD;
+            regfile_we         = 1'b0;
+
+            if (instr_new_i) begin
+              jump_set_o       = 1'b1;
+            end
+          end
+          default: begin
+            illegal_insn       = 1'b1;
+          end
+        endcase
       end
 
       OPCODE_SYSTEM: begin

rtl/ibex_prefetch_buffer.sv

@@ -74,7 +74,9 @@ module ibex_prefetch_buffer (
   // PMP errors are generated in the address phase, and registered into a fake data phase
   assign instr_or_pmp_err = instr_err_i | rdata_pmp_err_q[0];
 
-  // A branch will invalidate any previously fetched instructions
+  // A branch will invalidate any previously fetched instructions.
+  // Note that the FENCE.I instruction relies on this flushing behaviour on branch. If it is
+  // altered the FENCE.I implementation may require changes.
   assign fifo_clear = branch_i;
 
   ibex_fetch_fifo #(