massive feat: added a GDB stub for debugging

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

.github/workflows/clang.yml

@@ -21,7 +21,7 @@ jobs:
 
 
       - name: setup
-        run: nix develop .#matar-clang -c meson setup $BUILDDIR
+        run: nix develop .#matar-clang -c meson setup $BUILDDIR -Dgdb_debug=true
 
       - name: fmt
         run: nix develop .#matar-clang -c ninja clang-format-check -C $BUILDDIR

.github/workflows/gcc.yml

@@ -21,7 +21,7 @@ jobs:
 
 
       - name: setup
-        run: nix develop .#matar -c meson setup $BUILDDIR
+        run: nix develop .#matar -c meson setup $BUILDDIR -Dgdb_debug=true
 
       - name: build
         run: nix develop .#matar -c ninja -C $BUILDDIR

include/bus.hh

@@ -31,7 +31,6 @@ class Bus {
 
     uint32_t read_word(uint32_t, bool = true);
     void write_word(uint32_t, uint32_t, bool = true);
-
     // not sure what else to do?
     inline void internal_cycle() { cycles++; }
 

include/cpu/cpu.hh

@@ -4,11 +4,19 @@
 #include "bus.hh"
 #include "cpu/psr.hh"
 #include "thumb/instruction.hh"
+#include <cstdint>
 #include <memory>
 
-#include <cstdint>
+#ifdef GDB_DEBUG
+#include <unordered_set>
+#endif
 
 namespace matar {
+
+#ifdef GDB_DEBUG
+class GdbRsp;
+#endif
+
 class Cpu {
   public:
     Cpu(std::shared_ptr<Bus>) noexcept;
@@ -16,24 +24,12 @@ class Cpu {
     void step();
     void chg_mode(const Mode to);
 
-    bool is_flushed = false;
+    inline bool is_halted() { return halted; }
-    inline void flush_pipeline() {
+    inline void resume() { halted = false; }
-        is_flushed = true;
-        if (cpsr.state() == State::Arm) {
-            opcodes[0] = bus->read_word(pc, false);
-            advance_pc_arm();
-            opcodes[1] = bus->read_word(pc);
-            advance_pc_arm();
-        } else {
-            opcodes[0] = bus->read_halfword(pc, false);
-            advance_pc_thumb();
-            opcodes[1] = bus->read_halfword(pc);
-            advance_pc_thumb();
-        }
-        sequential = true;
-    };
 
   private:
+    bool halted = false;
+
     friend void arm::Instruction::exec(Cpu& cpu);
     friend void thumb::Instruction::exec(Cpu& cpu);
 
@@ -93,5 +89,26 @@ class Cpu {
 
     inline void advance_pc_arm() { pc += arm::INSTRUCTION_SIZE; };
     inline void advance_pc_thumb() { pc += thumb::INSTRUCTION_SIZE; }
+
+    bool is_flushed = false;
+    inline void flush_pipeline() {
+        if (cpsr.state() == State::Arm) {
+            opcodes[0] = bus->read_word(pc, false);
+            advance_pc_arm();
+            opcodes[1] = bus->read_word(pc);
+            advance_pc_arm();
+        } else {
+            opcodes[0] = bus->read_halfword(pc, false);
+            advance_pc_thumb();
+            opcodes[1] = bus->read_halfword(pc);
+            advance_pc_thumb();
+        }
+        sequential = true;
+    };
+
+#ifdef GDB_DEBUG
+    friend class GdbRsp;
+    std::unordered_set<uint32_t> breakpoints = {};
+#endif
 };
 }

meson.build

@@ -7,8 +7,18 @@ project('matar', 'cpp',
                      'cpp_std=c++23',
                      'default_library=static'])
 
+lib_cpp_args = []
 compiler = meson.get_compiler('cpp')
 
+if get_option('disassembler')
+  lib_cpp_args += '-DDISASSEMBLER'
+endif
+
+if get_option('gdb_debug')
+  lib_cpp_args += '-DGDB_DEBUG'
+endif
+
+
 subdir('include')
 subdir('src')
 subdir('apps')

meson.options

@@ -1,2 +1,3 @@
 option('tests', type : 'boolean', value : true, description: 'enable tests')
 option('disassembler', type: 'boolean', value: true, description: 'enable disassembler')
+option('gdb_debug', type: 'boolean', value: false, description: 'enable GDB RSP server')

src/bus.cc

@@ -136,6 +136,7 @@ Bus::write(uint32_t address) {
 #undef MATCHES
     }
 
+    glogger.error("Invalid memory region written");
     return {};
 }
 

src/cpu/cpu.cc

@@ -129,6 +129,14 @@ Cpu::step() {
         // word align
         rst_bit(pc, 1);
 
+#ifdef GDB_DEBUG
+        if (breakpoints.contains(pc - 2 * arm::INSTRUCTION_SIZE)) {
+            glogger.info_bold("CPU halted");
+            halted = true;
+            return;
+        }
+#endif
+
         arm::Instruction instruction(opcodes[0]);
 
         opcodes[0] = opcodes[1];
@@ -148,6 +156,15 @@ Cpu::step() {
         } else
             advance_pc_arm();
     } else {
+
+#ifdef GDB_DEBUG
+        if (breakpoints.contains(pc - 2 * thumb::INSTRUCTION_SIZE)) {
+            glogger.info_bold("CPU halted");
+            halted = true;
+            return;
+        }
+#endif
+
         thumb::Instruction instruction(opcodes[0]);
 
         opcodes[0] = opcodes[1];

src/gdb_rsp.cc

@@ -0,0 +1,481 @@
+#include "gdb_rsp.hh"
+#include "util/log.hh"
+#include <csignal>
+#include <numeric>
+#include <regex>
+#include <stdexcept>
+#include <string>
+
+namespace matar {
+
+template<typename... Args>
+static inline constexpr void
+gdb_log(const std::format_string<Args...>& fmt, Args&&... args) {
+    glogger.debug("GDB: {}", std::format(fmt, std::forward<Args>(args)...));
+}
+
+static inline void
+append_le(std::string& str, uint32_t value) {
+    // little endian only
+    str += std::format("{:02x}", value & 0xFF);
+    str += std::format("{:02x}", value >> 8 & 0xFF);
+    str += std::format("{:02x}", value >> 16 & 0xFF);
+    str += std::format("{:02x}", value >> 24 & 0xFF);
+}
+
+static inline std::string
+be_to_le(std::string str) {
+    if (str.length() != 8)
+        throw std::out_of_range("string is supposed to be 8 bytes");
+
+    std::string current;
+
+    for (int i = 7; i >= 0; i -= 2) {
+        current += str[i - 1];
+        current += str[i];
+    }
+
+    return current;
+}
+
+GdbRsp::GdbRsp(std::shared_ptr<Cpu> cpu)
+  : cpu(cpu) {}
+
+void
+GdbRsp::start(const uint port) {
+    server.start(port);
+    server.run();
+
+    std::string msg;
+
+    while (!attached) {
+        step();
+    }
+}
+
+void
+GdbRsp::satisfy_client() {
+    while (server.client_waiting() && attached) {
+        step();
+    }
+}
+
+void
+GdbRsp::step() {
+    std::string msg = receive();
+
+    switch (msg[0]) {
+        case '+':
+            break;
+        case '-':
+            break;
+        case '\x03':
+            gdb_log("ctrl+c interrupt received");
+            cmd_halted();
+            break;
+        case '$': {
+            if (msg.starts_with("$qSupported")) {
+                acknowledge();
+                cmd_supported(msg);
+            } else if (msg.starts_with("$qAttached")) {
+                acknowledge();
+                cmd_attached();
+            } else {
+                switch (msg[1]) {
+                    case '?':
+                        acknowledge();
+                        cmd_halted();
+                        break;
+                    case 'g':
+                        acknowledge();
+                        cmd_read_registers();
+                        break;
+                    case 'G':
+                        acknowledge();
+                        cmd_write_registers(msg);
+                        break;
+                    case 'p':
+                        acknowledge();
+                        cmd_read_register(msg);
+                        break;
+                    case 'P':
+                        acknowledge();
+                        cmd_write_register(msg);
+                        break;
+                    case 'm':
+                        acknowledge();
+                        cmd_read_memory(msg);
+                        break;
+                    case 'M':
+                        acknowledge();
+                        cmd_write_memory(msg);
+                        break;
+                    case 'z':
+                        acknowledge();
+                        cmd_rm_breakpoint(msg);
+                        break;
+                    case 'Z':
+                        acknowledge();
+                        cmd_add_breakpoint(msg);
+                        break;
+                    case 'c':
+                        acknowledge();
+                        cmd_continue();
+                        break;
+                    case 'D':
+                        acknowledge();
+                        cmd_detach();
+                        break;
+                    default:
+                        gdb_log("unknown command");
+                        send_empty();
+                }
+            }
+
+            break;
+        }
+        default:
+            gdb_log("unknown message received");
+    }
+}
+
+std::string
+GdbRsp::receive() {
+    std::string msg = server.receive(1);
+    char ch         = msg[0];
+    int checksum    = 0;
+
+    if (ch == '$') {
+        while ((ch = server.receive(1)[0]) != '#') {
+            checksum += static_cast<uint>(ch);
+            msg += ch;
+            if (msg.length() > MAX_MSG_LEN) {
+                throw std::logic_error("GDB: received message is too long");
+            }
+        }
+
+        if (std::stoul(server.receive(2), nullptr, 16) != (checksum & 0xFF)) {
+            gdb_log("{}", msg);
+            throw std::logic_error("GDB: bad message checksum");
+        }
+    }
+
+    gdb_log("received message \"{}\"", msg);
+    return msg;
+}
+
+std::string
+GdbRsp::make_packet(std::string raw) {
+    uint checksum = std::accumulate(raw.begin(), raw.end(), 0);
+    return std::format("${}#{:02x}", raw, checksum & 0xFF);
+}
+
+void
+GdbRsp::acknowledge() {
+    server.send("+");
+}
+
+void
+GdbRsp::send_empty() {
+    acknowledge();
+    server.send(make_packet(""));
+}
+
+void
+GdbRsp::notify_breakpoint_reached() {
+    gdb_log("reached breakpoint, sending signal");
+    server.send(make_packet(std::format("S{:02x}", SIGTRAP)));
+}
+
+void
+GdbRsp::cmd_attached() {
+    attached = true;
+
+    gdb_log("server is now attached");
+    server.send(make_packet("1"));
+}
+
+void
+GdbRsp::cmd_supported(std::string msg) {
+    std::string response;
+
+    if (msg.find("hwbreak+;") != std::string::npos)
+        response += "hwbreak+;";
+
+    gdb_log("sending response for qSupported");
+    server.send(make_packet(response));
+}
+
+void
+GdbRsp::cmd_halted() {
+    gdb_log("sending reason for upcoming halt");
+    server.send(make_packet(std::format("S{:02x}", SIGTRAP)));
+}
+
+void
+GdbRsp::cmd_read_registers() {
+    std::string response;
+
+    for (int i = 0; i < cpu->GPR_COUNT - 1; i++)
+        append_le(response, cpu->gpr[i]);
+
+    // for some reason this PC needs to be the address of executing instruction
+    // i.e, two instructions behind actual PC
+    append_le(response,
+              cpu->pc - 2 * (cpu->cpsr.state() == State::Arm
+                               ? arm::INSTRUCTION_SIZE
+                               : thumb::INSTRUCTION_SIZE));
+
+    gdb_log("sending register values");
+    server.send(make_packet(response));
+}
+
+void
+GdbRsp::cmd_write_registers(std::string msg) {
+    static std::regex rgx("\\$G([0-9A-Fa-f]+)");
+    std::smatch sm;
+    regex_match(msg, sm, rgx);
+
+    if (sm.size() != 2 || sm[1].str().size() != 16 * 8) {
+        gdb_log("invalid arguments to write registers");
+        send_empty();
+        return;
+    }
+
+    try {
+        std::string values = sm[1].str();
+
+        for (uint i = 0, j = 0; i < values.length() - 8; i += 8, j++) {
+            cpu->gpr[i] = std::stoul(sm[i + 1].str(), nullptr, 16);
+            cpu->gpr[j] =
+              std::stoul(be_to_le(values.substr(i, 8)), nullptr, 16);
+        }
+
+        gdb_log("register values written");
+        server.send(OK_MSG);
+    } catch (const std::exception& e) {
+        gdb_log("{}", e.what());
+        send_empty();
+    }
+}
+
+void
+GdbRsp::cmd_read_register(std::string msg) {
+    std::string response;
+
+    try {
+        uint reg = std::stoul(msg.substr(2), nullptr, 16);
+        // 25th register is CPSR in gdb ARM
+        if (reg == 25)
+            append_le(response, cpu->cpsr.raw());
+        else if (reg < cpu->GPR_COUNT)
+            append_le(response, cpu->gpr[reg]);
+        else
+            response += "xxxxxxxx";
+
+        gdb_log("sending single register value");
+        server.send(make_packet(response));
+    } catch (const std::exception& e) {
+        gdb_log("{}", e.what());
+        send_empty();
+    }
+}
+
+void
+GdbRsp::cmd_write_register(std::string msg) {
+    static std::regex rgx("\\$P([0-9A-Fa-f]+)\\=([0-9A-Fa-f]+)");
+    std::smatch sm;
+    regex_match(msg, sm, rgx);
+
+    if (sm.size() != 3 && sm[2].str().length() != 8) {
+        gdb_log("invalid arguments to write single register");
+        send_empty();
+        return;
+    }
+
+    try {
+        uint reg       = std::stoul(sm[1].str(), nullptr, 16);
+        uint32_t value = std::stoul(be_to_le(sm[2].str()), nullptr, 16);
+
+        dbg(value);
+
+        if (reg == 25)
+            cpu->cpsr.set_all(value);
+        else if (reg < cpu->GPR_COUNT)
+            cpu->gpr[reg] = value;
+
+        gdb_log("single register value written");
+        server.send(OK_MSG);
+    } catch (const std::exception& e) {
+        gdb_log("{}", e.what());
+        send_empty();
+    }
+}
+
+void
+GdbRsp::cmd_read_memory(std::string msg) {
+    std::string response;
+    bool sequential = false;
+
+    static std::regex rgx("\\$m([0-9A-Fa-f]+),([0-9A-Fa-f]+)");
+    std::smatch sm;
+    regex_match(msg, sm, rgx);
+
+    if (sm.size() != 3) {
+        gdb_log("invalid arguments to read memory");
+        send_empty();
+        return;
+    }
+
+    uint32_t address = 0, length = 0;
+
+    try {
+        address = std::stoul(sm[1].str(), nullptr, 16);
+        length  = std::stoul(sm[2].str(), nullptr, 16);
+    } catch (const std::exception& e) {
+        gdb_log("{}", e.what());
+        send_empty();
+        return;
+    }
+
+    for (uint i = 0; i < length; i++) {
+        response +=
+          std::format("{:02x}", cpu->bus->read_byte(address + i), sequential);
+        sequential = true;
+    }
+
+    cpu->sequential = false;
+    gdb_log("sending memory values values");
+    server.send(make_packet(response));
+}
+
+void
+GdbRsp::cmd_write_memory(std::string msg) {
+    bool sequential = false;
+
+    static std::regex rgx("\\$M([0-9A-Fa-f]+),([0-9A-Fa-f]+):([0-9A-Fa-f]+)");
+    std::smatch sm;
+    regex_match(msg, sm, rgx);
+
+    if (sm.size() != 4) {
+        gdb_log("invalid arguments to write memory");
+        send_empty();
+        return;
+    }
+
+    try {
+        uint32_t address = std::stoul(sm[1].str(), nullptr, 16);
+        uint32_t length  = std::stoul(sm[2].str(), nullptr, 16);
+
+        std::string values = sm[3].str();
+
+        for (uint i = 0, j = 0; i < length && j < values.size(); i++, j += 2) {
+            cpu->bus->write_byte(address + i,
+                                 std::stoul(values.substr(j, 2), nullptr, 16) &
+                                   0xFF,
+                                 sequential);
+            glogger.warn("hi {:02x}", cpu->bus->read_byte(address + i));
+            sequential = true;
+        }
+
+        cpu->sequential = false;
+        gdb_log("register values written");
+        server.send(OK_MSG);
+    } catch (const std::exception& e) {
+        gdb_log("{}", e.what());
+        send_empty();
+    }
+}
+
+void
+GdbRsp::cmd_rm_breakpoint(std::string msg) {
+    static std::regex rgx("\\$z(0|1),([0-9A-Fa-f]+),(2|3|4)");
+    std::smatch sm;
+    regex_match(msg, sm, rgx);
+
+    if (sm.size() != 4) {
+        gdb_log("invalid arguments to remove breakpoint");
+        send_empty();
+        return;
+    }
+
+    if (sm[1].str() != "0" && sm[0].str() != "1") {
+        gdb_log("unrecognized breakpoint type encountered");
+        send_empty();
+        return;
+    }
+
+    if (sm[3].str() != "3" && sm[3].str() != "4") {
+        gdb_log("only 32 bit breakpoints supported");
+        send_empty();
+        return;
+    }
+
+    try {
+        uint32_t address = std::stoul(sm[2].str(), nullptr, 16);
+
+        cpu->breakpoints.erase(address);
+        gdb_log("breakpoint {:#08x} removed", address);
+        server.send(OK_MSG);
+    } catch (const std::exception& e) {
+        gdb_log("{}", e.what());
+        send_empty();
+    }
+}
+
+void
+GdbRsp::cmd_add_breakpoint(std::string msg) {
+    static std::regex rgx("\\$Z(0|1),([0-9A-Fa-f]+),(2|3|4)");
+    std::smatch sm;
+    regex_match(msg, sm, rgx);
+    dbg(sm.size());
+    dbg(sm[0].str());
+
+    if (sm.size() != 4) {
+        gdb_log("invalid arguments to add breakpoint");
+        send_empty();
+        return;
+    }
+
+    if (sm[1].str() != "0" && sm[0].str() != "1") {
+        gdb_log("unrecognized breakpoint type encountered");
+        send_empty();
+        return;
+    }
+
+    if (sm[3].str() != "3" && sm[3].str() != "4") {
+        gdb_log("only 32 bit breakpoints supported");
+        send_empty();
+        return;
+    }
+
+    try {
+        uint32_t address = std::stoul(sm[2].str(), nullptr, 16);
+
+        cpu->breakpoints.insert(address);
+        gdb_log("breakpoint {:#08x} added", address);
+        server.send(OK_MSG);
+    } catch (const std::exception& e) {
+        gdb_log("{}", e.what());
+        send_empty();
+    }
+}
+
+void
+GdbRsp::cmd_detach() {
+    attached = false;
+    cpu->resume();
+    gdb_log("detached");
+    server.send(OK_MSG);
+}
+
+void
+GdbRsp::cmd_continue() {
+    cpu->resume();
+    gdb_log("continued");
+    server.send(OK_MSG);
+    while (true) {
+        cpu->step();
+    }
+}
+}

src/gdb_rsp.hh

@@ -0,0 +1,43 @@
+#include "cpu/cpu.hh"
+#include "util/tcp_server.hh"
+
+namespace matar {
+class GdbRsp {
+  public:
+    GdbRsp(std::shared_ptr<Cpu> cpu);
+    void start(const uint port);
+    void satisfy_client();
+    void step();
+
+  private:
+    bool attached = false;
+
+    std::shared_ptr<Cpu> cpu;
+    net::TcpServer server;
+    std::string receive();
+    std::string make_packet(std::string raw);
+    void acknowledge();
+    void send_empty();
+    void notify_breakpoint_reached();
+
+    // Commands
+    void cmd_attached();
+    void cmd_supported(std::string msg);
+    void cmd_halted();
+    void cmd_read_registers();
+    void cmd_write_registers(std::string msg);
+    void cmd_read_register(std::string msg);
+    void cmd_write_register(std::string msg);
+    void cmd_read_memory(std::string msg);
+    void cmd_write_memory(std::string msg);
+    void cmd_rm_breakpoint(std::string msg);
+    void cmd_add_breakpoint(std::string msg);
+    void cmd_detach();
+    void cmd_continue();
+
+    static constexpr std::string ATTACHED_MSG = "$qAttached#8f";
+    static constexpr std::string OK_MSG       = "+$OK#9a";
+
+    static constexpr uint MAX_MSG_LEN = 4096;
+};
+}

src/meson.build

@@ -2,16 +2,14 @@ lib_sources = files(
   'bus.cc',
 )
 
+if get_option('gdb_debug')
+  lib_sources += files('gdb_rsp.cc')
+endif
+
 subdir('util')
 subdir('cpu')
 subdir('io')
 
-lib_cpp_args = []
-
-if get_option('disassembler')
-  lib_cpp_args += '-DDISASSEMBLER'
-endif
-
 lib = library(
   meson.project_name(),
   lib_sources,

src/util/meson.build

@@ -1,3 +1,8 @@
 lib_sources += files(
-  'log.cc'
+  'log.cc',
+  'tcp_server.cc'
 )
+
+if get_option('gdb_debug')
+  lib_sources += files('tcp_server.cc')
+endif

src/util/tcp_server.cc

@@ -0,0 +1,80 @@
+#include "tcp_server.hh"
+
+#include <cstring>
+#include <format>
+#include <sys/ioctl.h>
+#include <unistd.h>
+
+namespace net {
+TcpServer::TcpServer()
+  : server_fd(0)
+  , client_fd(0) {}
+
+TcpServer::~TcpServer() {
+    close(server_fd);
+    close(client_fd);
+}
+
+bool
+TcpServer::client_waiting() {
+    int count = 0;
+    ioctl(client_fd, FIONREAD, &count);
+    return static_cast<bool>(count);
+}
+
+void
+TcpServer::run() {
+    socklen_t cli_addr_size = sizeof(client_addr);
+
+    client_fd = ::accept(
+      server_fd, reinterpret_cast<sockaddr*>(&client_addr), &cli_addr_size);
+
+    if (client_fd == -1)
+        throw std::runtime_error("accept failed");
+}
+
+void
+TcpServer::start(uint port) {
+    server_fd = socket(PF_INET, SOCK_STREAM, 0);
+    if (server_fd == -1) {
+        throw std::runtime_error("creating socket failed");
+    }
+
+    int option = 1;
+    setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR, &option, sizeof(option));
+
+    std::memset(&server_addr, 0, sizeof(server_addr));
+    server_addr.sin_family      = PF_INET;
+    server_addr.sin_addr.s_addr = htonl(INADDR_ANY);
+    server_addr.sin_port        = htons(port);
+
+    if (::bind(server_fd,
+               reinterpret_cast<sockaddr*>(&server_addr),
+               sizeof(server_addr)) == -1) {
+        throw std::runtime_error("binding socket failed");
+    }
+    if (::listen(server_fd, 1) == -1) {
+        throw std::runtime_error("listening failed");
+    }
+}
+
+void
+TcpServer::send(std::string msg) {
+    if (::send(client_fd, msg.data(), msg.length(), 0) == -1) {
+        throw std::runtime_error(
+          std::format("failed to send message: {}\n", strerror(errno)));
+    }
+}
+
+std::string
+TcpServer::receive(uint length) {
+    char msg[MAX_PACKET_SIZE];
+    ssize_t num_bytes = recv(client_fd, msg, length, 0);
+    msg[length]       = '\0';
+    if (num_bytes < 0) {
+        throw std::runtime_error(
+          std::format("failed to receive messages: {}\n", strerror(errno)));
+    }
+    return std::string(msg);
+}
+}

src/util/tcp_server.hh

@@ -0,0 +1,26 @@
+#pragma once
+
+#include <netinet/in.h>
+#include <string>
+
+namespace net {
+
+class TcpServer {
+  public:
+    TcpServer();
+    ~TcpServer();
+
+    void run();
+    void start(uint port);
+    void send(std::string msg);
+    std::string receive(uint length);
+    bool client_waiting();
+
+  private:
+    static constexpr uint MAX_PACKET_SIZE = 4096;
+    int server_fd;
+    int client_fd;
+    sockaddr_in server_addr;
+    sockaddr_in client_addr;
+};
+}

tests/meson.build

@@ -9,15 +9,11 @@ tests_sources = files(
   'bus.cc'
 )
 
+tests_cpp_args = lib_cpp_args
+
 subdir('cpu')
 subdir('util')
 
-tests_cpp_args = []
-
-if get_option('disassembler')
-  tests_cpp_args += '-DDISASSEMBLER'
-endif
-
 catch2 = dependency('catch2', version: '>=3.4.0', static: true)
 catch2_tests = executable(
   'matar_tests',