perf Documentation: Add event parameters

Event parameters are a basic way for partial events to be specified in
sysfs with per-event names given to the fields that need to be filled in
when using a particular event.

It is intended for supporting cases where the single 'cpu' parameter is
insufficient. For example, POWER 8 has events for physical
sockets/cores/cpus that are accessible from with virtual machines. To
keep using the single 'cpu' parameter we'd need to perform a mapping
between Linux's cpus and the physical machine's cpus (in this case Linux
is running under a hypervisor). This isn't possible because bindings
between our cpus and physical cpus may not be fixed, and we probably
won't have a "cpu" on each physical cpu.

Signed-off-by: Cody P Schafer <[e-mail]>
Signed-off-by: Sukadev Bhattiprolu <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Cc: Cody P Schafer <[e-mail]>
Cc: Haren Myneni <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Michael Ellerman <[e-mail]>
Cc: Paul Mackerras <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Sukadev Bhattiprolu <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

Documentation/ABI/testing/sysfs-bus-event_source-devices-events

···
 			event=0x2abc
 			event=0x423,inv,cmask=0x3
 			domain=0x1,offset=0x8,starting_index=0xffff
+			domain=0x1,offset=0x8,core=?
 
 		Each of the assignments indicates a value to be assigned to a
 		particular set of bits (as defined by the format file
 		corresponding to the <term>) in the perf_event structure passed
 		to the perf_open syscall.
+
+		In the case of the last example, a value replacing "?" would
+		need to be provided by the user selecting the particular event.
+		This is referred to as "event parameterization". Event
+		parameters have the format 'param=?'.
 
 What: /sys/bus/event_source/devices/<pmu>/events/<event>.unit
 Date: 2014/02/24