server/gdb_server.c: support unavailable registers

According to gdb documentation, `g` and `p` packets can report a
register being unavailable by a string of 'x' instead of register's
value.

Change-Id: I8ef279f1357c2e612f5d3290eb0022c1b47d9fa7
Signed-off-by: Evgeniy Naydanov <[email protected]>
Reviewed-on: https://review.openocd.org/c/openocd/+/7876
Reviewed-by: Marek Vrbka <[email protected]>
Reviewed-by: Antonio Borneo <[email protected]>
Reviewed-by: Anatoly Parshintsev <[email protected]>
Tested-by: jenkins

src/server/gdb_server.c

@@ -1207,6 +1207,27 @@ static void gdb_target_to_reg(struct target *target,
 	}
 }
 
+/* get register value if needed and fill the buffer accordingly */
+static int gdb_get_reg_value_as_str(struct target *target, char *tstr, struct reg *reg)
+{
+	int retval = ERROR_OK;
+
+	if (!reg->valid)
+		retval = reg->type->get(reg);
+
+	const unsigned int len = DIV_ROUND_UP(reg->size, 8) * 2;
+	switch (retval) {
+		case ERROR_OK:
+			gdb_str_to_target(target, tstr, reg);
+			return ERROR_OK;
+		case ERROR_TARGET_RESOURCE_NOT_AVAILABLE:
+			memset(tstr, 'x', len);
+			tstr[len] = '\0';
+			return ERROR_OK;
+	}
+	return ERROR_FAIL;
+}
+
 static int gdb_get_registers_packet(struct connection *connection,
 		char const *packet, int packet_size)
 {
@@ -1248,16 +1269,11 @@ static int gdb_get_registers_packet(struct connection *connection,
 	for (i = 0; i < reg_list_size; i++) {
 		if (!reg_list[i] || reg_list[i]->exist == false || reg_list[i]->hidden)
 			continue;
-		if (!reg_list[i]->valid) {
-			retval = reg_list[i]->type->get(reg_list[i]);
-			if (retval != ERROR_OK && gdb_report_register_access_error) {
-				LOG_DEBUG("Couldn't get register %s.", reg_list[i]->name);
-				free(reg_packet);
-				free(reg_list);
-				return gdb_error(connection, retval);
-			}
+		if (gdb_get_reg_value_as_str(target, reg_packet_p, reg_list[i]) != ERROR_OK) {
+			free(reg_packet);
+			free(reg_list);
+			return gdb_error(connection, retval);
 		}
-		gdb_str_to_target(target, reg_packet_p, reg_list[i]);
 		reg_packet_p += DIV_ROUND_UP(reg_list[i]->size, 8) * 2;
 	}
 
@@ -1369,18 +1385,13 @@ static int gdb_get_register_packet(struct connection *connection,
 		return ERROR_SERVER_REMOTE_CLOSED;
 	}
 
-	if (!reg_list[reg_num]->valid) {
-		retval = reg_list[reg_num]->type->get(reg_list[reg_num]);
-		if (retval != ERROR_OK && gdb_report_register_access_error) {
-			LOG_DEBUG("Couldn't get register %s.", reg_list[reg_num]->name);
-			free(reg_list);
-			return gdb_error(connection, retval);
-		}
-	}
-
 	reg_packet = calloc(DIV_ROUND_UP(reg_list[reg_num]->size, 8) * 2 + 1, 1); /* plus one for string termination null */
 
-	gdb_str_to_target(target, reg_packet, reg_list[reg_num]);
+	if (gdb_get_reg_value_as_str(target, reg_packet, reg_list[reg_num]) != ERROR_OK) {
+		free(reg_packet);
+		free(reg_list);
+		return gdb_error(connection, retval);
+	}
 
 	gdb_put_packet(connection, reg_packet, DIV_ROUND_UP(reg_list[reg_num]->size, 8) * 2);