perf tools: Update topdown documentation to permit rdpmc calls

Update Topdown documentation to permit calls to rdpmc, and describe
interaction with system calls.
Signed-off-by: Ray Kinsella <mdr@ashroe.eu>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Link: http://lore.kernel.org/lkml/20210421091009.1711565-1-mdr@ashroe.euSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/topdown.txt

@@ -72,6 +72,7 @@ For example, the perf_event_attr structure can be initialized with
 The Fixed counter 3 must be the leader of the group.
 
 #include <linux/perf_event.h>
+#include <sys/mman.h>
 #include <sys/syscall.h>
 #include <unistd.h>
 
@@ -95,6 +96,11 @@ int slots_fd = perf_event_open(&slots, 0, -1, -1, 0);
 if (slots_fd < 0)
 	... error ...
 
+/* Memory mapping the fd permits _rdpmc calls from userspace */
+void *slots_p = mmap(0, getpagesize(), PROT_READ, MAP_SHARED, slots_fd, 0);
+if (!slot_p)
+	.... error ...
+
 /*
  * Open metrics event file descriptor for current task.
  * Set slots event as the leader of the group.
@@ -110,6 +116,14 @@ int metrics_fd = perf_event_open(&metrics, 0, -1, slots_fd, 0);
 if (metrics_fd < 0)
 	... error ...
 
+/* Memory mapping the fd permits _rdpmc calls from userspace */
+void *metrics_p = mmap(0, getpagesize(), PROT_READ, MAP_SHARED, metrics_fd, 0);
+if (!metrics_p)
+	... error ...
+
+Note: the file descriptors returned by the perf_event_open calls must be memory
+mapped to permit calls to the _rdpmd instruction. Permission may also be granted
+by writing the /sys/devices/cpu/rdpmc sysfs node.
 
 The RDPMC instruction (or _rdpmc compiler intrinsic) can now be used
 to read slots and the topdown metrics at different points of the program:
@@ -141,6 +155,10 @@ as the parallelism and overlap in the CPU program execution will
 cause too much measurement inaccuracy. For example instrumenting
 individual basic blocks is definitely too fine grained.
 
+_rdpmc calls should not be mixed with reading the metrics and slots counters
+through system calls, as the kernel will reset these counters after each system
+call.
+
 Decoding metrics values
 =======================