feat: export gdbstub api to be used by difftest

.gitmodules

@@ -2,3 +2,6 @@
 	path = am-kernels
 	url = https://git.xinyang.life/xin/am-kernels.git
 	branch = dev
+[submodule "diffu"]
+	path = diffu
+	url = https://git.xinyang.life/xin/diffu.git

diffu

@@ -0,0 +1 @@
+Subproject commit 0c590d218be4b2df4f8f827a9fd40278c227a1ca

nemu/src/isa/riscv32/reg.c

@@ -14,9 +14,10 @@
 ***************************************************************************************/
 
 #include "local-include/reg.h"
-#include "gdbstub.h"
 #include "macro.h"
+#include <difftest-def.h>
 #include <errno.h>
+#include <gdbstub.h>
 #include <isa.h>
 
 const char *regs[] = {"$0", "ra", "sp",  "gp",  "tp", "t0", "t1", "t2",
@@ -76,6 +77,6 @@ int isa_write_reg(void *args, int regno, size_t data) {
   return 0;
 }
 
-arch_info_t isa_arch_info = {.reg_num = 33,
+__EXPORT arch_info_t isa_arch_info = {.reg_num = 32,
-                             .reg_byte = MUXDEF(CONFIG_RV64, 8, 4),
+                                      .reg_byte = MUXDEF(CONFIG_RV64, 8, 4),
-                             .target_desc = TARGET_RV32};
+                                      .target_desc = TARGET_RV32};

nemu/src/monitor/filelist.mk

@@ -1,3 +1,4 @@
 DIRS-y += src/monitor
 
 CXXSRC += src/monitor/gdbstub.cc
+LIBS += -lgdbstub

nemu/src/monitor/gdbstub.cc

@@ -4,27 +4,27 @@
 extern "C" {
 #include <cpu/cpu.h>
 #include <debug.h>
+#include <difftest-def.h>
 #include <errno.h>
 #include <gdbstub.h>
 #include <isa.h>
 #include <memory/paddr.h>
 #include <stddef.h>
 #include <stdlib.h>
-}
 
 typedef struct {
   std::vector<breakpoint_t> *bp;
   bool halt;
 } DbgState;
 
-int nemu_read_mem(void *args, size_t addr, size_t len, void *val) {
+__EXPORT int nemu_read_mem(void *args, size_t addr, size_t len, void *val) {
   if (!in_pmem(addr))
     return EINVAL;
   memcpy(val, guest_to_host(addr), len);
   return 0;
 }
 
-int nemu_write_mem(void *args, size_t addr, size_t len, void *val) {
+__EXPORT int nemu_write_mem(void *args, size_t addr, size_t len, void *val) {
   if (!in_pmem(addr))
     return EINVAL;
   memcpy(guest_to_host(addr), val, len);
@@ -35,21 +35,25 @@ static void nemu_is_stopped(gdb_action_t *act, breakpoint_t *stopped_at) {
   switch (nemu_state.state) {
   case NEMU_RUNNING:
     nemu_state.state = NEMU_STOP;
-    switch (stopped_at->type) {
+    if (stopped_at == NULL) {
-    case BP_SOFTWARE:
+      act->reason = gdb_action_t::ACT_NONE;
-      act->reason = gdb_action_t::ACT_BREAKPOINT;
+    } else {
-      break;
+      switch (stopped_at->type) {
-    case BP_ACCESS:
+      case BP_SOFTWARE:
-      act->reason = gdb_action_t::ACT_WATCH;
+        act->reason = gdb_action_t::ACT_BREAKPOINT;
-      break;
+        break;
-    case BP_WRITE:
+      case BP_ACCESS:
-      act->reason = gdb_action_t::ACT_WWATCH;
+        act->reason = gdb_action_t::ACT_WATCH;
-      break;
+        break;
-    case BP_READ:
+      case BP_WRITE:
-      act->reason = gdb_action_t::ACT_RWATCH;
+        act->reason = gdb_action_t::ACT_WWATCH;
-      break;
+        break;
+      case BP_READ:
+        act->reason = gdb_action_t::ACT_RWATCH;
+        break;
+      }
+      act->data = stopped_at->addr;
     }
-    act->data = stopped_at->addr;
     break;
 
   default:
@@ -58,21 +62,21 @@ static void nemu_is_stopped(gdb_action_t *act, breakpoint_t *stopped_at) {
   }
 }
 
-void nemu_cont(void *args, gdb_action_t *res) {
+__EXPORT void nemu_cont(void *args, gdb_action_t *res) {
   DbgState *dbg_state = (DbgState *)args;
   breakpoint_t *stopped_at =
       cpu_exec_with_bp(-1, dbg_state->bp->data(), dbg_state->bp->size());
   nemu_is_stopped(res, stopped_at);
 }
 
-void nemu_stepi(void *args, gdb_action_t *res) {
+__EXPORT void nemu_stepi(void *args, gdb_action_t *res) {
   DbgState *dbg_state = (DbgState *)args;
   breakpoint_t *stopped_at =
       cpu_exec_with_bp(1, dbg_state->bp->data(), dbg_state->bp->size());
   nemu_is_stopped(res, stopped_at);
 }
 
-bool nemu_set_bp(void *args, size_t addr, bp_type_t type) {
+__EXPORT bool nemu_set_bp(void *args, size_t addr, bp_type_t type) {
   DbgState *dbg_state = (DbgState *)args;
   for (const auto &bp : *dbg_state->bp) {
     if (bp.addr == addr && bp.type == type) {
@@ -83,7 +87,7 @@ bool nemu_set_bp(void *args, size_t addr, bp_type_t type) {
   return true;
 }
 
-bool nemu_del_bp(void *args, size_t addr, bp_type_t type) {
+__EXPORT bool nemu_del_bp(void *args, size_t addr, bp_type_t type) {
   DbgState *dbg_state = (DbgState *)args;
   for (auto it = dbg_state->bp->begin(); it != dbg_state->bp->end(); it++) {
     if (it->addr == addr && it->type == type) {
@@ -95,24 +99,44 @@ bool nemu_del_bp(void *args, size_t addr, bp_type_t type) {
   return false;
 }
 
-void nemu_on_interrupt(void *args) {
+__EXPORT void nemu_on_interrupt(void *args) {
   // fputs("Not implemented", stderr);
 }
 
+__EXPORT int nemu_read_reg(void *args, int regno, size_t *data) {
+  return isa_read_reg(args, regno, data);
+}
+__EXPORT int nemu_write_reg(void *args, int regno, size_t data) {
+  return isa_write_reg(args, regno, data);
+}
+__EXPORT size_t argsize = sizeof(DbgState);
+
 static struct target_ops nemu_gdbstub_ops = {.cont = nemu_cont,
                                              .stepi = nemu_stepi,
-                                             .read_reg = isa_read_reg,
+                                             .read_reg = nemu_read_reg,
-                                             .write_reg = isa_write_reg,
+                                             .write_reg = nemu_write_reg,
                                              .read_mem = nemu_read_mem,
                                              .write_mem = nemu_write_mem,
                                              .set_bp = nemu_set_bp,
                                              .del_bp = nemu_del_bp,
                                              .on_interrupt = NULL};
 static DbgState dbg;
-extern "C" {
 static gdbstub_t gdbstub_priv;
 #define SOCKET_ADDR "127.0.0.1:1234"
-int nemu_gdbstub_init() {
+
+__EXPORT void nemu_init(void *args) {
+  DbgState *dbg_state = (DbgState *)args;
+  dbg_state->bp = new std::vector<breakpoint_t>();
+  dbg_state->halt = 0;
+  Assert(dbg_state->bp != NULL, "Failed to allocate breakpoint");
+
+  void init_mem();
+  init_mem();
+  /* Perform ISA dependent initialization. */
+  init_isa();
+}
+
+__EXPORT int nemu_gdbstub_init() {
   dbg.bp = new std::vector<breakpoint_t>();
   assert(dbg.bp);
   if (!gdbstub_init(&gdbstub_priv, &nemu_gdbstub_ops,
@@ -121,7 +145,8 @@ int nemu_gdbstub_init() {
   }
   return 0;
 }
-int nemu_gdbstub_run() {
+
+__EXPORT int nemu_gdbstub_run() {
   puts("Waiting for gdb connection at " SOCKET_ADDR);
   bool success = gdbstub_run(&gdbstub_priv, &dbg);
   gdbstub_close(&gdbstub_priv);

nemu/src/monitor/monitor.c

@@ -25,7 +25,7 @@ void init_mem();
 void init_difftest(char *ref_so_file, long img_size, int port);
 void init_device();
 void init_disasm(const char *triple);
-int nemu_gdbstub_init();
+void nemu_init();
 
 static void welcome() {
   Log("Trace: %s", MUXDEF(CONFIG_TRACE, ANSI_FMT("ON", ANSI_FG_GREEN),
@@ -60,7 +60,8 @@ static long load_img() {
   // Image file is searched from paths in environment variable NEMU_IMAGES_PATH if it's a relative path
   if (img_file[0] != '/') {
     char *search_paths = getenv("NEMU_IMAGES_PATH");
-    if(search_paths == NULL) search_paths = "./";
+    if (search_paths == NULL)
+      search_paths = "./";
     search_paths = strdup(search_paths);
     Trace("NEMU_IMAGES_PATH=%s", search_paths);
 
@@ -68,8 +69,10 @@ static long load_img() {
     char *p_start = search_paths;
     do {
       char *p = strchr(p_start, ':');
-      if (p != NULL) *p = '\0';
+      if (p != NULL)
-      else p = paths_end;
+        *p = '\0';
+      else
+        p = paths_end;
 
       char *file_path = malloc(p - p_start + img_filename_len + 2);
       strcpy(file_path, p_start);
@@ -86,7 +89,7 @@ static long load_img() {
 
       Assert(fp != NULL || errno == ENOENT, "Cannot open '%s'", img_file);
       p_start = p + 1;
-    } while(p_start < paths_end);
+    } while (p_start < paths_end);
     free(search_paths);
 
     Assert(fp, "Cannot find '%s'", img_file);
@@ -178,10 +181,11 @@ void init_monitor(int argc, char *argv[]) {
   init_difftest(diff_so_file, img_size, difftest_port);
 
   /* Initialize debugger */
-  if (nemu_gdbstub_init()) {
+  // if (nemu_init()) {
-    Error("Failed to init");
+  //   Error("Failed to init");
-    exit(1);
+  //   exit(1);
-  }
+  // }
+  nemu_init();
 
   // printf("elf_file: %s\n", elf_file);
   if (elf_file != NULL) {

npc/.envrc

@@ -1 +1 @@
-use flake ..#npc
+use flake ".?submodules=1#npc"

npc/CMakeLists.txt

@@ -42,7 +42,7 @@ if(BUILD_SIM_NVBOARD_TARGET)
 endif()
 find_package(CLI11 CONFIG REQUIRED)
 
-option(ENABLE_SDB "Enable simple debugger" ON)
+option(ENABLE_SDB "Enable simple debugger" OFF)
 
 find_library(NVBOARD_LIBRARY NAMES nvboard)
 find_path(NVBOARD_INCLUDE_DIR NAMES nvboard.h)

npc/csrc/Flow/CMakeLists.txt

@@ -1,6 +1,6 @@
 include(ChiselBuild)
 add_executable(V${TOPMODULE} config.cpp main.cpp)
-target_link_libraries(V${TOPMODULE} PRIVATE sdb devices)
+target_link_libraries(V${TOPMODULE} PRIVATE devices gdbstub)
 target_include_directories(V${TOPMODULE} PRIVATE ${CMAKE_SOURCE_DIR}/include)
 
 verilate(

npc/csrc/Flow/main.cpp

@@ -1,30 +1,25 @@
+extern "C" {
+#include <gdbstub.h>
+}
 #include "VFlow___024root.h"
 #include "components.hpp"
 #include <VFlow.h>
-#include <array>
 #include <config.hpp>
 #include <cstdint>
 #include <cstdlib>
 #include <devices.hpp>
-#include <disasm.hpp>
-#include <filesystem>
-#include <fstream>
 #include <memory>
-#include <sdb.hpp>
-#include <trm_difftest.hpp>
-#include <trm_interface.hpp>
 #include <types.h>
+#include <vector>
 #include <vl_wrapper.hpp>
 #include <vpi_user.h>
 #include <vpi_wrapper.hpp>
 
-using VlModule = VlModuleInterfaceCommon<VFlow>;
 using Registers = _RegistersVPI<uint32_t, 32>;
+using VlModule = VlModuleInterfaceCommon<VFlow, Registers>;
 
 // SDB::SDB<NPC::npc_interface> sdb_dut;
-using CPUState = CPUStateBase<uint32_t, 32>;
 bool g_skip_memcheck = false;
-CPUState npc_cpu;
 VlModule *top;
 Registers *regs;
 vpiHandle pc = nullptr;
@@ -32,7 +27,13 @@ vpiHandle pc = nullptr;
 const size_t PMEM_START = 0x80000000;
 const size_t PMEM_END = 0x87ffffff;
 
+struct DbgState {
+  std::vector<Breakpoint> bp;
+};
+
 extern "C" {
+
+/* === Memory Access === */
 using MMap = MemoryMap<Memory<128 * 1024>, Devices::DeviceMap>;
 void *pmem_get() {
   static Devices::DeviceMap devices{
@@ -65,127 +66,99 @@ void pmem_write(int waddr, int wdata, char wmask) {
     mem->trace((std::size_t)waddr, false, regs->get_pc(), wdata);
   return mem->write((std::size_t)waddr, wdata, wmask);
 }
+
+/* === For gdbstub === */
+
+int npc_read_mem(void *args, size_t addr, size_t len, void *val) {
+  void *pmem = pmem_get();
+  auto mmap = static_cast<MMap *>(pmem);
+  mmap->copy_to(addr, (uint8_t *)val, len);
+  return 0;
 }
 
-namespace NPC {
+int npc_write_mem(void *args, size_t addr, size_t len, void *val) {
-void npc_memcpy(paddr_t addr, void *buf, size_t sz, bool direction) {
+  void *pmem = pmem_get();
-  if (direction == TRM_FROM_MACHINE) {
+  auto mmap = static_cast<MMap *>(pmem);
-    static_cast<MMap *>(pmem_get())->copy_to(addr, (uint8_t *)buf, sz);
+  mmap->copy_from(addr, (uint8_t *)val, len);
-  }
+  return 0;
-};
+}
 
-void npc_regcpy(void *p, bool direction) {
+int npc_read_reg(void *args, int regno, size_t *value) {
+  if (regno == 32)
+    *value = regs->get_pc();
+  else
+    *value = (*regs)[regno];
+  return 0;
+}
 
-  if (direction == TRM_FROM_MACHINE) {
+int npc_write_reg(void *args, int regno, size_t value) { return 1; }
-    ((CPUState *)p)->pc = regs->get_pc();
+
-    for (int i = 0; i < 32; i++) {
+void npc_cont(void *args, gdb_action_t *res) {
-      ((CPUState *)p)->reg[i] = (*regs)[i];
+  DbgState *dbg = (DbgState *)args;
+  *res = top->eval(dbg->bp);
+}
+
+void npc_stepi(void *args, gdb_action_t *res) {
+  DbgState *dbg = (DbgState *)args;
+  *res = top->eval(dbg->bp);
+}
+
+bool npc_set_bp(void *args, size_t addr, bp_type_t type) {
+  DbgState *dbg = (DbgState *)args;
+  for (const auto &bp : dbg->bp) {
+    if (bp.addr == addr && bp.type == type) {
+      return true;
     }
   }
+  dbg->bp.push_back({.addr = addr, .type = type});
+  return true;
 }
 
-void npc_exec(uint64_t n) {
+bool npc_del_bp(void *args, size_t addr, bp_type_t type) {
-  while (n--) {
+  DbgState *dbg = (DbgState *)args;
-    for (int i = 0; i < 2; i++) {
+  for (auto it = dbg->bp.begin(); it != dbg->bp.end(); it++) {
-      if (top->is_posedge()) {
+    if (it->addr == addr && it->type == type) {
-        // Posedge
+      std::swap(*it, *dbg->bp.rbegin());
-        regs->update();
+      dbg->bp.pop_back();
-      }
+      return true;
-      top->eval();
     }
   }
+  return false;
 }
 
-void npc_atexit(void) {
+static target_ops npc_gdbstub_ops = {.cont = npc_cont,
-  delete top;
+                                     .stepi = npc_stepi,
-  delete regs;
+                                     .read_reg = npc_read_reg,
-}
+                                     .write_reg = npc_write_reg,
+                                     .read_mem = npc_read_mem,
+                                     .write_mem = npc_write_mem,
+                                     .set_bp = npc_set_bp,
+                                     .del_bp = npc_del_bp,
+                                     .on_interrupt = NULL};
 
-void npc_init(int port) {
+static gdbstub_t gdbstub_priv;
-  top = new VlModule{config.wavefile};
+static DbgState dbg;
-  regs = new Registers("TOP.Flow.reg_0.regFile_", "TOP.Flow.pc.out");
+arch_info_t isa_arch_info = {
-  atexit(npc_atexit);
+    .target_desc = strdup(TARGET_RV32), .reg_num = 33, .reg_byte = 4};
-  top->reset_eval(10);
-}
 
-class DutTrmInterface : public TrmInterface {
+int gdbstub_loop() {
-public:
+  if (!gdbstub_init(&gdbstub_priv, &npc_gdbstub_ops, (arch_info_t)isa_arch_info,
-  DutTrmInterface(memcpy_t f_memcpy, regcpy_t f_regcpy, exec_t f_exec,
+                    strdup("127.0.0.1:1234"))) {
-                  init_t f_init, void *cpu_state)
+    return EINVAL;
-      : TrmInterface{f_memcpy, f_regcpy, f_exec, f_init, cpu_state} {}
-  word_t at(std::string name) const override {
-    return ((CPUState *)cpu_state)->at(name);
   }
-
+  bool success = gdbstub_run(&gdbstub_priv, &dbg);
-  word_t at(paddr_t addr) const override {
+  gdbstub_close(&gdbstub_priv);
-    word_t buf;
+  return !success;
-    this->memcpy(addr, &buf, sizeof(word_t), TRM_FROM_MACHINE);
-    return buf;
-  }
-  void print(std::ostream &os) const override {
-    this->regcpy(cpu_state, TRM_FROM_MACHINE);
-    os << *(CPUState *)cpu_state << std::endl;
-  }
-};
-
-DutTrmInterface npc_interface =
-    DutTrmInterface{&npc_memcpy, &npc_regcpy, &npc_exec, &npc_init, &npc_cpu};
-}; // namespace NPC
-
-extern "C" {
-word_t reg_str2val(const char *name, bool *success) {
-  return npc_cpu.reg_str2val(name, success);
-}
 }
+} // extern "C"
 
 int main(int argc, char **argv, char **env) {
   config.cli_parse(argc, argv);
 
-  if (config.lib_ref.empty()) {
+  top = new VlModule;
-    if (config.interactive) {
+  regs = new Registers("TOP.Flow.reg_0.regFile_", "TOP.Flow.pc.out");
-      SDB::SDB sdb_npc{NPC::npc_interface};
+  top->setup(config.wavefile, regs);
-      sdb_npc.main_loop();
+  top->reset_eval(10);
-      return 0;
-    } else {
-      NPC::npc_interface.init(0);
-      while (true) {
-        word_t inst = NPC::npc_interface.at(regs->get_pc());
-        if (inst == 1048691) {
-          return 0;
-        }
-        NPC::npc_interface.exec(1);
-      }
-    }
-  }
 
-  /* -- Difftest -- */
+  return gdbstub_loop();
-  std::filesystem::path ref{config.lib_ref};
-  RefTrmInterface ref_interface{ref};
-  DifftestTrmInterface diff_interface{
-      NPC::npc_interface, ref_interface,
-      static_cast<MMap *>(pmem_get())->get_pmem(), 128 * 1024};
-  if (config.interactive) {
-    SDB::SDB sdb_diff{diff_interface};
-    sdb_diff.main_loop();
-    return 0;
-  }
-
-  try {
-    diff_interface.init(0);
-    diff_interface.exec(-1);
-  } catch (TrmRuntimeException &e) {
-    switch (e.error_code()) {
-    case TrmRuntimeException::EBREAK:
-      return 0;
-    case TrmRuntimeException::DIFFTEST_FAILED:
-      std::cout << "Difftest Failed" << std::endl;
-      diff_interface.print(std::cout);
-      return 1;
-    default:
-      std::cerr << "Unknown error happened" << std::endl;
-      return 1;
-    }
-  }
-
-  return 0;
 }

npc/include/components.hpp

@@ -20,12 +20,12 @@
 template <typename T, std::size_t nr> class _RegistersBase {
   std::array<T, nr> regs;
   T pc;
-  virtual T fetch_pc();
+  virtual T fetch_pc() const;
-  virtual T fetch_reg(std::size_t id);
+  virtual T fetch_reg(std::size_t id) const;
 
 public:
-  T operator[](size_t id) { return fetch_reg(id); }
+  T operator[](size_t id) const { return fetch_reg(id); }
-  T get_pc() { return fetch_pc(); }
+  T get_pc() const { return fetch_pc(); }
   void update() {
     for (int i = 0; i < regs.size(); i++) {
       regs[i] = fetch_reg(i);
@@ -112,14 +112,14 @@ public:
     if (ram->in_pmem(addr)) {
       ram->transfer(addr, buf, len, false);
     } else {
-      std::cerr << "Not in pmem" << std::endl;
+      std::cerr << "0x" << std::hex << addr << " not in pmem" << std::endl;
     }
   }
-  void copy_from(paddr_t addr, const uint8_t *buf, size_t len) {
+  void copy_from(paddr_t addr, uint8_t *buf, size_t len) {
     if (ram->in_pmem(addr)) {
       ram->transfer(addr, buf, len, true);
     } else {
-      std::cerr << "Not in pmem" << std::endl;
+      std::cerr << "0x" << std::hex << addr << " not in pmem" << std::endl;
     }
   }
   void *get_pmem() { return ram->mem.data(); }

npc/include/types.h

@@ -29,4 +29,10 @@ const std::map<std::string, int> riscv32_regs_by_name{
     {"t5", 30}, {"t6", 31}};
 #endif
 
+#include <gdbstub.h>
+struct Breakpoint {
+  size_t addr;
+  bp_type_t type;
+};
+
 #endif

npc/include/vl_wrapper.hpp

@@ -1,7 +1,10 @@
 #ifndef _NPC_TRACER_H_
 #define _NPC_TRACER_H_
 #include "components.hpp"
+#include "types.h"
 #include <filesystem>
+#include <gdbstub.h>
+#include <sys/types.h>
 #include <verilated_vcd_c.h>
 
 template <class T> class Tracer {
@@ -26,16 +29,24 @@ public:
   void update() { m_trace->dump(cycle++); }
 };
 
-template <typename T> class VlModuleInterfaceCommon : public T {
+template <typename T, typename R> class VlModuleInterfaceCommon : public T {
   uint64_t sim_time = 0;
   uint64_t posedge_cnt = 0;
   std::unique_ptr<Tracer<T>> tracer;
 
 public:
-  VlModuleInterfaceCommon<T>(std::filesystem::path wavefile) {
+  const R *registers;
+  VlModuleInterfaceCommon<T, R>() {
+    tracer = nullptr;
+    registers = nullptr;
+  }
+
+  void setup(std::filesystem::path wavefile, const R *r) {
     if (!wavefile.empty())
       tracer = std::make_unique<Tracer<T>>(this, wavefile);
+    registers = r;
   }
+
   void eval() {
     if (this->is_posedge()) {
       posedge_cnt++;
@@ -51,6 +62,25 @@ public:
       this->eval();
     }
   }
+
+  gdb_action_t eval(const std::vector<Breakpoint> &breakpoints) {
+    gdb_action_t res;
+    do {
+      this->eval();
+      size_t pc = registers->get_pc();
+      for (const auto &bp : breakpoints) {
+        if (pc == bp.addr) {
+          res.data = bp.addr;
+          switch (bp.type) {
+          default:
+            res.reason = gdb_action_t::ACT_BREAKPOINT;
+          }
+          return res;
+        }
+      }
+    } while (true);
+  }
+
   void reset_eval(int n) {
     extern bool g_skip_memcheck;
     g_skip_memcheck = true;

npc/include/vpi_wrapper.hpp

@@ -7,14 +7,14 @@ template <typename T, std::size_t nr>
 class _RegistersVPI : public _RegistersBase<T, nr> {
   std::array<vpiHandle, nr> reg_handles;
   vpiHandle pc_handle;
-  T vpi_get(vpiHandle vh) {
+  T vpi_get(vpiHandle vh) const {
     s_vpi_value v;
     v.format = vpiIntVal;
     vpi_get_value(vh, &v);
     return v.value.integer;
   }
-  T fetch_pc(void) { return vpi_get(pc_handle); }
+  T fetch_pc(void) const { return vpi_get(pc_handle); }
-  T fetch_reg(std::size_t id) { return vpi_get(reg_handles[id]); }
+  T fetch_reg(std::size_t id) const { return vpi_get(reg_handles[id]); }
 
 public:
   _RegistersVPI<T, nr>(const std::string regs_prefix,