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
+