Update proposal to v1.5
[ctf.git] / common-trace-format-linux-proposal.txt
1
2 RFC: Common Trace Format Proposal for Linux (v1.5)
3
4 Mathieu Desnoyers, EfficiOS Inc.
5
6 The goal of the present document is to propose a trace format that suits the
7 needs of the embedded, telecom, high-performance and kernel communities. It is
8 based on the Common Trace Format Requirements (v1.4) document. It is designed to
9 allow tracing that is natively generated by the Linux kernel and Linux
10 user-space applications written in C/C++.
11
12 A reference implementation of a library to read and write this trace format is
13 being implemented within the BabelTrace project, a converter between trace
14 formats. The development tree is available at:
15
16 git tree: git://git.efficios.com/babeltrace.git
17 gitweb: http://git.efficios.com/?p=babeltrace.git
18
19
20 1. Preliminary definitions
21
22 - Event Trace: An ordered sequence of events.
23 - Event Stream: An ordered sequence of events, containing a subset of the
24 trace event types.
25 - Event Packet: A sequence of physically contiguous events within an event
26 stream.
27 - Event: This is the basic entry in a trace. (aka: a trace record).
28 - An event identifier (ID) relates to the class (a type) of event within
29 an event stream.
30 e.g. event: irq_entry.
31 - An event (or event record) relates to a specific instance of an event
32 class.
33 e.g. event: irq_entry, at time X, on CPU Y
34 - Source Architecture: Architecture writing the trace.
35 - Reader Architecture: Architecture reading the trace.
36
37
38 2. High-level representation of a trace
39
40 A trace is divided into multiple event streams. Each event stream contains a
41 subset of the trace event types.
42
43 The final output of the trace, after its generation and optional transport over
44 the network, is expected to be either on permanent or temporary storage in a
45 virtual file system. Because each event stream is appended to while a trace is
46 being recorded, each is associated with a separate file for output. Therefore,
47 a stored trace can be represented as a directory containing one file per stream.
48
49 A metadata event stream contains information on trace event types. It describes:
50
51 - Trace version.
52 - Types available.
53 - Per-stream event header description.
54 - Per-stream event header selection.
55 - Per-stream event context fields.
56 - Per-event
57 - Event type to stream mapping.
58 - Event type to name mapping.
59 - Event type to ID mapping.
60 - Event fields description.
61
62
63 3. Event stream
64
65 An event stream is divided in contiguous event packets of variable size. These
66 subdivisions have a variable size. An event packet can contain a certain amount
67 of padding at the end. The rationale for the event stream design choices is
68 explained in Appendix B. Stream Header Rationale.
69
70 An event stream is divided in contiguous event packets of variable size. These
71 subdivisions have a variable size. An event packet can contain a certain amount
72 of padding at the end. The stream header is repeated at the beginning of each
73 event packet.
74
75 The event stream header will therefore be referred to as the "event packet
76 header" throughout the rest of this document.
77
78
79 4. Types
80
81 4.1 Basic types
82
83 A basic type is a scalar type, as described in this section.
84
85 4.1.1 Type inheritance
86
87 Type specifications can be inherited to allow deriving concrete types from an
88 abstract type. For example, see the uint32_t type derived from the "integer"
89 abstract type below ("Integers" section). Concrete types have a precise binary
90 representation in the trace. Abstract types have methods to read and write these
91 types, but must be derived into a concrete type to be usable in an event field.
92
93 Concrete types inherit from abstract types. Abstract types can inherit from
94 other abstract types.
95
96 4.1.2 Alignment
97
98 We define "byte-packed" types as aligned on the byte size, namely 8-bit.
99 We define "bit-packed" types as following on the next bit, as defined by the
100 "bitfields" section.
101
102 All basic types, except bitfields, are either aligned on an architecture-defined
103 specific alignment or byte-packed, depending on the architecture preference.
104 Architectures providing fast unaligned write byte-packed basic types to save
105 space, aligning each type on byte boundaries (8-bit). Architectures with slow
106 unaligned writes align types on specific alignment values. If no specific
107 alignment is declared for a type nor its parents, it is assumed to be bit-packed
108 for bitfields and byte-packed for other types.
109
110 Metadata attribute representation of a specific alignment:
111
112 align = value; /* value in bits */
113
114 4.1.3 Byte order
115
116 By default, the native endianness of the source architecture the trace is used.
117 Byte order can be overridden for a basic type by specifying a "byte_order"
118 attribute. Typical use-case is to specify the network byte order (big endian:
119 "be") to save data captured from the network into the trace without conversion.
120 If not specified, the byte order is native.
121
122 Metadata representation:
123
124 byte_order = native OR network OR be OR le; /* network and be are aliases */
125
126 4.1.4 Size
127
128 Type size, in bits, for integers and floats is that returned by "sizeof()" in C
129 multiplied by CHAR_BIT.
130 We require the size of "char" and "unsigned char" types (CHAR_BIT) to be fixed
131 to 8 bits for cross-endianness compatibility.
132
133 Metadata representation:
134
135 size = value; (value is in bits)
136
137 4.1.5 Integers
138
139 Signed integers are represented in two-complement. Integer alignment, size,
140 signedness and byte ordering are defined in the metadata. Integers aligned on
141 byte size (8-bit) and with length multiple of byte size (8-bit) correspond to
142 the C99 standard integers. In addition, integers with alignment and/or size that
143 are _not_ a multiple of the byte size are permitted; these correspond to the C99
144 standard bitfields, with the added specification that the CTF integer bitfields
145 have a fixed binary representation. A MIT-licensed reference implementation of
146 the CTF portable bitfields is available at:
147
148 http://git.efficios.com/?p=babeltrace.git;a=blob;f=include/babeltrace/bitfield.h
149
150 Binary representation of integers:
151
152 - On little and big endian:
153 - Within a byte, high bits correspond to an integer high bits, and low bits
154 correspond to low bits.
155 - On little endian:
156 - Integer across multiple bytes are placed from the less significant to the
157 most significant.
158 - Consecutive integers are placed from lower bits to higher bits (even within
159 a byte).
160 - On big endian:
161 - Integer across multiple bytes are placed from the most significant to the
162 less significant.
163 - Consecutive integers are placed from higher bits to lower bits (even within
164 a byte).
165
166 This binary representation is derived from the bitfield implementation in GCC
167 for little and big endian. However, contrary to what GCC does, integers can
168 cross units boundaries (no padding is required). Padding can be explicitely
169 added (see 4.1.6 GNU/C bitfields) to follow the GCC layout if needed.
170
171 Metadata representation:
172
173 abstract_type integer {
174 signed = true OR false; /* default false */
175 byte_order = native OR network OR be OR le; /* default native */
176 size = value; /* value in bits, no default */
177 align = value; /* value in bits */
178 };
179
180 Example of type inheritance (creation of a concrete type uint32_t):
181
182 type uint32_t {
183 parent = integer;
184 size = 8;
185 signed = false;
186 align = 32;
187 };
188
189 Definition of a 5-bit signed bitfield:
190
191 type int5_t {
192 parent = integer;
193 size = 5;
194 signed = true;
195 align = 1;
196 };
197
198 4.1.6 GNU/C bitfields
199
200 The GNU/C bitfields follow closely the integer representation, with a
201 particularity on alignment: if a bitfield cannot fit in the current unit, the
202 unit is padded and the bitfield starts at the following unit. We therefore need
203 to express the extra "unit size" information.
204
205 Metadata representation:
206
207 abstract_type gcc_bitfield {
208 parent = integer;
209 unit_size = value;
210 };
211
212 As an example, the following structure declared in C compiled by GCC:
213
214 struct example {
215 short a:12;
216 short b:5;
217 };
218
219 Would correspond to the following structure, aligned on the largest element
220 (short). The second bitfield would be aligned on the next unit boundary, because
221 it would not fit in the current unit.
222
223 type struct_example {
224 parent = struct;
225 fields = {
226 type {
227 parent = gcc_bitfield;
228 unit_size = 16; /* sizeof(short) */
229 size = 12;
230 signed = true;
231 align = 1;
232 } a;
233 type {
234 parent = gcc_bitfield;
235 unit_size = 16; /* sizeof(short) */
236 size = 5;
237 signed = true;
238 align = 1;
239 } b;
240 };
241 };
242
243 4.1.7 Floating point
244
245 The floating point values byte ordering is defined in the metadata.
246
247 Floating point values follow the IEEE 754-2008 standard interchange formats.
248 Description of the floating point values include the exponent and mantissa size
249 in bits. Some requirements are imposed on the floating point values:
250
251 - FLT_RADIX must be 2.
252 - mant_dig is the number of digits represented in the mantissa. It is specified
253 by the ISO C99 standard, section 5.2.4, as FLT_MANT_DIG, DBL_MANT_DIG and
254 LDBL_MANT_DIG as defined by <float.h>.
255 - exp_dig is the number of digits represented in the exponent. Given that
256 mant_dig is one bit more than its actual size in bits (leading 1 is not
257 needed) and also given that the sign bit always takes one bit, exp_dig can be
258 specified as:
259
260 - sizeof(float) * CHAR_BIT - FLT_MANT_DIG
261 - sizeof(double) * CHAR_BIT - DBL_MANT_DIG
262 - sizeof(long double) * CHAR_BIT - LDBL_MANT_DIG
263
264 Metadata representation:
265
266 abstract_type floating_point {
267 exp_dig = value;
268 mant_dig = value;
269 byte_order = native OR network OR be OR le;
270 };
271
272 Example of type inheritance:
273
274 type float {
275 exp_dig = 8; /* sizeof(float) * CHAR_BIT - FLT_MANT_DIG */
276 mant_dig = 24; /* FLT_MANT_DIG */
277 byte_order = native;
278 };
279
280 TODO: define NaN, +inf, -inf behavior.
281
282 4.1.8 Enumerations
283
284 Enumerations are a mapping between an integer type and a table of strings. The
285 numerical representation of the enumeration follows the integer type specified
286 by the metadata. The enumeration mapping table is detailed in the enumeration
287 description within the metadata. The mapping table maps inclusive value ranges
288 (or single values) to strings. Instead of being limited to simple
289 "value -> string" mappings, these enumerations map
290 "[ start_value .. end_value ] -> string", which map inclusive ranges of
291 values to strings. An enumeration from the C language can be represented in
292 this format by having the same start_value and end_value for each element, which
293 is in fact a range of size 1. This single-value range is supported without
294 repeating the start and end values with the { value, string } declaration.
295
296 abstract_type enum {
297 parent = integer;
298 map = {
299 { { start_value, end_value }, string },
300 { { start_value, end_value }, string },
301 { { start_value, end_value }, string },
302 { value, string },
303 { value, string },
304 ...
305 };
306 };
307
308
309 4.2 Compound types
310
311 4.2.1 Structures
312
313 Structures are aligned on the largest alignment required by basic types
314 contained within the structure. (This follows the ISO/C standard for structures)
315
316 Metadata representation:
317
318 abstract_type struct {
319 fields = {
320 field_type field_name;
321 field_type field_name;
322 ...
323 };
324 }
325
326 Example:
327
328 type struct_example {
329 parent = struct;
330 fields = {
331 type { /* Nameless type */
332 parent = integer;
333 size = 16;
334 signed = true;
335 align = 16;
336 } first_field_name;
337 uint64_t second_field_name; /* Named type declared in the metadata */
338 };
339 };
340
341 The fields are placed in a sequence next to each other. They each possess a
342 field name, which is a unique identifier within the structure.
343
344 4.2.2 Arrays
345
346 Arrays are fixed-length. Their length is declared in the type declaration within
347 the metadata. They contain an array of "inner type" elements, which can refer to
348 any type not containing the type of the array being declared (no circular
349 dependency). The length is the number of elements in an array.
350
351 Metadata representation:
352
353 abstract_type array {
354 length = value;
355 elem_type = type;
356 };
357
358 E.g.:
359
360 type example_array {
361 parent = array;
362 length = 10;
363 elem_type = uint32_t;
364 };
365
366 4.2.3 Sequences
367
368 Sequences are dynamically-sized arrays. They start with an integer that specify
369 the length of the sequence, followed by an array of "inner type" elements.
370 The length is the number of elements in the sequence.
371
372 abstract_type sequence {
373 length_type = type; /* Inheriting from integer */
374 elem_type = type;
375 };
376
377 The integer type follows the integer types specifications, and the sequence
378 elements follow the "array" specifications.
379
380 4.2.4 Strings
381
382 Strings are an array of bytes of variable size and are terminated by a '\0'
383 "NULL" character. Their encoding is described in the metadata. In absence of
384 encoding attribute information, the default encoding is UTF-8.
385
386 abstract_type string {
387 encoding = UTF8 OR ASCII;
388 };
389
390
391 5. Event Packet Header
392
393 The event packet header consists of two part: one is mandatory and have a fixed
394 layout. The second part, the "event packet context", has its layout described in
395 the metadata.
396
397 - Aligned on page size. Fixed size. Fields either aligned or packed (depending
398 on the architecture preference).
399 No padding at the end of the event packet header. Native architecture byte
400 ordering.
401
402 Fixed layout (event packet header):
403
404 - Magic number (CTF magic numbers: 0xC1FC1FC1 and its reverse endianness
405 representation: 0xC11FFCC1) It needs to have a non-symmetric bytewise
406 representation. Used to distinguish between big and little endian traces (this
407 information is determined by knowing the endianness of the architecture
408 reading the trace and comparing the magic number against its value and the
409 reverse, 0xC11FFCC1). This magic number specifies that we use the CTF metadata
410 description language described in this document. Different magic numbers
411 should be used for other metadata description languages.
412 - Trace UUID, used to ensure the event packet match the metadata used.
413 (note: we cannot use a metadata checksum because metadata can be appended to
414 while tracing is active)
415 - Stream ID, used as reference to stream description in metadata.
416
417 Metadata-defined layout (event packet context):
418
419 - Event packet content size (in bytes).
420 - Event packet size (in bytes, includes padding).
421 - Event packet content checksum (optional). Checksum excludes the event packet
422 header.
423 - Per-stream event packet sequence count (to deal with UDP packet loss). The
424 number of significant sequence counter bits should also be present, so
425 wrap-arounds are deal with correctly.
426 - Timestamp at the beginning and timestamp at the end of the event packet.
427 Both timestamps are written in the packet header, but sampled respectively
428 while (or before) writing the first event and while (or after) writing the
429 last event in the packet. The inclusive range between these timestamps should
430 include all event timestamps assigned to events contained within the packet.
431 - Events discarded count
432 - Snapshot of a per-stream free-running counter, counting the number of
433 events discarded that were supposed to be written in the stream prior to
434 the first event in the event packet.
435 * Note: producer-consumer buffer full condition should fill the current
436 event packet with padding so we know exactly where events have been
437 discarded.
438 - Lossless compression scheme used for the event packet content. Applied
439 directly to raw data. New types of compression can be added in following
440 versions of the format.
441 0: no compression scheme
442 1: bzip2
443 2: gzip
444 3: xz
445 - Cypher used for the event packet content. Applied after compression.
446 0: no encryption
447 1: AES
448 - Checksum scheme used for the event packet content. Applied after encryption.
449 0: no checksum
450 1: md5
451 2: sha1
452 3: crc32
453
454 5.1 Event Packet Header Fixed Layout Description
455
456 type event_packet_header {
457 parent = struct;
458 fields = {
459 uint32_t magic;
460 type {
461 parent = array;
462 length = 16;
463 elem_type = uint8_t;
464 } trace_uuid;
465 uint32_t stream_id;
466 }
467
468 5.2 Event Packet Context Description
469
470 Event packet context example. These are declared within the stream declaration
471 in the metadata. All these fields are optional except for "content_size" and
472 "packet_size", which must be present in the context.
473
474 An example event packet context type:
475
476 type event_packet_context {
477 uint64_t timestamp_begin;
478 uint64_t timestamp_end;
479 uint32_t checksum;
480 uint32_t stream_packet_count;
481 uint32_t events_discarded;
482 uint32_t cpu_id;
483 uint32_t/uint16_t content_size;
484 uint32_t/uint16_t packet_size;
485 uint8_t stream_packet_count_bits; /* Significant counter bits */
486 uint8_t compression_scheme;
487 uint8_t encryption_scheme;
488 uint8_t checksum;
489 };
490
491 6. Event Structure
492
493 The overall structure of an event is:
494
495 - Event Header (as specifed by the stream metadata)
496 - Extended Event Header (as specified by the event header)
497 - Event Context (as specified by the stream metadata)
498 - Event Payload (as specified by the event metadata)
499
500
501 6.1 Event Header
502
503 One major factor can vary between streams: the number of event IDs assigned to
504 a stream. Luckily, this information tends to stay relatively constant (modulo
505 event registration while trace is being recorded), so we can specify different
506 representations for streams containing few event IDs and streams containing
507 many event IDs, so we end up representing the event ID and timestamp as densely
508 as possible in each case.
509
510 We therefore provide two types of events headers. Type 1 accommodates streams
511 with less than 31 event IDs. Type 2 accommodates streams with 31 or more event
512 IDs.
513
514 The "extended headers" are used in the rare occasions where the information
515 cannot be represented in the ranges available in the event header. They are also
516 used in the rare occasions where the data required for a field could not be
517 collected: the flag corresponding to the missing field within the missing_fields
518 array is then set to 1.
519
520 Types uintX_t represent an X-bit unsigned integer.
521
522
523 6.1.1 Type 1 - Few event IDs
524
525 - Aligned on 32-bit (or 8-bit if byte-packed, depending on the architecture
526 preference).
527 - Fixed size: 32 bits.
528 - Native architecture byte ordering.
529
530 type event_header_1 {
531 parent = struct;
532 fields = {
533 uint5_t id; /*
534 * id: range: 0 - 30.
535 * id 31 is reserved to indicate a following
536 * extended header.
537 */
538 uint27_t timestamp;
539 };
540 };
541
542 The end of a type 1 header is aligned on a 32-bit boundary (or packed).
543
544
545 6.1.2 Extended Type 1 Event Header
546
547 - Follows struct event_header_1, which is aligned on 32-bit, so no need to
548 realign.
549 - Variable size (depends on the number of fields per event).
550 - Native architecture byte ordering.
551
552 type event_header_1_ext {
553 parent = struct;
554 fields = {
555 uint32_t id; /* 32-bit event IDs */
556 uint64_t timestamp; /* 64-bit timestamps */
557 type {
558 parent = array;
559 length = NR_FIELDS; /* Number of fields within the event */
560 elem_type = uint1_t; /* 1-bit bitfield */
561 } missing_fields;
562 };
563 };
564
565
566 6.1.3 Type 2 - Many event IDs
567
568 - Aligned on 32-bit (or 8-bit if byte-packed, depending on the architecture
569 preference).
570 - Fixed size: 48 bits.
571 - Native architecture byte ordering.
572
573 type event_header_2 {
574 parent = struct;
575 fields = {
576 uint32_t timestamp;
577 uint16_t id; /*
578 * id: range: 0 - 65534.
579 * id 65535 is reserved to indicate a following
580 * extended header.
581 */
582 };
583 };
584
585 The end of a type 2 header is aligned on a 16-bit boundary (or 8-bit if
586 byte-packed).
587
588
589 6.1.4 Extended Type 2 Event Header
590
591 - Follows struct event_header_2, which alignment end on a 16-bit boundary, so
592 we need to align on 64-bit integer architecture alignment (or 8-bit if
593 byte-packed).
594 - Variable size (depends on the number of fields per event).
595 - Native architecture byte ordering.
596
597 type event_header_2_ext {
598 parent = struct;
599 fields = {
600 uint64_t timestamp; /* 64-bit timestamps */
601 uint32_t id; /* 32-bit event IDs */
602 type {
603 parent = array;
604 length = NR_FIELDS; /* Number of fields within the event */
605 elem_type = uint1_t; /* 1-bit bitfield */
606 } missing_fields;
607 };
608 };
609
610
611 6.2 Event Context
612
613 The event context contains information relative to the current event. The choice
614 and meaning of this information is specified by the metadata "stream"
615 information. For this trace format, event context is usually empty, except when
616 the metadata "stream" information specifies otherwise by declaring a non-empty
617 structure for the event context. An example of event context is to save the
618 event payload size with each event, or to save the current PID with each event.
619 These are declared within the stream declaration within the metadata.
620
621 An example event context type:
622
623 type event_context {
624 parent = struct;
625 fields = {
626 uint pid;
627 uint16_t payload_size;
628 };
629 };
630
631
632 6.3 Event Payload
633
634 An event payload contains fields specific to a given event type. The fields
635 belonging to an event type are described in the event-specific metadata
636 within a structure type.
637
638 6.3.1 Padding
639
640 No padding at the end of the event payload. This differs from the ISO/C standard
641 for structures, but follows the CTF standard for structures. In a trace, even
642 though it makes sense to align the beginning of a structure, it really makes no
643 sense to add padding at the end of the structure, because structures are usually
644 not followed by a structure of the same type.
645
646 This trick can be done by adding a zero-length "end" field at the end of the C
647 structures, and by using the offset of this field rather than using sizeof()
648 when calculating the size of a structure (see Appendix "A. Helper macros").
649
650 6.3.2 Alignment
651
652 The event payload is aligned on the largest alignment required by types
653 contained within the payload. (This follows the ISO/C standard for structures)
654
655
656
657 7. Metadata
658
659 The meta-data is located in a stream named "metadata". It is made of "event
660 packets", which each start with an event packet header. The event type within
661 the metadata stream have no event header nor event context. Each event only
662 contains a null-terminated "string" payload, which is a metadata description
663 entry. The events are packed one next to another. Each event packet start with
664 an event packet header, which contains, amongst other fields, the magic number
665 and trace UUID.
666
667 The metadata can be parsed by reading through the metadata strings, skipping
668 newlines and null-characters. Type names may contain spaces.
669
670 trace {
671 major = value; /* Trace format version */
672 minor = value;
673 uuid = value; /* Trace UUID */
674 word_size = value;
675 };
676
677 stream {
678 id = stream_id;
679 event {
680 /* Type 1 - Few event IDs; Type 2 - Many event IDs. See section 6.1. */
681 header_type = event_header_1 OR event_header_2;
682 /*
683 * Extended event header type. Only present if specified in event header
684 * on a per-event basis.
685 */
686 header_type_ext = event_header_1_ext OR event_header_2_ext;
687 context_type = type inheriting from "struct" abstract type;
688 };
689 packet {
690 context_type = type inheriting from "struct" abstract type;
691 };
692 };
693
694 event {
695 name = eventname;
696 id = value; /* Numeric identifier within the stream */
697 stream = stream_id;
698 fields = type inheriting from "struct" abstract type;
699 };
700
701 /* More detail on types in section 4. Types */
702
703 /* Named types */
704 type {
705 name = typename;
706 ...
707 };
708
709 /* Unnamed types, contained within compound type fields or type assignments. */
710 type {
711 ...
712 };
713
714
715 Structure types used for fields and context_type implicitly inherit from
716 "struct" and require no "type" identifier before the braces. E.g.:
717
718 fields = {
719 typeA name1;
720 typeB name2;
721 ...
722 };
723
724
725 A. Helper macros
726
727 The two following macros keep track of the size of a GNU/C structure without
728 padding at the end by placing HEADER_END as the last field. A one byte end field
729 is used for C90 compatibility (C99 flexible arrays could be used here). Note
730 that this does not affect the effective structure size, which should always be
731 calculated with the header_sizeof() helper.
732
733 #define HEADER_END char end_field
734 #define header_sizeof(type) offsetof(typeof(type), end_field)
735
736
737 B. Stream Header Rationale
738
739 An event stream is divided in contiguous event packets of variable size. These
740 subdivisions allow the trace analyzer to perform a fast binary search by time
741 within the stream (typically requiring to index only the event packet headers)
742 without reading the whole stream. These subdivisions have a variable size to
743 eliminate the need to transfer the event packet padding when partially filled
744 event packets must be sent when streaming a trace for live viewing/analysis.
745 An event packet can contain a certain amount of padding at the end. Dividing
746 streams into event packets is also useful for network streaming over UDP and
747 flight recorder mode tracing (a whole event packet can be swapped out of the
748 buffer atomically for reading).
749
750 The stream header is repeated at the beginning of each event packet to allow
751 flexibility in terms of:
752
753 - streaming support,
754 - allowing arbitrary buffers to be discarded without making the trace
755 unreadable,
756 - allow UDP packet loss handling by either dealing with missing event packet
757 or asking for re-transmission.
758 - transparently support flight recorder mode,
759 - transparently support crash dump.
760
761 The event stream header will therefore be referred to as the "event packet
762 header" throughout the rest of this document.
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