Hide unknown registers, which probably don't exist

Change-Id: Iffa8fa5ff4b0a01abd30fa302b7087e2011337bf

src/target/riscv/riscv-013.c

@@ -1249,6 +1249,12 @@ static int init_registers(struct target *target)
 				r->name = csr_info[csr_info_index].name;
 			} else {
 				sprintf(reg_name, "csr%d", csr_number);
+				// Assume unnamed registers don't exist, unless we have some
+				// configuration that tells us otherwise. That's important
+				// because eg. Eclipse crashes if a target has too many
+				// registers, and apparently has no way of only showing a
+				// subset of registers in any case.
+				r->exist = false;
 			}
 
 			switch (csr_number) {

src/target/target.c

@@ -2742,21 +2742,23 @@ COMMAND_HANDLER(handle_reg_command)
 					i < cache->num_regs;
 					i++, reg++, count++) {
 				/* only print cached values if they are valid */
-				if (reg->valid) {
-					value = buf_to_str(reg->value,
-							reg->size, 16);
-					command_print(CMD_CTX,
-							"(%i) %s (/%" PRIu32 "): 0x%s%s",
-							count, reg->name,
-							reg->size, value,
-							reg->dirty
+				if (reg->exist) {
+					if (reg->valid) {
+						value = buf_to_str(reg->value,
+								reg->size, 16);
+						command_print(CMD_CTX,
+								"(%i) %s (/%" PRIu32 "): 0x%s%s",
+								count, reg->name,
+								reg->size, value,
+								reg->dirty
 								? " (dirty)"
 								: "");
-					free(value);
-				} else {
-					command_print(CMD_CTX, "(%i) %s (/%" PRIu32 ")",
-							  count, reg->name,
-							  reg->size) ;
+						free(value);
+					} else {
+						command_print(CMD_CTX, "(%i) %s (/%" PRIu32 ")",
+								count, reg->name,
+								reg->size) ;
+					}
 				}
 			}
 			cache = cache->next;