Add --config-override flag for configuration overrides

README.md

@@ -111,6 +111,11 @@ use a custom configuration, save the default configuration into a
 file, edit it as needed, and pass it to the simulator using the
 `--config` option.
 
+To override only a small subset of options while using the default configuration
+or a custom configuration file as a base, the `--config-override` option can be
+used. This option allows one or more additional JSON configuration files to be specified,
+whose fields take precedence over those in the base configuration.
+
 Information on other options for the simulator is available from
 `build/c_emulator/sail_riscv_sim -h`.
 

c_emulator/CMakeLists.txt

@@ -38,6 +38,8 @@ add_library(riscv_model
     riscv_softfloat.h
     config_utils.cpp
     config_utils.h
+    file_utils.cpp
+    file_utils.h
     symbol_table.cpp
     symbol_table.h
     sail_riscv_version.h

c_emulator/config_utils.cpp

@@ -1,12 +1,13 @@
 #include "config_utils.h"
 
 #include <fstream>
-#include <jsoncons/json.hpp>
 #include <jsoncons_ext/jsonschema/jsonschema.hpp>
+#include <sstream>
 #include <stdexcept>
 
 #include "config_schema.h"
 #include "default_config.h"
+#include "file_utils.h"
 #include "sail.h"
 #include "sail_config.h"
 #include "sail_riscv_model.h"
@@ -71,7 +72,7 @@ const char *get_config_schema() {
   return CONFIG_SCHEMA;
 }
 
-void validate_config_schema(const std::string &conf_file) {
+void validate_config_schema(const jsoncons::json &json_config, const std::string &source_desc) {
   using jsoncons::json;
   namespace jsonschema = jsoncons::jsonschema;
 
@@ -81,30 +82,15 @@ void validate_config_schema(const std::string &conf_file) {
   // Throws schema_error if compilation fails.
   jsonschema::json_schema<json> compiled = jsonschema::make_json_schema(std::move(schema), options);
 
-  std::string source = conf_file.empty() ? "default configuration" : "configuration in " + conf_file;
-
-  // Parse the config.
-  json config;
-  try {
-    if (conf_file.empty()) {
-      config = json::parse(get_default_config());
-    } else {
-      std::ifstream is(conf_file);
-      config = json::parse(is);
-    }
-  } catch (const std::exception &ex) {
-    throw std::runtime_error("Cannot parse " + source + ": " + ex.what() + ".");
-  }
-
-  // Validate.
   bool is_valid = true;
-  auto report = [&is_valid](const jsonschema::validation_message &msg) -> jsonschema::walk_result {
+  std::ostringstream error_stream;
+  auto report = [&is_valid, &error_stream](const jsonschema::validation_message &msg) -> jsonschema::walk_result {
     is_valid = false;
-    std::cerr << msg.instance_location().string() << ": " << msg.message() << std::endl;
+    error_stream << "- " << msg.instance_location().string() << ": " << msg.message() << "\n";
     return jsonschema::walk_result::advance;
   };
-  compiled.validate(config, report);
+  compiled.validate(json_config, report);
   if (!is_valid) {
-    throw std::runtime_error("Schema conformance check failed for the " + source + ".");
+    throw std::runtime_error("Schema conformance check failed for " + source_desc + ":\n" + error_stream.str());
   }
 }

c_emulator/config_utils.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <cstdint>
+#include <jsoncons/json.hpp>
 #include <string>
 #include <vector>
 
@@ -13,4 +14,4 @@ uint64_t get_config_uint64(const std::vector<const char *> &keypath);
 const char *get_default_config();
 const char *get_config_schema();
 
-void validate_config_schema(const std::string &conf_file);
+void validate_config_schema(const jsoncons::json &json_config, const std::string &source_desc);

c_emulator/file_utils.cpp

@@ -0,0 +1,26 @@
+#include "file_utils.h"
+#include <cstdint>
+#include <fstream>
+#include <sstream>
+#include <stdexcept>
+
+std::string read_file_to_string(const std::string &file_path) {
+  std::ifstream instream(file_path);
+  if (!instream) {
+    throw std::runtime_error("Failed to open file: " + file_path);
+  }
+
+  std::ostringstream ss;
+  ss << instream.rdbuf();
+  return ss.str();
+}
+
+std::vector<uint8_t> read_file(const std::string &file_path) {
+  std::ifstream instream(file_path, std::ios::in | std::ios::binary);
+  if (!instream) {
+    throw std::runtime_error("Failed to open file: " + file_path);
+  }
+
+  std::vector<uint8_t> data{std::istreambuf_iterator<char>(instream), std::istreambuf_iterator<char>()};
+  return data;
+}

c_emulator/file_utils.h

@@ -0,0 +1,6 @@
+#include <cstdint>
+#include <string>
+#include <vector>
+
+std::string read_file_to_string(const std::string &file_path);
+std::vector<uint8_t> read_file(const std::string &file_path);

c_emulator/riscv_sim.cpp

@@ -21,6 +21,7 @@
 #include "CLI11.hpp"
 #include "elf_loader.h"
 #include "jsoncons/config/version.hpp"
+#include "jsoncons/json.hpp"
 #include "rts.h"
 #include "sail.h"
 #include "sail_config.h"
@@ -29,6 +30,7 @@
 #include "sail_coverage.h"
 #endif
 #include "config_utils.h"
+#include "file_utils.h"
 #include "riscv_callbacks_log.h"
 #include "riscv_callbacks_rvfi.h"
 #include "riscv_model_impl.h"
@@ -87,9 +89,29 @@ static void print_build_info() {
   std::cout << "JSONCONS: " << jsoncons::version() << std::endl;
 }
 
-std::vector<uint8_t> read_file(const std::string &file_path) {
-  std::ifstream instream(file_path, std::ios::in | std::ios::binary);
-  return {std::istreambuf_iterator<char>(instream), std::istreambuf_iterator<char>()};
+static jsoncons::json parse_json_or_exit(const std::string &json_text, const std::string &source_desc) {
+  try {
+    return jsoncons::json::parse(json_text);
+  } catch (const jsoncons::json_exception &e) {
+    std::cerr << "JSON parse error in " << source_desc << ":\n" << e.what() << "\n\n";
+    exit(EXIT_FAILURE);
+  }
+}
+
+// JSON objects are merged by replacing/adding fields from the override config.
+// If a given field is an object in the base and override then instead of
+// replacing the field entirely the merging process recurses into it.
+// All other field types (including arrays) are simply replaced.
+void deep_merge_json(jsoncons::json &base, const jsoncons::json &json_override) {
+  for (const auto &entry : json_override.object_range()) {
+    const auto &key = entry.key();
+    const auto &value = entry.value();
+    if (base.contains(key) && base[key].is_object() && value.is_object()) {
+      deep_merge_json(base[key], value);
+    } else {
+      base[key] = value;
+    }
+  }
 }
 
 const unsigned DEFAULT_SIGNATURE_GRANULARITY = 4;
@@ -105,6 +127,7 @@ struct CLIOptions {
   bool do_print_isa = false;
 
   std::string config_file;
+  std::vector<std::string> config_overrides;
   std::string term_log;
   std::string trace_log_path;
   std::string dtb_file;
@@ -169,6 +192,16 @@ static CLIOptions parse_cli(int argc, char **argv) {
   app.add_option("--terminal-log", opts.term_log, "Terminal log output file")->option_text("<file>");
   app.add_option("--test-signature", opts.sig_file, "Test signature file")->option_text("<file>");
   app.add_option("--config", opts.config_file, "Configuration file")->check(CLI::ExistingFile)->option_text("<file>");
+  app
+    .add_option(
+      "--config-override",
+      opts.config_overrides,
+      "Configuration override file (repeatable; later files override earlier ones). Use this when you only want to "
+      "change a small part of the base configuration."
+    )
+    ->check(CLI::ExistingFile)
+    ->option_text("<file>")
+    ->allow_extra_args(false);
   app.add_option("--trace-output", opts.trace_log_path, "Trace output file")->option_text("<file>");
 
   app.add_option("--signature-granularity", opts.signature_granularity, "Signature granularity")->option_text("<uint>");
@@ -598,15 +631,39 @@ int inner_main(int argc, char **argv) {
 
   model.set_config_print_step(opts.config_print_step);
 
-  // Always validate the schema conformance of the config.
-  validate_config_schema(opts.config_file);
-
-  // Initialize the model.
+  std::string config_json_string;
   if (!opts.config_file.empty()) {
-    sail_config_set_file(opts.config_file.c_str());
+    config_json_string = read_file_to_string(opts.config_file);
   } else {
-    sail_config_set_string(get_default_config());
+    config_json_string = get_default_config();
+  }
+
+  // Check json config and merge overrides
+  const std::string base_source_desc =
+    opts.config_file.empty() ? "default configuration" : "configuration file " + opts.config_file;
+  jsoncons::json config_json = parse_json_or_exit(config_json_string, base_source_desc);
+  for (const auto &override_path : opts.config_overrides) {
+    std::string override_json_string = read_file_to_string(override_path);
+    jsoncons::json override_item = parse_json_or_exit(override_json_string, "override file " + override_path);
+    deep_merge_json(config_json, override_item);
+  }
+
+  std::ostringstream os;
+  os << config_json;
+  config_json_string = os.str();
+
+  // Always validate the schema conformance of the config.
+  std::string config_source_desc = opts.config_file.empty() ? "default configuration" : opts.config_file;
+  if (!opts.config_overrides.empty()) {
+    config_source_desc = "merged configuration from " + config_source_desc;
+    for (const auto &override_path : opts.config_overrides) {
+      config_source_desc = config_source_desc + ", " + override_path;
+    }
   }
+  validate_config_schema(config_json, config_source_desc);
+
+  // Initialize the model.
+  sail_config_set_string(config_json_string.c_str());
 
   // Initialize platform.
   init_platform_constants(model);

doc/ChangeLog.md

@@ -7,6 +7,9 @@
 
 - A configuration parameter to specify the size of the reservation set
   for Zalrsc atomics has been added.
+- A `--config-override` cli option has been added to specify one or more
+  additional JSON configuration files that override the corresponding fields
+  in a configuration.
 
 - The following extensions have been added:
   - Za64rs, Za128rs