sw/device/lib/runtime/ibex.h

// Copyright lowRISC contributors.
// Licensed under the Apache License, Version 2.0, see LICENSE for details.
// SPDX-License-Identifier: Apache-2.0

#ifndef OPENTITAN_SW_DEVICE_LIB_RUNTIME_IBEX_H_
#define OPENTITAN_SW_DEVICE_LIB_RUNTIME_IBEX_H_

#include <stddef.h>

#include "sw/device/lib/base/stdasm.h"

/**
 * @file
 * @brief This header provides Ibex-specific functions, such as cycle-accurate
 * busy loops.
 */

/**
 * Read the cycle counter.
 *
 * The value of the counter is stored across two 32-bit registers: `mcycle` and
 * `mcycleh`. This function is guaranteed to return a valid 64-bit cycle
 * counter value, even if `mcycle` overflows before reading `mcycleh`.
 *
 * Adapted from: The RISC-V Instruction Set Manual, Volume I: Unprivileged ISA
 * V20191213, pp. 61.
 */
inline uint64_t ibex_mcycle_read() {
  uint32_t cycle_low = 0;
  uint32_t cycle_high = 0;
  uint32_t cycle_high_2 = 0;
  asm volatile(
      "read%=:"
      "  csrr %0, mcycleh;"     // Read `mcycleh`.
      "  csrr %1, mcycle;"      // Read `mcycle`.
      "  csrr %2, mcycleh;"     // Read `mcycleh` again.
      "  bne  %0, %2, read%=;"  // Try again if `mcycle` overflowed before
                                // reading `mcycleh`.
      : "+r"(cycle_high), "=r"(cycle_low), "+r"(cycle_high_2)
      :);
  return (uint64_t)cycle_high << 32 | cycle_low;
}

#endif  // OPENTITAN_SW_DEVICE_LIB_RUNTIME_IBEX_H_