Hello Josh,

thank you for your feedback!

On 12/7/2025 4:10 PM, Josh Poimboeuf wrote:

On Fri, Dec 05, 2025 at 06:14:45PM +0100, Jens Remus wrote:

quoted

@@ -159,6 +165,10 @@ static int unwind_user_next(struct unwind_user_state *state)
 			if (!unwind_user_next_fp(state))
 				return 0;
 			continue;
+		case UNWIND_USER_TYPE_BACKCHAIN:
+			if (!unwind_user_next_backchain(state))
+				return 0;
+			continue;		/* Try next method. */
 		default:
 			WARN_ONCE(1, "Undefined unwind bit %d", bit);
 			break;

@@ -187,6 +197,8 @@ static int unwind_user_start(struct unwind_user_state *state)
 		state->available_types |= UNWIND_USER_TYPE_SFRAME;
 	if (IS_ENABLED(CONFIG_HAVE_UNWIND_USER_FP))
 		state->available_types |= UNWIND_USER_TYPE_FP;
+	if (IS_ENABLED(CONFIG_HAVE_UNWIND_USER_BACKCHAIN))
+		state->available_types |= UNWIND_USER_TYPE_BACKCHAIN;

Any reason not to just use the existing CONFIG_HAVE_UNWIND_USER_FP hook
here rather than create the new BACKCHAIN one?

At first I thought this would not be a good idea, as my unwind user
backchain implementation relies on being standalone without using
unwind_user_next_common().  Mainly because s390 back chain unwinding
does not have fixed CFA, FP, and RA offsets/locations.  But then I gave
it a try and it does not look that bad actually.

I'll send a RFC v3 soon.

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@de.ibm.com

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