.I cap_usr_rdpmc
field in the mmap page; documentation on how
to calculate event values can be found in that section.
+
+Originally, when rdpmc support was enabled, any process (not just ones
+with an active perf event) could use the rdpmc instruction to access
+the counters.
+Starting with Linux 4.0
+.\" 7911d3f7af14a614617e38245fedf98a724e46a9
+rdpmc support is only allowed if an event is currently enabled
+in a process' context.
+To restore the old behavior, write the value 2 to
+.IR /sys/devices/cpu/rdpmc .
.SS perf_event ioctl calls
.PP
Various ioctls act on
.I perf_event_attr
to indicate that you wish to use this PMU.
.TP
-.IR /sys/bus/event_source/devices/*/rdpmc " (since Linux 3.4)"
+.IR /sys/bus/event_source/devices/cpu/rdpmc " (since Linux 3.4)"
.\" commit 0c9d42ed4cee2aa1dfc3a260b741baae8615744f
If this file is 1, then direct user-space access to the
performance counter registers is allowed via the rdpmc instruction.
This can be disabled by echoing 0 to the file.
+
+As of Linux 4.0
+.\" a66734297f78707ce39d756b656bfae861d53f62
+.\" 7911d3f7af14a614617e38245fedf98a724e46a9
+the behavior has changed, so that 1 now means only allow access
+to processes with active perf events, with 2 indicating the old
+allow-anyone-access behavior.
.TP
.IR /sys/bus/event_source/devices/*/format/ " (since Linux 3.4)"
.\" commit 641cc938815dfd09f8fa1ec72deb814f0938ac33