perf tools: Add README for info on parsing JSON/map files

The contents of this directory allow users to specify PMU events in their

CPUs by their symbolic names rather than raw event codes (see example below).

The main program in this directory, is the 'jevents', which is built and

executed _BEFORE_ the perf binary itself is built.

The 'jevents' program tries to locate and process JSON files in the directory

tree tools/perf/pmu-events/arch/foo.

	- Regular files with '.json' extension in the name are assumed to be

	  JSON files, each of which describes a set of PMU events.

	- Regular files with basename starting with 'mapfile.csv' are assumed

	  to be a CSV file that maps a specific CPU to its set of PMU events.

	  (see below for mapfile format)

	- Directories are traversed, but all other files are ignored.

The PMU events supported by a CPU model are expected to grouped into topics

such as Pipelining, Cache, Memory, Floating-point etc. All events for a topic

should be placed in a separate JSON file - where the file name identifies

the topic. Eg: "Floating-point.json".

All the topic JSON files for a CPU model/family should be in a separate

sub directory. Thus for the Silvermont X86 CPU:

	$ ls tools/perf/pmu-events/arch/x86/Silvermont_core

	Cache.json 	Memory.json 	Virtual-Memory.json

	Frontend.json 	Pipeline.json

Using the JSON files and the mapfile, 'jevents' generates the C source file,

'pmu-events.c', which encodes the two sets of tables:

	- Set of 'PMU events tables' for all known CPUs in the architecture,

	  (one table like the following, per JSON file; table name 'pme_power8'

	  is derived from JSON file name, 'power8.json').

		struct pmu_event pme_power8[] = {

			...

			{

				.name = "pm_1plus_ppc_cmpl",

				.event = "event=0x100f2",

				.desc = "1 or more ppc insts finished,",

			},

			...

		}

	- A 'mapping table' that maps each CPU of the architecture, to its

	  'PMU events table'

		struct pmu_events_map pmu_events_map[] = {

		{

			.cpuid = "004b0000",

			.version = "1",

			.type = "core",

			.table = pme_power8

		},

			...

		};

After the 'pmu-events.c' is generated, it is compiled and the resulting

'pmu-events.o' is added to 'libperf.a' which is then used to build perf.

NOTES:

	1. Several CPUs can support same set of events and hence use a common

	   JSON file. Hence several entries in the pmu_events_map[] could map

	   to a single 'PMU events table'.

	2. The 'pmu-events.h' has an extern declaration for the mapping table

	   and the generated 'pmu-events.c' defines this table.

	3. _All_ known CPU tables for architecture are included in the perf

	   binary.

At run time, perf determines the actual CPU it is running on, finds the

matching events table and builds aliases for those events. This allows

users to specify events by their name:

	$ perf stat -e pm_1plus_ppc_cmpl sleep 1

where 'pm_1plus_ppc_cmpl' is a Power8 PMU event.

In case of errors when processing files in the tools/perf/pmu-events/arch

directory, 'jevents' tries to create an empty mapping file to allow the perf

build to succeed even if the PMU event aliases cannot be used.

However some errors in processing may cause the perf build to fail.

Mapfile format

===============

The mapfile enables multiple CPU models to share a single set of PMU events.

It is required even if such mapping is 1:1.

The mapfile.csv format is expected to be:

	Header line

	CPUID,Version,Dir/path/name,Type

where:

	Comma:

		is the required field delimiter (i.e other fields cannot

		have commas within them).

	Comments:

		Lines in which the first character is either '\n' or '#'

		are ignored.

	Header line

		The header line is the first line in the file, which is

		always _IGNORED_. It can empty.

	CPUID:

		CPUID is an arch-specific char string, that can be used

		to identify CPU (and associate it with a set of PMU events

		it supports). Multiple CPUIDS can point to the same

		File/path/name.json.

		Example:

			CPUID == 'GenuineIntel-6-2E' (on x86).

			CPUID == '004b0100' (PVR value in Powerpc)

	Version:

		is the Version of the mapfile.

	Dir/path/name:

		is the pathname to the directory containing the CPU's JSON

		files, relative to the directory containing the mapfile.csv

	Type:

		indicates whether the events or "core" or "uncore" events.

	Eg:

	$ grep Silvermont tools/perf/pmu-events/arch/x86/mapfile.csv

	GenuineIntel-6-37,V13,Silvermont_core,core

	GenuineIntel-6-4D,V13,Silvermont_core,core

	GenuineIntel-6-4C,V13,Silvermont_core,core

	i.e the three CPU models use the JSON files (i.e PMU events) listed

	in the directory 'tools/perf/pmu-events/arch/x86/Silvermont_core'.