Implement memory write and register write

The `mini-gdbstub` library supports the functionality to write register and write
memory now. After this PR, `rv32emu` is enabled to do those commands that will
try to write register and memory, e.g. the `load` command.

Author: RinHizakura

README.md

@@ -149,17 +149,6 @@ $ riscv32-unknown-elf-gdb
 Congratulate yourself if `riscv-gdb` does not produce an error message. Now that the GDB
 command line is available, you can communicate with `rv32emu`.
 
-### Limitation
-
-The development of `rv32emu`'s GDBRSP functionality is still ongoing. Due to the current
-unfinished design, various restrictions on this feature are known.
-* Since the 'G' packet is not supported yet, writing emulator registers with GDB is
-not permitted.
-* Consequently, the packets for binary download (the "X" packet) and memory writing
-(the "M" packet) specified in GDBRSP are not allowed. It is forbidden to use GDB commands
-such as "load" that aim to modify the emulator's internal memory.
-* You can only set one breakpoint when debugging due to the inadequate breakpoint handling design.
-
 ## Contributing
 See [CONTRIBUTING.md](CONTRIBUTING.md) for contribution guidelines.
 

src/gdbstub.c

@@ -18,6 +18,18 @@ static size_t rv_read_reg(void *args, int regno)
     return -1;
 }
 
+static void rv_write_reg(void *args, int regno, size_t data)
+{
+    if (unlikely(regno > 32))
+        return;
+
+    struct riscv_t *rv = (struct riscv_t *) args;
+    if (regno == 32)
+        rv_set_pc(rv, data);
+    else
+        rv_set_reg(rv, regno, data);
+}
+
 static void rv_read_mem(void *args, size_t addr, size_t len, void *val)
 {
     struct riscv_t *rv = (struct riscv_t *) args;
@@ -26,6 +38,14 @@ static void rv_read_mem(void *args, size_t addr, size_t len, void *val)
         *((uint8_t *) val + i) = rv->io.mem_read_b(rv, addr + i);
 }
 
+static void rv_write_mem(void *args, size_t addr, size_t len, void *val)
+{
+    struct riscv_t *rv = (struct riscv_t *) args;
+
+    for (size_t i = 0; i < len; i++)
+        rv->io.mem_write_b(rv, addr + i, *((uint8_t *) val + i));
+}
+
 static gdb_action_t rv_cont(void *args)
 {
     struct riscv_t *rv = (struct riscv_t *) args;
@@ -69,7 +89,9 @@ static bool rv_del_bp(void *args, size_t addr, bp_type_t type)
 
 const struct target_ops gdbstub_ops = {
     .read_reg = rv_read_reg,
+    .write_reg = rv_write_reg,
     .read_mem = rv_read_mem,
+    .write_mem = rv_write_mem,
     .cont = rv_cont,
     .stepi = rv_stepi,
     .set_bp = rv_set_bp,