perf/hw_breakpoint: Return EOPNOTSUPP for unsupported breakpoint type

Currently, __reserve_bp_slot() returns -ENOSPC for unsupported
breakpoint types on the architecture. For example, powerpc
does not support hardware instruction breakpoints. This causes
the perf_skip BPF selftest to fail, as neither ENOENT nor
EOPNOTSUPP is returned by perf_event_open for unsupported
breakpoint types. As a result, the test that should be skipped
for this arch is not correctly identified.

To resolve this, hw_breakpoint_event_init() should exit early by
checking for unsupported breakpoint types using
hw_breakpoint_slots_cached() and return the appropriate error
(-EOPNOTSUPP).

Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Marco Elver <elver@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: https://lore.kernel.org/r/20250303092451.1862862-1-skb99@linux.ibm.com

kernel/events/hw_breakpoint.c

@@ -950,9 +950,10 @@ static int hw_breakpoint_event_init(struct perf_event *bp)
 		return -ENOENT;
 
 	/*
-	 * no branch sampling for breakpoint events
+	 * Check if breakpoint type is supported before proceeding.
+	 * Also, no branch sampling for breakpoint events.
 	 */
-	if (has_branch_stack(bp))
+	if (!hw_breakpoint_slots_cached(find_slot_idx(bp->attr.bp_type)) || has_branch_stack(bp))
 		return -EOPNOTSUPP;
 
 	err = register_perf_hw_breakpoint(bp);