feat: add possibility for custom CSRs

Currently in riscv-isa-sim there's no way to make a custom extension
that adds new CSRs. This simple patch makes it possible via new
virtual function in extension_t class.

ci-tests/create-ci-binary-tarball

@@ -16,6 +16,10 @@ mkdir -p build/dummy-slliuw && cd "$_"
 riscv64-unknown-elf-gcc -O2 -o dummy-slliuw `git rev-parse --show-toplevel`/ci-tests/dummy-slliuw.c
 cd -
 
+mkdir -p build/dummycsr && cd "$_"
+riscv64-unknown-elf-gcc -O2 -o customcsr `git rev-parse --show-toplevel`/ci-tests/customcsr.c
+cd -
+
 mv build/pk/pk .
 mv build/hello/hello .
 mv build/dummy-slliuw/dummy-slliuw .

ci-tests/custom-csr.cc

@@ -0,0 +1,81 @@
+#include <riscv/extension.h>
+#include <riscv/sim.h>
+
+
+class dummycsr_t: public csr_t {
+  public:
+  dummycsr_t(processor_t *proc, const reg_t addr): csr_t(proc, addr) {}
+
+  reg_t read() const noexcept override {
+    return 42;
+  }
+
+  void verify_permissions(insn_t insn, bool write) const override {}
+
+  protected:
+  bool unlogged_write(const reg_t val) noexcept override {
+    return true;
+  }
+};
+
+// dummy extension with dummy CSRs. Nice.
+struct xdummycsr_t : public extension_t {
+  const char *name() { return "dummycsr"; }
+
+  xdummycsr_t() {}
+
+  std::vector<insn_desc_t> get_instructions() override {
+    return {};
+  }
+
+  std::vector<disasm_insn_t *> get_disasms() override {
+    return {};
+  }
+
+  std::vector<csr_t_p> get_csrs(processor_t &proc) const override {
+    return {std::make_shared<dummycsr_t>(&proc, /*Addr*/ 0xfff)};
+  }
+};
+
+REGISTER_EXTENSION(dummycsr, []() { return new xdummycsr_t; })
+
+// Copied from spike main.
+// TODO: This should really be provided in libriscv
+static std::vector<std::pair<reg_t, abstract_mem_t *>>
+make_mems(const std::vector<mem_cfg_t> &layout) {
+  std::vector<std::pair<reg_t, abstract_mem_t *>> mems;
+  mems.reserve(layout.size());
+  for (const auto &cfg : layout) {
+    mems.push_back(std::make_pair(cfg.get_base(), new mem_t(cfg.get_size())));
+  }
+  return mems;
+}
+
+int main(int argc, char **argv) {
+  cfg_t cfg;
+  cfg.isa = "RV64IMAFDCV_Zicsr_xdummycsr";
+  std::vector<device_factory_sargs_t> plugin_devices;
+  if (argc != 3) {
+    std::cerr << "Error: invalid arguments\n";
+    exit(1);
+  }
+  std::vector<std::string> htif_args{argv[1] /* pk */, argv[2] /* executable */};
+  debug_module_config_t dm_config = {.progbufsize = 2,
+                                     .max_sba_data_width = 0,
+                                     .require_authentication = false,
+                                     .abstract_rti = 0,
+                                     .support_hasel = true,
+                                     .support_abstract_csr_access = true,
+                                     .support_abstract_fpr_access = true,
+                                     .support_haltgroups = true,
+                                     .support_impebreak = true};
+  std::vector<std::pair<reg_t, abstract_mem_t *>> mems =
+      make_mems(cfg.mem_layout);
+  sim_t sim(&cfg, false, mems, plugin_devices, htif_args, dm_config,
+            nullptr,  // log_path
+            true,     // dtb_enabled
+            nullptr,  // dtb_file
+            false,    // socket_enabled
+            nullptr); // cmd_file
+  sim.run();
+}

ci-tests/customcsr.c

@@ -0,0 +1,12 @@
+#include <stdio.h>
+
+int main() {
+  int x = 1;
+  //            dummycsr
+  asm("csrr %0, 0xfff" : "=r"(x));
+  if (x == 42)
+    printf("Executed successfully\n");
+  else
+    printf("FAIL. Got value: %d instead of 42\n", x);
+  return 0;
+}

ci-tests/test-spike

@@ -15,9 +15,11 @@ time ../install/bin/spike --isa=rv64gc pk hello | grep "Hello, world!  Pi is app
 # check that including sim.h in an external project works
 g++ -std=c++2a -I../install/include -L../install/lib $DIR/testlib.cc -lriscv -o test-libriscv
 g++ -std=c++2a -I../install/include -L../install/lib $DIR/test-customext.cc -lriscv -o test-customext
+g++ -std=c++2a -I../install/include -L../install/lib $DIR/customext-csr.cc -lriscv -o test-custom-csr
 
 # check that all installed headers are functional
 g++ -std=c++2a -I../install/include -L../install/lib $DIR/testlib.cc -lriscv -o /dev/null -include ../install-hdrs-list.h
 
 LD_LIBRARY_PATH=../install/lib ./test-libriscv pk hello| grep "Hello, world!  Pi is approximately 3.141588."
 LD_LIBRARY_PATH=../install/lib ./test-customext pk dummy-slliuw | grep "Executed successfully"
+LD_LIBRARY_PATH=../install/lib ./test-custom-csr pk customcsr | grep "Executed successfully"

riscv/extension.h

@@ -13,6 +13,7 @@ class extension_t
  public:
   virtual std::vector<insn_desc_t> get_instructions() = 0;
   virtual std::vector<disasm_insn_t*> get_disasms() = 0;
+  virtual std::vector<csr_t_p> get_csrs ([[maybe_unused]] processor_t &proc) const { return {}; };
   virtual const char* name() = 0;
   virtual void reset() {};
   virtual void set_debug(bool UNUSED value) {}

riscv/processor.cc

@@ -711,6 +711,8 @@ void processor_t::register_extension(extension_t *x) {
     fprintf(stderr, "extensions must have unique names (got two named \"%s\"!)\n", x->name());
     abort();
   }
+  for (auto &csr: x->get_csrs(*this))
+    state.add_csr(csr->address, csr);
   x->set_processor(this);
 }