libbpf: Add the ability to suppress perf event enablement

Automatically enabling a perf event after attaching a BPF prog to it is
not always desirable.

Add a new "dont_enable" field to struct bpf_perf_event_opts. While
introducing "enable" instead would be nicer in that it would avoid
a double negation in the implementation, it would make
DECLARE_LIBBPF_OPTS() less efficient.

Acked-by: Eduard Zingerman <[e-mail]>
Suggested-by: Jiri Olsa <[e-mail]>
Tested-by: Thomas Richter <[e-mail]>
Co-developed-by: Thomas Richter <[e-mail]>
Signed-off-by: Thomas Richter <[e-mail]>
Signed-off-by: Ilya Leoshkevich <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/lib/bpf/libbpf.c

···
 		}
 		link->link.fd = pfd;
 	}
-	if (ioctl(pfd, PERF_EVENT_IOC_ENABLE, 0) < 0) {
-		err = -errno;
-		pr_warn("prog '%s': failed to enable perf_event FD %d: %s\n",
-			prog->name, pfd, errstr(err));
-		goto err_out;
+
+	if (!OPTS_GET(opts, dont_enable, false)) {
+		if (ioctl(pfd, PERF_EVENT_IOC_ENABLE, 0) < 0) {
+			err = -errno;
+			pr_warn("prog '%s': failed to enable perf_event FD %d: %s\n",
+				prog->name, pfd, errstr(err));
+			goto err_out;
+		}
 	}
 
 	return &link->link;

tools/lib/bpf/libbpf.h

···
 	__u64 bpf_cookie;
 	/* don't use BPF link when attach BPF program */
 	bool force_ioctl_attach;
+	/* don't automatically enable the event */
+	bool dont_enable;
 	size_t :0;
 };
-#define bpf_perf_event_opts__last_field force_ioctl_attach
+#define bpf_perf_event_opts__last_field dont_enable
 
 LIBBPF_API struct bpf_link *
 bpf_program__attach_perf_event(const struct bpf_program *prog, int pfd);