rtos: server/gdb_server: fix missing thread ID in stop reply

Cherry-picked from [1].

To replicate the issue that this fixes:

1. Connect to a multi-hart RISC-V target configured as an SMP group.
2. Start a GDB instance against the running OpenOCD.
3. Observe that GDB might display "warning: multi-threaded target
   stopped without sending a thread-id, using first non-exited thread."
4. Set a breakpoint in code that any non-hart-0 hart is expected to
   reach (but hart 0 is not expected to reach).
5. Allow a non-hart-0 hart to reach the breakpoint.
6. Remove the breakpoint.
7. Do a few sequential `stepi` commands in GDB.
8. Observe that GDB displays "Switching to Thread 1" even though the
   thread that was just single stepped was not Thread 1 in GDB. Also
   observe that the register values in GDB correspond to the thread that
   was single-stepped, not Thread 1. Basically GDB erroneously starts to
   consider thread 1 to be current, when in fact the thread that was
   single-stepped is still current.

The changes in this pull request are intended to avoid the erroneous
"Switching to Thread 1" described in (8) above.

What was happening was that, in a couple areas of code, non-hart-0 harts
weren't seen as belonging to an RTOS module, and this had the effect of
(1) bypassing `hwthread_update_threads()` being called after a halt; (2)
omitting a thread ID in a stop reply over GDB remote protocol connection
(requiring GDB to take an arbitrary guess of current thread id, a guess
that is wrong unless the current thread happens to be hart 0).

Link: https://github.com/riscv-collab/riscv-openocd/pull/675 [1]
Change-Id: I9872062dfa0e3f1ca531d282d52a1b04c527546a
Signed-off-by: Greg Savin <greg.savin@sifive.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/9183
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

src/rtos/rtos.c

@@ -11,6 +11,7 @@
 
 #include "rtos.h"
 #include "target/target.h"
+#include "target/smp.h"
 #include "helper/log.h"
 #include "helper/binarybuffer.h"
 #include "helper/types.h"
@@ -715,10 +716,24 @@ int rtos_generic_stack_read(struct target *target,
 	return ERROR_OK;
 }
 
+struct rtos *rtos_from_target(struct target *target)
+{
+	if (target->rtos && target->rtos->type)
+		return target->rtos;
+
+	struct target_list *pos;
+	foreach_smp_target(pos, target->smp_targets)
+		if (pos->target->rtos && pos->target->rtos->type)
+			return pos->target->rtos;
+
+	return NULL;
+}
+
 int rtos_update_threads(struct target *target)
 {
-	if ((target->rtos) && (target->rtos->type))
-		target->rtos->type->update_threads(target->rtos);
+	struct rtos *rtos = rtos_from_target(target);
+	if (rtos)
+		rtos->type->update_threads(rtos);
 	return ERROR_OK;
 }
 

src/rtos/rtos.h

@@ -154,6 +154,11 @@ int rtos_write_buffer(struct target *target, target_addr_t address,
 bool rtos_needs_fake_step(struct target *target, int64_t thread_id);
 struct target *rtos_swbp_target(struct target *target, target_addr_t address,
 				uint32_t length, enum breakpoint_type type);
+/**
+ * Get the RTOS from the target itself, or from one of the targets in
+ * the same SMP node, or NULL when no RTOS is set.
+ */
+struct rtos *rtos_from_target(struct target *target);
 
 // Keep in alphabetic order this list of rtos
 extern const struct rtos_type chibios_rtos;

src/server/gdb_server.c

@@ -813,9 +813,12 @@ static void gdb_signal_reply(struct target *target, struct connection *connectio
 		sig_reply_len = snprintf(sig_reply, sizeof(sig_reply), "W00");
 	} else {
 		struct target *ct;
-		if (target->rtos) {
-			target->rtos->current_threadid = target->rtos->current_thread;
-			target->rtos->gdb_target_for_threadid(connection, target->rtos->current_threadid, &ct);
+		struct rtos *rtos;
+
+		rtos = rtos_from_target(target);
+		if (rtos) {
+			rtos->current_threadid = rtos->current_thread;
+			rtos->gdb_target_for_threadid(connection, rtos->current_threadid, &ct);
 		} else {
 			ct = target;
 		}
@@ -853,9 +856,9 @@ static void gdb_signal_reply(struct target *target, struct connection *connectio
 		}
 
 		current_thread[0] = '\0';
-		if (target->rtos)
+		if (rtos)
 			snprintf(current_thread, sizeof(current_thread), "thread:%" PRIx64 ";",
-					target->rtos->current_thread);
+					rtos->current_thread);
 
 		sig_reply_len = snprintf(sig_reply, sizeof(sig_reply), "T%2.2x%s%s",
 				signal_var, stop_reason, current_thread);