Revert "Xray docs with description of Flight Data Recorder binary format."

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-======================================
-XRay Flight Data Recorder Trace Format
-======================================
-
-:Version: 1 as of 2017-07-20
-
-.. contents::
-   :local:
-
-
-Introduction
-============
-
-When gathering XRay traces in Flight Data Recorder mode, each thread of an
-application will claim buffers to fill with trace data, which at some point
-is finalized and flushed.
-
-A goal of the profiler is to minimize overhead, so the flushed data directly
-corresponds to the buffer.
-
-This document describes the format of a trace file.
-
-
-General
-=======
-
-Each trace file corresponds to a sequence of events in a particular thread.
-
-The file has a header followed by a sequence of discriminated record types.
-
-The endianess of byte fields matches the endianess of the platform which
-produced the trace file.
-
-
-Header Section
-==============
-
-A trace file begins with a 32 byte header.
-
-+-------------------+-----------------+----------------------------------------+
-| Field             | Size (bytes)    | Description                            |
-+===================+=================+========================================+
-| version           | ``2``           | Anticipates versioned  readers. This   |
-|                   |                 | document describes the format when     |
-|                   |                 | version == 1                           |
-+-------------------+-----------------+----------------------------------------+
-| type              | ``2``           | An enumeration encoding the type of    |
-|                   |                 | trace. Flight Data Recorder mode       |
-|                   |                 | traces have type == 1                  |
-+-------------------+-----------------+----------------------------------------+
-| bitfield          | ``4``           | Holds parameters that are not aligned  |
-|                   |                 | to bytes. Further described below.     |
-+-------------------+-----------------+----------------------------------------+
-| cycle_frequency   | ``8``           | The frequency in hertz of the CPU      |
-|                   |                 | oscillator used to measure duration of |
-|                   |                 | events in ticks.                       |
-+-------------------+-----------------+----------------------------------------+
-| buffer_size       | ``8``           | The size in bytes of the data portion  |
-|                   |                 | of the trace following the header.     |
-+-------------------+-----------------+----------------------------------------+
-| reserved          | ``8``           | Reserved for future use.               |
-+-------------------+-----------------+----------------------------------------+
-
-The bitfield parameter of the file header is composed of the following fields.
-
-+-------------------+----------------+-----------------------------------------+
-| Field             | Size (bits)    | Description                             |
-+===================+================+=========================================+
-| constant_tsc      | ``1``          | Whether the platform's timestamp        |
-|                   |                | counter used to record ticks between    |
-|                   |                | events ticks at a constant frequency    |
-|                   |                | despite CPU frequency changes.          |
-|                   |                | 0 == non-constant. 1 == constant.       |
-+-------------------+----------------+-----------------------------------------+
-| nonstop_tsc       | ``1``          | Whether the tsc continues to count      |
-|                   |                | despite whether the CPU is in a low     |
-|                   |                | power state. 0 == stop. 1 == non-stop.  |
-+-------------------+----------------+-----------------------------------------+
-| reserved          | ``30``         | Not meaningful.                         |
-+-------------------+----------------+-----------------------------------------+
-
-
-Data Section
-============
-
-Following the header in a trace is a data section with size matching the
-buffer_size field in the header.
-
-The data section is a stream of elements of different types.
-
-There are a few categories of data in the sequence.
-
-- ``Function Records``: Function Records contain the timing of entry into and
-  exit from function execution. Function Records have 8 bytes each.
-
-- ``Metadata Records``: Metadata records serve many purposes. Mostly, they
-  capture information that may be too costly to record for each function, but
-  that is required to contextualize the fine-grained timings. They also are used
-  as markers for user-defined Event Data payloads. Metadata records have 16
-  bytes each.
-
-- ``Event Data``: Free form data may be associated with events that are traced
-  by the binary and encode data defined by a handler function. Event data is
-  always preceded with a marker record which indicates how large it is.
-
-- ``Function Arguments``: The arguments to some functions are included in the
-  trace. These are either pointer addresses or primitives that are read and
-  logged independently of their types in a high level language. To the tracer,
-  they are all simply numbers. Function Records that have attached arguments
-  will indicate their presence on the function entry record. We only support
-  logging contiguous function argument sequences starting with argument zero,
-  which will be the "this" pointer for member function invocations. For example,
-  we don't support logging the first and third argument.
-
-A reader of the memory format must maintain a state machine. The format makes no
-attempt to pad for alignment, and it is not seekable.
-
-
-Function Records
-----------------
-
-Function Records have an 8 byte layout. This layout encodes information to
-reconstruct a call stack of instrumented function and their durations.
-
-+---------------+--------------+-----------------------------------------------+
-| Field         | Size (bits)  | Description                                   |
-+===============+==============+===============================================+
-| discriminant  | ``1``        | Indicates whether a reader should read a      |
-|               |              | Function or Metadata record. Set to ``0`` for |
-|               |              | Function records.                             |
-+---------------+--------------+-----------------------------------------------+
-| action        | ``3``        | Specifies whether the function is being       |
-|               |              | entered, exited, or is a non-standard entry   |
-|               |              | or exit produced by optimizations.            |
-+---------------+--------------+-----------------------------------------------+
-| function_id   | ``28``       | A numeric ID for the function. Resolved to a  |
-|               |              | name via the xray instrumentation map. The    |
-|               |              | instrumentation map is built by xray at       |
-|               |              | compile time into an object file and pairs    |
-|               |              | the function ids to addresses. It is used for |
-|               |              | patching and as a lookup into the binary's    |
-|               |              | symbols to obtain names.                      |
-+---------------+--------------+-----------------------------------------------+
-| tsc_delta     | ``32``       | The number of ticks of the timestamp counter  |
-|               |              | since a previous record recorded a delta or   |
-|               |              | other TSC resetting event.                    |
-+---------------+--------------+-----------------------------------------------+
-
-On little-endian machines, the bitfields are ordered from least significant bit
-bit to most significant bit. A reader can read an 8 bit value and apply the mask
-``0x01`` for the discriminant. Similarly, they can read 32 bits and unsigned
-shift right by ``0x04`` to obtain the function_id field.
-
-On big-endian machine, the bitfields are written in order from most significant
-bit to least significant bit. A reader would read an 8 bit value and unsigned
-shift right by 7 bits for the discriminant. The function_id field could be
-obtained by reading a 32 bit value and applying the mask ``0x0FFFFFFF``.
-
-Function action types are as follows.
-
-+---------------+--------------+-----------------------------------------------+
-| Type          | Number       | Description                                   |
-+===============+==============+===============================================+
-| Entry         | ``0``        | Typical function entry.                       |
-+---------------+--------------+-----------------------------------------------+
-| Exit          | ``1``        | Typical function exit.                        |
-+---------------+--------------+-----------------------------------------------+
-| Tail_Exit     | ``2``        | An exit from a function due to Tail call      |
-|               |              | optimization.                                 |
-+---------------+--------------+-----------------------------------------------+
-| Entry_Args    | ``3``        | A function entry that records arguments.      |
-+---------------+--------------+-----------------------------------------------+
-
-Entry_Args records do not contain the arguments themselves. Instead, metadata
-records for each of the logged args follow the function record in the stream.
-
-
-Metadata Records
-----------------
-
-Interspersed throughout the buffer are 16 byte Metadata records. For typically
-instrumented binaries, they will be sparser than Function records, and they
-provide a fuller picture of the binary execution state.
-
-Metadata record layout is partially record dependent, but they share a common
-structure.
-
-The same bit field rules described for function records apply to the first byte
-of MetadataRecords. Within this byte, little endian machines use lsb to msb
-ordering and big endian machines use msb to lsb ordering.
-
-+---------------+--------------+-----------------------------------------------+
-| Field         | Size         | Description                                   |
-+===============+==============+===============================================+
-| discriminant  | ``1 bit``    | Indicates whether a reader should read a      |
-|               |              | Function or Metadata record. Set to ``1`` for |
-|               |              | Metadata records.                             |
-+---------------+--------------+-----------------------------------------------+
-| record_kind   | ``7 bits``   | The type of Metadata record.                  |
-+---------------+--------------+-----------------------------------------------+
-| data          | ``15 bytes`` | A data field used differently for each record |
-|               |              | type.                                         |
-+---------------+--------------+-----------------------------------------------+
-
-Here is a table of the enumerated record kinds.
-
-======  ===========================
-Number  Type
-------  ---------------------------
-0       NewBuffer
-1       EndOfBuffer
-2       NewCPUId
-3       TSCWrap
-4       WallTimeMarker
-5       CustomEventMarker
-6       CallArgument
-
-
-NewBuffer Records
------------------
-
-Each buffer begins with a NewBuffer record immediately after the header.
-It records the thread ID of the thread that the trace belongs to.
-
-Its data segment is as follows.
-
-+---------------+--------------+-----------------------------------------------+
-| Field         | Size (bytes) | Description                                   |
-+===============+==============+===============================================+
-| thread_Id     | ``2``        | Thread ID for buffer.                         |
-+---------------+--------------+-----------------------------------------------+
-| reserved      | ``13``       | Unused.                                       |
-+---------------+--------------+-----------------------------------------------+
-
-
-WallClockTime Records
----------------------
-
-Following the NewBuffer record, each buffer records an absolute time as a frame
-of reference for the durations recorded by timestamp counter deltas.
-
-Its data segment is as follows.
-
-+---------------+--------------+-----------------------------------------------+
-| Field         | Size (bytes) | Description                                   |
-+===============+==============+===============================================+
-| seconds       | ``8``        | Seconds on absolute timescale. The starting   |
-|               |              | point is unspecified and depends on the       |
-|               |              | implementation and platform configured by the |
-|               |              | tracer.                                       |
-+---------------+--------------+-----------------------------------------------+
-| microseconds  | ``4``        | The microsecond component of the time.        |
-+---------------+--------------+-----------------------------------------------+
-| reserved      | ``3``        | Unused.                                       |
-+---------------+--------------+-----------------------------------------------+
-
-
-NewCpuId Records
-----------------
-
-Each function entry invokes a routine to determine what CPU is executing.
-Typically, this is done with readtscp, which reads the timestamp counter at the
-same time.
-
-If the tracing detects that the execution has switched CPUs or if this is the
-first instrumented entry point, the tracer will output a NewCpuId record.
-
-Its data segment is as follows.
-
-+---------------+--------------+-----------------------------------------------+
-| Field         | Size (bytes) | Description                                   |
-+===============+==============+===============================================+
-| cpu_id        | ``2``        | CPU Id.                                       |
-+---------------+--------------+-----------------------------------------------+
-| absolute_tsc  | ``8``        | The absolute value of the timestamp counter.  |
-+---------------+--------------+-----------------------------------------------+
-| reserved      | ``5``        | Unused.                                       |
-+---------------+--------------+-----------------------------------------------+
-
-
-TSCWrap Records
----------------
-
-Since each function record uses a 32 bit value to represent the number of ticks
-of the timestamp counter since the last reference, it is possible for this value
-to overflow, particularly for sparsely instrumented binaries.
-
-When this delta would not fit into a 32 bit representation, a reference absolute
-timestamp counter record is written in the form of a TSCWrap record.
-
-Its data segment is as follows.
-
-+---------------+--------------+-----------------------------------------------+
-| Field         | Size (bytes) | Description                                   |
-+===============+==============+===============================================+
-| absolute_tsc  | ``8``        | Timestamp counter value.                      |
-+---------------+--------------+-----------------------------------------------+
-| reserved      | ``7``        | Unused.                                       |
-+---------------+--------------+-----------------------------------------------+
-
-
-CallArgument Records
---------------------
-
-Immediately following an Entry_Args type function record, there may be one or
-more CallArgument records that contain the traced function's parameter values.
-
-The order of the CallArgument Record sequency corresponds one to one with the
-order of the function parameters.
-
-CallArgument data segment:
-
-+---------------+--------------+-----------------------------------------------+
-| Field         | Size (bytes) | Description                                   |
-+===============+==============+===============================================+
-| argument      | ``8``        | Numeric argument (may be pointer address).    |
-+---------------+--------------+-----------------------------------------------+
-| reserved      | ``7``        | Unused.                                       |
-+---------------+--------------+-----------------------------------------------+
-
-
-CustomEventMarker Records
--------------------------
-
-XRay provides the feature of logging custom events. This may be leveraged to
-record tracing info for RPCs or similarly trace data that is application
-specific.
-
-Custom Events themselves are an unstructured (application defined) segment of
-memory with arbitrary size within the buffer. They are preceded by
-CustomEventMarkers to indicate their presence and size.
-
-CustomEventMarker data segment:
-
-+---------------+--------------+-----------------------------------------------+
-| Field         | Size (bytes) | Description                                   |
-+===============+==============+===============================================+
-| event_size    | ``4``        | Size of preceded event.                       |
-+---------------+--------------+-----------------------------------------------+
-| absolute_tsc  | ``8``        | A timestamp counter of the event.             |
-+---------------+--------------+-----------------------------------------------+
-| reserved      | ``3``        | Unused.                                       |
-+---------------+--------------+-----------------------------------------------+
-
-
-EndOfBuffer Records
--------------------
-
-An EndOfBuffer record type indicates that there is no more trace data in this
-buffer. The reader is expected to seek past the remaining buffer_size expressed
-before the start of buffer and look for either another header or EOF.
-
-
-Format Grammar and Invariants
-=============================
-
-Not all sequences of Metadata records and Function records are valid data. A
-sequence should be parsed as a state machine. The expectations for a valid
-format can be expressed as a context free grammar.
-
-This is an attempt to explain the format with statements in EBNF format.
-
-- Format := Header ThreadBuffer* EOF
-
-- ThreadBuffer := NewBuffer WallClockTime NewCPUId BodySequence* End
-
-- BodySequence := NewCPUId | TSCWrap | Function | CustomEvent
-
-- Function := (Function_Entry_Args CallArgument*) | Function_Other_Type
-
-- CustomEvent := CustomEventMarker CustomEventUnstructuredMemory
-
-- End := EndOfBuffer RemainingBufferSizeToSkip
-
-
-Function Record Order
----------------------
-
-There are a few clarifications that may help understand what is expected of
-Function records.
-
-- Functions with an Exit are expected to have a corresponding Entry or
-  Entry_Args function record precede them in the trace.
-
-- Tail_Exit Function records record the Function ID of the function whose return
-  address the program counter will take. In other words, the final function that
-  would be popped off of the call stack if tail call optimization was not used.
-
-- Not all functions marked for instrumentation are necessarily in the trace. The
-  tracer uses heuristics to preserve the trace for non-trivial functions.
-
-- Not every entry must have a traced Exit or Tail Exit. The buffer may run out
-  of space or the program may request for the tracer to finalize toreturn the
-  buffer before an instrumented function exits.