kmemtrace: Additional documentation.

Documented kmemtrace's ABI, purpose and design. Also includes a short
usage guide, FAQ, as well as a link to the userspace application's Git
repository, which is currently hosted at repo.or.cz.

Signed-off-by: Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>

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+What:		/sys/kernel/debug/kmemtrace/
+Date:		July 2008
+Contact:	Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
+Description:
+
+In kmemtrace-enabled kernels, the following files are created:
+
+/sys/kernel/debug/kmemtrace/
+	cpu<n>		(0400)	Per-CPU tracing data, see below. (binary)
+	total_overruns	(0400)	Total number of bytes which were dropped from
+				cpu<n> files because of full buffer condition,
+				non-binary. (text)
+	abi_version	(0400)	Kernel's kmemtrace ABI version. (text)
+
+Each per-CPU file should be read according to the relay interface. That is,
+the reader should set affinity to that specific CPU and, as currently done by
+the userspace application (though there are other methods), use poll() with
+an infinite timeout before every read(). Otherwise, erroneous data may be
+read. The binary data has the following _core_ format:
+
+	Event ID	(1 byte)	Unsigned integer, one of:
+		0 - represents an allocation (KMEMTRACE_EVENT_ALLOC)
+		1 - represents a freeing of previously allocated memory
+		    (KMEMTRACE_EVENT_FREE)
+	Type ID		(1 byte)	Unsigned integer, one of:
+		0 - this is a kmalloc() / kfree()
+		1 - this is a kmem_cache_alloc() / kmem_cache_free()
+		2 - this is a __get_free_pages() et al.
+	Event size	(2 bytes)	Unsigned integer representing the
+					size of this event. Used to extend
+					kmemtrace. Discard the bytes you
+					don't know about.
+	Sequence number	(4 bytes)	Signed integer used to reorder data
+					logged on SMP machines. Wraparound
+					must be taken into account, although
+					it is unlikely.
+	Caller address	(8 bytes)	Return address to the caller.
+	Pointer to mem	(8 bytes)	Pointer to target memory area. Can be
+					NULL, but not all such calls might be
+					recorded.
+
+In case of KMEMTRACE_EVENT_ALLOC events, the next fields follow:
+
+	Requested bytes	(8 bytes)	Total number of requested bytes,
+					unsigned, must not be zero.
+	Allocated bytes (8 bytes)	Total number of actually allocated
+					bytes, unsigned, must not be lower
+					than requested bytes.
+	Requested flags	(4 bytes)	GFP flags supplied by the caller.
+	Target CPU	(4 bytes)	Signed integer, valid for event id 1.
+					If equal to -1, target CPU is the same
+					as origin CPU, but the reverse might
+					not be true.
+
+The data is made available in the same endianness the machine has.
+
+Other event ids and type ids may be defined and added. Other fields may be
+added by increasing event size, but see below for details.
+Every modification to the ABI, including new id definitions, are followed
+by bumping the ABI version by one.
+
+Adding new data to the packet (features) is done at the end of the mandatory
+data:
+	Feature size	(2 byte)
+	Feature ID	(1 byte)
+	Feature data	(Feature size - 4 bytes)
+
+
+Users:
+	kmemtrace-user - git://repo.or.cz/kmemtrace-user.git
+