FIX(src/target/riscv/riscv.c): CSR_MCOUNTEREN is not available w/o U or S

Author: Pavel S. Smirnov

src/target/riscv/riscv.c

@@ -2966,7 +2966,6 @@ riscv_init_registers(struct target *const target)
 			case CSR_INSTRET:
 			case CSR_MARCHID:
 			case CSR_MCAUSE:
-			case CSR_MCOUNTEREN:
 			case CSR_MCYCLE:
 			case CSR_MEPC:
 			case CSR_MHARTID:
@@ -2984,6 +2983,12 @@ riscv_init_registers(struct target *const target)
 				p_reg->exist = true;
 				break;
 
+			case CSR_MCOUNTEREN:
+				p_reg->exist =
+					riscv_supports_extension(target, riscv_current_hartid(target), 'S') ||
+					riscv_supports_extension(target, riscv_current_hartid(target), 'U');
+				break;
+
 			case CSR_FFLAGS:
 			case CSR_FRM:
 			case CSR_FCSR: