perf unwind: Don't show unwind error messages when augmenting frame pointer stack

Commit Fixes: b9f6fbb3b2c29736 ("perf arm64: Inject missing frames when
using 'perf record --call-graph=fp'") intended to add a 'best effort'
DWARF unwind that improved the frame pointer stack in most scenarios.

It's expected that the unwind will fail sometimes, but this shouldn't be
reported as an error. It only works when the return address can be
determined from the contents of the link register alone.

Fix the error shown when the unwinder requires extra registers by adding
a new flag that suppresses error messages. This flag is not set in the
normal --call-graph=dwarf unwind mode so that behavior is not changed.

Fixes: b9f6fbb3b2c29736 ("perf arm64: Inject missing frames when using 'perf record --call-graph=fp'")
Reported-by: John Garry <[e-mail]>
Signed-off-by: James Clark <[e-mail]>
Tested-by: John Garry <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Alexandre Truong <[e-mail]>
Cc: German Gomez <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/tests/dwarf-unwind.c

···
 	}
 
 	err = unwind__get_entries(unwind_entry, &cnt, thread,
-				  &sample, MAX_STACK);
+				  &sample, MAX_STACK, false);
 	if (err)
 		pr_debug("unwind failed\n");
 	else if (cnt != MAX_STACK) {

tools/perf/util/arm64-frame-pointer-unwind-support.c

···
 		sample->user_regs.cache_regs[PERF_REG_ARM64_SP] = 0;
 	}
 
-	ret = unwind__get_entries(add_entry, &entries, thread, sample, 2);
+	ret = unwind__get_entries(add_entry, &entries, thread, sample, 2, true);
 	sample->user_regs = old_regs;
 
 	if (ret || entries.length != 2)

tools/perf/util/machine.c

···
 		return 0;
 
 	return unwind__get_entries(unwind_entry, cursor,
-				   thread, sample, max_stack);
+				   thread, sample, max_stack, false);
 }
 
 int thread__resolve_callchain(struct thread *thread,

tools/perf/util/unwind-libdw.c

···
 	bool isactivation;
 
 	if (!dwfl_frame_pc(state, &pc, NULL)) {
-		pr_err("%s", dwfl_errmsg(-1));
+		if (!ui->best_effort)
+			pr_err("%s", dwfl_errmsg(-1));
 		return DWARF_CB_ABORT;
 	}
 
···
 	report_module(pc, ui);
 
 	if (!dwfl_frame_pc(state, &pc, &isactivation)) {
-		pr_err("%s", dwfl_errmsg(-1));
+		if (!ui->best_effort)
+			pr_err("%s", dwfl_errmsg(-1));
 		return DWARF_CB_ABORT;
 	}
 
···
 int unwind__get_entries(unwind_entry_cb_t cb, void *arg,
 			struct thread *thread,
 			struct perf_sample *data,
-			int max_stack)
+			int max_stack,
+			bool best_effort)
 {
 	struct unwind_info *ui, ui_buf = {
 		.sample		= data,
···
 		.cb		= cb,
 		.arg		= arg,
 		.max_stack	= max_stack,
+		.best_effort    = best_effort
 	};
 	Dwarf_Word ip;
 	int err = -EINVAL, i;

tools/perf/util/unwind-libdw.h

···
 	void			*arg;
 	int			max_stack;
 	int			idx;
+	bool			best_effort;
 	struct unwind_entry	entries[];
 };
 

tools/perf/util/unwind-libunwind-local.c

···
 	struct perf_sample	*sample;
 	struct machine		*machine;
 	struct thread		*thread;
+	bool			 best_effort;
 };
 
 #define dw_read(ptr, type, end) ({	\
···
 
 	ret = perf_reg_value(&val, &ui->sample->user_regs, id);
 	if (ret) {
-		pr_err("unwind: can't read reg %d\n", regnum);
+		if (!ui->best_effort)
+			pr_err("unwind: can't read reg %d\n", regnum);
 		return ret;
 	}
 
···
 			return -1;
 
 		ret = unw_init_remote(&c, addr_space, ui);
-		if (ret)
+		if (ret && !ui->best_effort)
 			display_error(ret);
 
 		while (!ret && (unw_step(&c) > 0) && i < max_stack) {
···
 
 static int _unwind__get_entries(unwind_entry_cb_t cb, void *arg,
 			struct thread *thread,
-			struct perf_sample *data, int max_stack)
+			struct perf_sample *data, int max_stack,
+			bool best_effort)
 {
 	struct unwind_info ui = {
 		.sample       = data,
 		.thread       = thread,
 		.machine      = thread->maps->machine,
+		.best_effort  = best_effort
 	};
 
 	if (!data->user_regs.regs)

tools/perf/util/unwind-libunwind.c

···
 
 int unwind__get_entries(unwind_entry_cb_t cb, void *arg,
 			 struct thread *thread,
-			 struct perf_sample *data, int max_stack)
+			 struct perf_sample *data, int max_stack,
+			 bool best_effort)
 {
 	if (thread->maps->unwind_libunwind_ops)
-		return thread->maps->unwind_libunwind_ops->get_entries(cb, arg, thread, data, max_stack);
+		return thread->maps->unwind_libunwind_ops->get_entries(cb, arg, thread, data,
+								       max_stack, best_effort);
 	return 0;
 }

tools/perf/util/unwind.h

···
 	void (*finish_access)(struct maps *maps);
 	int (*get_entries)(unwind_entry_cb_t cb, void *arg,
 			   struct thread *thread,
-			   struct perf_sample *data, int max_stack);
+			   struct perf_sample *data, int max_stack, bool best_effort);
 };
 
 #ifdef HAVE_DWARF_UNWIND_SUPPORT
+/*
+ * When best_effort is set, don't report errors and fail silently. This could
+ * be expanded in the future to be more permissive about things other than
+ * error messages.
+ */
 int unwind__get_entries(unwind_entry_cb_t cb, void *arg,
 			struct thread *thread,
-			struct perf_sample *data, int max_stack);
+			struct perf_sample *data, int max_stack,
+			bool best_effort);
 /* libunwind specific */
 #ifdef HAVE_LIBUNWIND_SUPPORT
 #ifndef LIBUNWIND__ARCH_REG_ID
···
 		    void *arg __maybe_unused,
 		    struct thread *thread __maybe_unused,
 		    struct perf_sample *data __maybe_unused,
-		    int max_stack __maybe_unused)
+		    int max_stack __maybe_unused,
+		    bool best_effort __maybe_unused)
 {
 	return 0;
 }