cpu: implement FENCE/FENCE.TSO/PAUSE as stub

cpu.go

@@ -464,7 +464,24 @@ func (c *CPUState) Step() error {
 
 	case 0b0001111:
 		// FENCE/FENCE.TSO/PAUSE
-		panic("todo")
+		funct3 := (opcode >> 12) & 0b111
+		if funct3 != 0b000 {
+			return ErrInvalidOpcode{}
+		}
+
+		if opcode == 0b00000001000000000000000000001111 {
+			// Zihintpause PAUSE extension
+			// PAUSE is encoded as a FENCE instruction with pred=W, succ=0, fm=0, rd=x0, and rs1=x0.
+			// PAUSE is encoded as a hint within the FENCE opcode because some implementations are ex-
+			// pected to deliberately stall the PAUSE instruction until outstanding memory transactions have
+			// completed. Because the successor set is null, however, PAUSE does not mandate any particular
+			// memory ordering - hence, it truly is a HINT.
+		}
+
+		err := c.w.MemFence(opcode)
+		if err != nil {
+			return err
+		}
 
 	case 0b1110011:
 		// ECALL/EBREAK

eei.go

@@ -14,4 +14,6 @@ type EEI interface {
 	Syscall()
 
 	Trap() (bool, error)
+
+	MemFence(opcode uint32) error
 }

virtualEei.go

@@ -116,4 +116,10 @@ func (ve *VirtualEEI) Trap() (bool, error) {
 	panic("trap")
 }
 
+func (ve *VirtualEEI) MemFence(opcode uint32) error {
+	// In the virtual EEI, a memory fence is a no-op.
+	// It matters for synchronizing multiple cores or for peripherals.
+	return nil
+}
+
 var _ EEI = &VirtualEEI{} // interface assertion