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1perf-intel-pt(1)
2================
3
4NAME
5----
6perf-intel-pt - Support for Intel Processor Trace within perf tools
7
8SYNOPSIS
9--------
10[verse]
11'perf record' -e intel_pt//
12
13DESCRIPTION
14-----------
15
16Intel Processor Trace (Intel PT) is an extension of Intel Architecture that
17collects information about software execution such as control flow, execution
18modes and timings and formats it into highly compressed binary packets.
19Technical details are documented in the Intel 64 and IA-32 Architectures
20Software Developer Manuals, Chapter 36 Intel Processor Trace.
21
22Intel PT is first supported in Intel Core M and 5th generation Intel Core
23processors that are based on the Intel micro-architecture code name Broadwell.
24
25Trace data is collected by 'perf record' and stored within the perf.data file.
26See below for options to 'perf record'.
27
28Trace data must be 'decoded' which involves walking the object code and matching
29the trace data packets. For example a TNT packet only tells whether a
30conditional branch was taken or not taken, so to make use of that packet the
31decoder must know precisely which instruction was being executed.
32
33Decoding is done on-the-fly. The decoder outputs samples in the same format as
34samples output by perf hardware events, for example as though the "instructions"
35or "branches" events had been recorded. Presently 3 tools support this:
36'perf script', 'perf report' and 'perf inject'. See below for more information
37on using those tools.
38
39The main distinguishing feature of Intel PT is that the decoder can determine
40the exact flow of software execution. Intel PT can be used to understand why
41and how did software get to a certain point, or behave a certain way. The
42software does not have to be recompiled, so Intel PT works with debug or release
43builds, however the executed images are needed - which makes use in JIT-compiled
44environments, or with self-modified code, a challenge. Also symbols need to be
45provided to make sense of addresses.
46
47A limitation of Intel PT is that it produces huge amounts of trace data
48(hundreds of megabytes per second per core) which takes a long time to decode,
49for example two or three orders of magnitude longer than it took to collect.
50Another limitation is the performance impact of tracing, something that will
51vary depending on the use-case and architecture.
52
53
54Quickstart
55----------
56
57It is important to start small. That is because it is easy to capture vastly
58more data than can possibly be processed.
59
60The simplest thing to do with Intel PT is userspace profiling of small programs.
61Data is captured with 'perf record' e.g. to trace 'ls' userspace-only:
62
63 perf record -e intel_pt//u ls
64
65And profiled with 'perf report' e.g.
66
67 perf report
68
69To also trace kernel space presents a problem, namely kernel self-modifying
70code. A fairly good kernel image is available in /proc/kcore but to get an
71accurate image a copy of /proc/kcore needs to be made under the same conditions
72as the data capture. 'perf record' can make a copy of /proc/kcore if the option
73--kcore is used, but access to /proc/kcore is restricted e.g.
74
75 sudo perf record -o pt_ls --kcore -e intel_pt// -- ls
76
77which will create a directory named 'pt_ls' and put the perf.data file (named
78simply 'data') and copies of /proc/kcore, /proc/kallsyms and /proc/modules into
79it. The other tools understand the directory format, so to use 'perf report'
80becomes:
81
82 sudo perf report -i pt_ls
83
84Because samples are synthesized after-the-fact, the sampling period can be
85selected for reporting. e.g. sample every microsecond
86
87 sudo perf report pt_ls --itrace=i1usge
88
89See the sections below for more information about the --itrace option.
90
91Beware the smaller the period, the more samples that are produced, and the
92longer it takes to process them.
93
94Also note that the coarseness of Intel PT timing information will start to
95distort the statistical value of the sampling as the sampling period becomes
96smaller.
97
98To represent software control flow, "branches" samples are produced. By default
99a branch sample is synthesized for every single branch. To get an idea what
100data is available you can use the 'perf script' tool with all itrace sampling
101options, which will list all the samples.
102
103 perf record -e intel_pt//u ls
104 perf script --itrace=ibxwpe
105
106An interesting field that is not printed by default is 'flags' which can be
107displayed as follows:
108
109 perf script --itrace=ibxwpe -F+flags
110
111The flags are "bcrosyiABExgh" which stand for branch, call, return, conditional,
112system, asynchronous, interrupt, transaction abort, trace begin, trace end,
113in transaction, VM-entry, and VM-exit respectively.
114
115perf script also supports higher level ways to dump instruction traces:
116
117 perf script --insn-trace --xed
118
119Dump all instructions. This requires installing the xed tool (see XED below)
120Dumping all instructions in a long trace can be fairly slow. It is usually better
121to start with higher level decoding, like
122
123 perf script --call-trace
124
125or
126
127 perf script --call-ret-trace
128
129and then select a time range of interest. The time range can then be examined
130in detail with
131
132 perf script --time starttime,stoptime --insn-trace --xed
133
134While examining the trace it's also useful to filter on specific CPUs using
135the -C option
136
137 perf script --time starttime,stoptime --insn-trace --xed -C 1
138
139Dump all instructions in time range on CPU 1.
140
141Another interesting field that is not printed by default is 'ipc' which can be
142displayed as follows:
143
144 perf script --itrace=be -F+ipc
145
146There are two ways that instructions-per-cycle (IPC) can be calculated depending
147on the recording.
148
149If the 'cyc' config term (see config terms section below) was used, then IPC is
150calculated using the cycle count from CYC packets, otherwise MTC packets are
151used - refer to the 'mtc' config term. When MTC is used, however, the values
152are less accurate because the timing is less accurate.
153
154Because Intel PT does not update the cycle count on every branch or instruction,
155the values will often be zero. When there are values, they will be the number
156of instructions and number of cycles since the last update, and thus represent
157the average IPC since the last IPC for that event type. Note IPC for "branches"
158events is calculated separately from IPC for "instructions" events.
159
160Also note that the IPC instruction count may or may not include the current
161instruction. If the cycle count is associated with an asynchronous branch
162(e.g. page fault or interrupt), then the instruction count does not include the
163current instruction, otherwise it does. That is consistent with whether or not
164that instruction has retired when the cycle count is updated.
165
166Another note, in the case of "branches" events, non-taken branches are not
167presently sampled, so IPC values for them do not appear e.g. a CYC packet with a
168TNT packet that starts with a non-taken branch. To see every possible IPC
169value, "instructions" events can be used e.g. --itrace=i0ns
170
171While it is possible to create scripts to analyze the data, an alternative
172approach is available to export the data to a sqlite or postgresql database.
173Refer to script export-to-sqlite.py or export-to-postgresql.py for more details,
174and to script exported-sql-viewer.py for an example of using the database.
175
176There is also script intel-pt-events.py which provides an example of how to
177unpack the raw data for power events and PTWRITE. The script also displays
178branches, and supports 2 additional modes selected by option:
179
180 --insn-trace - instruction trace
181 --src-trace - source trace
182
183As mentioned above, it is easy to capture too much data. One way to limit the
184data captured is to use 'snapshot' mode which is explained further below.
185Refer to 'new snapshot option' and 'Intel PT modes of operation' further below.
186
187Another problem that will be experienced is decoder errors. They can be caused
188by inability to access the executed image, self-modified or JIT-ed code, or the
189inability to match side-band information (such as context switches and mmaps)
190which results in the decoder not knowing what code was executed.
191
192There is also the problem of perf not being able to copy the data fast enough,
193resulting in data lost because the buffer was full. See 'Buffer handling' below
194for more details.
195
196
197perf record
198-----------
199
200new event
201~~~~~~~~~
202
203The Intel PT kernel driver creates a new PMU for Intel PT. PMU events are
204selected by providing the PMU name followed by the "config" separated by slashes.
205An enhancement has been made to allow default "config" e.g. the option
206
207 -e intel_pt//
208
209will use a default config value. Currently that is the same as
210
211 -e intel_pt/tsc,noretcomp=0/
212
213which is the same as
214
215 -e intel_pt/tsc=1,noretcomp=0/
216
217Note there are now new config terms - see section 'config terms' further below.
218
219The config terms are listed in /sys/devices/intel_pt/format. They are bit
220fields within the config member of the struct perf_event_attr which is
221passed to the kernel by the perf_event_open system call. They correspond to bit
222fields in the IA32_RTIT_CTL MSR. Here is a list of them and their definitions:
223
224 $ grep -H . /sys/bus/event_source/devices/intel_pt/format/*
225 /sys/bus/event_source/devices/intel_pt/format/cyc:config:1
226 /sys/bus/event_source/devices/intel_pt/format/cyc_thresh:config:19-22
227 /sys/bus/event_source/devices/intel_pt/format/mtc:config:9
228 /sys/bus/event_source/devices/intel_pt/format/mtc_period:config:14-17
229 /sys/bus/event_source/devices/intel_pt/format/noretcomp:config:11
230 /sys/bus/event_source/devices/intel_pt/format/psb_period:config:24-27
231 /sys/bus/event_source/devices/intel_pt/format/tsc:config:10
232
233Note that the default config must be overridden for each term i.e.
234
235 -e intel_pt/noretcomp=0/
236
237is the same as:
238
239 -e intel_pt/tsc=1,noretcomp=0/
240
241So, to disable TSC packets use:
242
243 -e intel_pt/tsc=0/
244
245It is also possible to specify the config value explicitly:
246
247 -e intel_pt/config=0x400/
248
249Note that, as with all events, the event is suffixed with event modifiers:
250
251 u userspace
252 k kernel
253 h hypervisor
254 G guest
255 H host
256 p precise ip
257
258'h', 'G' and 'H' are for virtualization which is not supported by Intel PT.
259'p' is also not relevant to Intel PT. So only options 'u' and 'k' are
260meaningful for Intel PT.
261
262perf_event_attr is displayed if the -vv option is used e.g.
263
264 ------------------------------------------------------------
265 perf_event_attr:
266 type 6
267 size 112
268 config 0x400
269 { sample_period, sample_freq } 1
270 sample_type IP|TID|TIME|CPU|IDENTIFIER
271 read_format ID
272 disabled 1
273 inherit 1
274 exclude_kernel 1
275 exclude_hv 1
276 enable_on_exec 1
277 sample_id_all 1
278 ------------------------------------------------------------
279 sys_perf_event_open: pid 31104 cpu 0 group_fd -1 flags 0x8
280 sys_perf_event_open: pid 31104 cpu 1 group_fd -1 flags 0x8
281 sys_perf_event_open: pid 31104 cpu 2 group_fd -1 flags 0x8
282 sys_perf_event_open: pid 31104 cpu 3 group_fd -1 flags 0x8
283 ------------------------------------------------------------
284
285
286config terms
287~~~~~~~~~~~~
288
289The June 2015 version of Intel 64 and IA-32 Architectures Software Developer
290Manuals, Chapter 36 Intel Processor Trace, defined new Intel PT features.
291Some of the features are reflect in new config terms. All the config terms are
292described below.
293
294tsc Always supported. Produces TSC timestamp packets to provide
295 timing information. In some cases it is possible to decode
296 without timing information, for example a per-thread context
297 that does not overlap executable memory maps.
298
299 The default config selects tsc (i.e. tsc=1).
300
301noretcomp Always supported. Disables "return compression" so a TIP packet
302 is produced when a function returns. Causes more packets to be
303 produced but might make decoding more reliable.
304
305 The default config does not select noretcomp (i.e. noretcomp=0).
306
307psb_period Allows the frequency of PSB packets to be specified.
308
309 The PSB packet is a synchronization packet that provides a
310 starting point for decoding or recovery from errors.
311
312 Support for psb_period is indicated by:
313
314 /sys/bus/event_source/devices/intel_pt/caps/psb_cyc
315
316 which contains "1" if the feature is supported and "0"
317 otherwise.
318
319 Valid values are given by:
320
321 /sys/bus/event_source/devices/intel_pt/caps/psb_periods
322
323 which contains a hexadecimal value, the bits of which represent
324 valid values e.g. bit 2 set means value 2 is valid.
325
326 The psb_period value is converted to the approximate number of
327 trace bytes between PSB packets as:
328
329 2 ^ (value + 11)
330
331 e.g. value 3 means 16KiB bytes between PSBs
332
333 If an invalid value is entered, the error message
334 will give a list of valid values e.g.
335
336 $ perf record -e intel_pt/psb_period=15/u uname
337 Invalid psb_period for intel_pt. Valid values are: 0-5
338
339 If MTC packets are selected, the default config selects a value
340 of 3 (i.e. psb_period=3) or the nearest lower value that is
341 supported (0 is always supported). Otherwise the default is 0.
342
343 If decoding is expected to be reliable and the buffer is large
344 then a large PSB period can be used.
345
346 Because a TSC packet is produced with PSB, the PSB period can
347 also affect the granularity to timing information in the absence
348 of MTC or CYC.
349
350mtc Produces MTC timing packets.
351
352 MTC packets provide finer grain timestamp information than TSC
353 packets. MTC packets record time using the hardware crystal
354 clock (CTC) which is related to TSC packets using a TMA packet.
355
356 Support for this feature is indicated by:
357
358 /sys/bus/event_source/devices/intel_pt/caps/mtc
359
360 which contains "1" if the feature is supported and
361 "0" otherwise.
362
363 The frequency of MTC packets can also be specified - see
364 mtc_period below.
365
366mtc_period Specifies how frequently MTC packets are produced - see mtc
367 above for how to determine if MTC packets are supported.
368
369 Valid values are given by:
370
371 /sys/bus/event_source/devices/intel_pt/caps/mtc_periods
372
373 which contains a hexadecimal value, the bits of which represent
374 valid values e.g. bit 2 set means value 2 is valid.
375
376 The mtc_period value is converted to the MTC frequency as:
377
378 CTC-frequency / (2 ^ value)
379
380 e.g. value 3 means one eighth of CTC-frequency
381
382 Where CTC is the hardware crystal clock, the frequency of which
383 can be related to TSC via values provided in cpuid leaf 0x15.
384
385 If an invalid value is entered, the error message
386 will give a list of valid values e.g.
387
388 $ perf record -e intel_pt/mtc_period=15/u uname
389 Invalid mtc_period for intel_pt. Valid values are: 0,3,6,9
390
391 The default value is 3 or the nearest lower value
392 that is supported (0 is always supported).
393
394cyc Produces CYC timing packets.
395
396 CYC packets provide even finer grain timestamp information than
397 MTC and TSC packets. A CYC packet contains the number of CPU
398 cycles since the last CYC packet. Unlike MTC and TSC packets,
399 CYC packets are only sent when another packet is also sent.
400
401 Support for this feature is indicated by:
402
403 /sys/bus/event_source/devices/intel_pt/caps/psb_cyc
404
405 which contains "1" if the feature is supported and
406 "0" otherwise.
407
408 The number of CYC packets produced can be reduced by specifying
409 a threshold - see cyc_thresh below.
410
411cyc_thresh Specifies how frequently CYC packets are produced - see cyc
412 above for how to determine if CYC packets are supported.
413
414 Valid cyc_thresh values are given by:
415
416 /sys/bus/event_source/devices/intel_pt/caps/cycle_thresholds
417
418 which contains a hexadecimal value, the bits of which represent
419 valid values e.g. bit 2 set means value 2 is valid.
420
421 The cyc_thresh value represents the minimum number of CPU cycles
422 that must have passed before a CYC packet can be sent. The
423 number of CPU cycles is:
424
425 2 ^ (value - 1)
426
427 e.g. value 4 means 8 CPU cycles must pass before a CYC packet
428 can be sent. Note a CYC packet is still only sent when another
429 packet is sent, not at, e.g. every 8 CPU cycles.
430
431 If an invalid value is entered, the error message
432 will give a list of valid values e.g.
433
434 $ perf record -e intel_pt/cyc,cyc_thresh=15/u uname
435 Invalid cyc_thresh for intel_pt. Valid values are: 0-12
436
437 CYC packets are not requested by default.
438
439pt Specifies pass-through which enables the 'branch' config term.
440
441 The default config selects 'pt' if it is available, so a user will
442 never need to specify this term.
443
444branch Enable branch tracing. Branch tracing is enabled by default so to
445 disable branch tracing use 'branch=0'.
446
447 The default config selects 'branch' if it is available.
448
449ptw Enable PTWRITE packets which are produced when a ptwrite instruction
450 is executed.
451
452 Support for this feature is indicated by:
453
454 /sys/bus/event_source/devices/intel_pt/caps/ptwrite
455
456 which contains "1" if the feature is supported and
457 "0" otherwise.
458
459fup_on_ptw Enable a FUP packet to follow the PTWRITE packet. The FUP packet
460 provides the address of the ptwrite instruction. In the absence of
461 fup_on_ptw, the decoder will use the address of the previous branch
462 if branch tracing is enabled, otherwise the address will be zero.
463 Note that fup_on_ptw will work even when branch tracing is disabled.
464
465pwr_evt Enable power events. The power events provide information about
466 changes to the CPU C-state.
467
468 Support for this feature is indicated by:
469
470 /sys/bus/event_source/devices/intel_pt/caps/power_event_trace
471
472 which contains "1" if the feature is supported and
473 "0" otherwise.
474
475
476AUX area sampling option
477~~~~~~~~~~~~~~~~~~~~~~~~
478
479To select Intel PT "sampling" the AUX area sampling option can be used:
480
481 --aux-sample
482
483Optionally it can be followed by the sample size in bytes e.g.
484
485 --aux-sample=8192
486
487In addition, the Intel PT event to sample must be defined e.g.
488
489 -e intel_pt//u
490
491Samples on other events will be created containing Intel PT data e.g. the
492following will create Intel PT samples on the branch-misses event, note the
493events must be grouped using {}:
494
495 perf record --aux-sample -e '{intel_pt//u,branch-misses:u}'
496
497An alternative to '--aux-sample' is to add the config term 'aux-sample-size' to
498events. In this case, the grouping is implied e.g.
499
500 perf record -e intel_pt//u -e branch-misses/aux-sample-size=8192/u
501
502is the same as:
503
504 perf record -e '{intel_pt//u,branch-misses/aux-sample-size=8192/u}'
505
506but allows for also using an address filter e.g.:
507
508 perf record -e intel_pt//u --filter 'filter * @/bin/ls' -e branch-misses/aux-sample-size=8192/u -- ls
509
510It is important to select a sample size that is big enough to contain at least
511one PSB packet. If not a warning will be displayed:
512
513 Intel PT sample size (%zu) may be too small for PSB period (%zu)
514
515The calculation used for that is: if sample_size <= psb_period + 256 display the
516warning. When sampling is used, psb_period defaults to 0 (2KiB).
517
518The default sample size is 4KiB.
519
520The sample size is passed in aux_sample_size in struct perf_event_attr. The
521sample size is limited by the maximum event size which is 64KiB. It is
522difficult to know how big the event might be without the trace sample attached,
523but the tool validates that the sample size is not greater than 60KiB.
524
525
526new snapshot option
527~~~~~~~~~~~~~~~~~~~
528
529The difference between full trace and snapshot from the kernel's perspective is
530that in full trace we don't overwrite trace data that the user hasn't collected
531yet (and indicated that by advancing aux_tail), whereas in snapshot mode we let
532the trace run and overwrite older data in the buffer so that whenever something
533interesting happens, we can stop it and grab a snapshot of what was going on
534around that interesting moment.
535
536To select snapshot mode a new option has been added:
537
538 -S
539
540Optionally it can be followed by the snapshot size e.g.
541
542 -S0x100000
543
544The default snapshot size is the auxtrace mmap size. If neither auxtrace mmap size
545nor snapshot size is specified, then the default is 4MiB for privileged users
546(or if /proc/sys/kernel/perf_event_paranoid < 0), 128KiB for unprivileged users.
547If an unprivileged user does not specify mmap pages, the mmap pages will be
548reduced as described in the 'new auxtrace mmap size option' section below.
549
550The snapshot size is displayed if the option -vv is used e.g.
551
552 Intel PT snapshot size: %zu
553
554
555new auxtrace mmap size option
556~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
557
558Intel PT buffer size is specified by an addition to the -m option e.g.
559
560 -m,16
561
562selects a buffer size of 16 pages i.e. 64KiB.
563
564Note that the existing functionality of -m is unchanged. The auxtrace mmap size
565is specified by the optional addition of a comma and the value.
566
567The default auxtrace mmap size for Intel PT is 4MiB/page_size for privileged users
568(or if /proc/sys/kernel/perf_event_paranoid < 0), 128KiB for unprivileged users.
569If an unprivileged user does not specify mmap pages, the mmap pages will be
570reduced from the default 512KiB/page_size to 256KiB/page_size, otherwise the
571user is likely to get an error as they exceed their mlock limit (Max locked
572memory as shown in /proc/self/limits). Note that perf does not count the first
573512KiB (actually /proc/sys/kernel/perf_event_mlock_kb minus 1 page) per cpu
574against the mlock limit so an unprivileged user is allowed 512KiB per cpu plus
575their mlock limit (which defaults to 64KiB but is not multiplied by the number
576of cpus).
577
578In full-trace mode, powers of two are allowed for buffer size, with a minimum
579size of 2 pages. In snapshot mode or sampling mode, it is the same but the
580minimum size is 1 page.
581
582The mmap size and auxtrace mmap size are displayed if the -vv option is used e.g.
583
584 mmap length 528384
585 auxtrace mmap length 4198400
586
587
588Intel PT modes of operation
589~~~~~~~~~~~~~~~~~~~~~~~~~~~
590
591Intel PT can be used in 3 modes:
592 full-trace mode
593 sample mode
594 snapshot mode
595
596Full-trace mode traces continuously e.g.
597
598 perf record -e intel_pt//u uname
599
600Sample mode attaches a Intel PT sample to other events e.g.
601
602 perf record --aux-sample -e intel_pt//u -e branch-misses:u
603
604Snapshot mode captures the available data when a signal is sent or "snapshot"
605control command is issued. e.g. using a signal
606
607 perf record -v -e intel_pt//u -S ./loopy 1000000000 &
608 [1] 11435
609 kill -USR2 11435
610 Recording AUX area tracing snapshot
611
612Note that the signal sent is SIGUSR2.
613Note that "Recording AUX area tracing snapshot" is displayed because the -v
614option is used.
615
616The advantage of using "snapshot" control command is that the access is
617controlled by access to a FIFO e.g.
618
619 $ mkfifo perf.control
620 $ mkfifo perf.ack
621 $ cat perf.ack &
622 [1] 15235
623 $ sudo ~/bin/perf record --control fifo:perf.control,perf.ack -S -e intel_pt//u -- sleep 60 &
624 [2] 15243
625 $ ps -e | grep perf
626 15244 pts/1 00:00:00 perf
627 $ kill -USR2 15244
628 bash: kill: (15244) - Operation not permitted
629 $ echo snapshot > perf.control
630 ack
631
632The 3 Intel PT modes of operation cannot be used together.
633
634
635Buffer handling
636~~~~~~~~~~~~~~~
637
638There may be buffer limitations (i.e. single ToPa entry) which means that actual
639buffer sizes are limited to powers of 2 up to 4MiB (MAX_ORDER). In order to
640provide other sizes, and in particular an arbitrarily large size, multiple
641buffers are logically concatenated. However an interrupt must be used to switch
642between buffers. That has two potential problems:
643 a) the interrupt may not be handled in time so that the current buffer
644 becomes full and some trace data is lost.
645 b) the interrupts may slow the system and affect the performance
646 results.
647
648If trace data is lost, the driver sets 'truncated' in the PERF_RECORD_AUX event
649which the tools report as an error.
650
651In full-trace mode, the driver waits for data to be copied out before allowing
652the (logical) buffer to wrap-around. If data is not copied out quickly enough,
653again 'truncated' is set in the PERF_RECORD_AUX event. If the driver has to
654wait, the intel_pt event gets disabled. Because it is difficult to know when
655that happens, perf tools always re-enable the intel_pt event after copying out
656data.
657
658
659Intel PT and build ids
660~~~~~~~~~~~~~~~~~~~~~~
661
662By default "perf record" post-processes the event stream to find all build ids
663for executables for all addresses sampled. Deliberately, Intel PT is not
664decoded for that purpose (it would take too long). Instead the build ids for
665all executables encountered (due to mmap, comm or task events) are included
666in the perf.data file.
667
668To see buildids included in the perf.data file use the command:
669
670 perf buildid-list
671
672If the perf.data file contains Intel PT data, that is the same as:
673
674 perf buildid-list --with-hits
675
676
677Snapshot mode and event disabling
678~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
679
680In order to make a snapshot, the intel_pt event is disabled using an IOCTL,
681namely PERF_EVENT_IOC_DISABLE. However doing that can also disable the
682collection of side-band information. In order to prevent that, a dummy
683software event has been introduced that permits tracking events (like mmaps) to
684continue to be recorded while intel_pt is disabled. That is important to ensure
685there is complete side-band information to allow the decoding of subsequent
686snapshots.
687
688A test has been created for that. To find the test:
689
690 perf test list
691 ...
692 23: Test using a dummy software event to keep tracking
693
694To run the test:
695
696 perf test 23
697 23: Test using a dummy software event to keep tracking : Ok
698
699
700perf record modes (nothing new here)
701~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
702
703perf record essentially operates in one of three modes:
704 per thread
705 per cpu
706 workload only
707
708"per thread" mode is selected by -t or by --per-thread (with -p or -u or just a
709workload).
710"per cpu" is selected by -C or -a.
711"workload only" mode is selected by not using the other options but providing a
712command to run (i.e. the workload).
713
714In per-thread mode an exact list of threads is traced. There is no inheritance.
715Each thread has its own event buffer.
716
717In per-cpu mode all processes (or processes from the selected cgroup i.e. -G
718option, or processes selected with -p or -u) are traced. Each cpu has its own
719buffer. Inheritance is allowed.
720
721In workload-only mode, the workload is traced but with per-cpu buffers.
722Inheritance is allowed. Note that you can now trace a workload in per-thread
723mode by using the --per-thread option.
724
725
726Privileged vs non-privileged users
727~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
728
729Unless /proc/sys/kernel/perf_event_paranoid is set to -1, unprivileged users
730have memory limits imposed upon them. That affects what buffer sizes they can
731have as outlined above.
732
733The v4.2 kernel introduced support for a context switch metadata event,
734PERF_RECORD_SWITCH, which allows unprivileged users to see when their processes
735are scheduled out and in, just not by whom, which is left for the
736PERF_RECORD_SWITCH_CPU_WIDE, that is only accessible in system wide context,
737which in turn requires CAP_PERFMON or CAP_SYS_ADMIN.
738
739Please see the 45ac1403f564 ("perf: Add PERF_RECORD_SWITCH to indicate context
740switches") commit, that introduces these metadata events for further info.
741
742When working with kernels < v4.2, the following considerations must be taken,
743as the sched:sched_switch tracepoints will be used to receive such information:
744
745Unless /proc/sys/kernel/perf_event_paranoid is set to -1, unprivileged users are
746not permitted to use tracepoints which means there is insufficient side-band
747information to decode Intel PT in per-cpu mode, and potentially workload-only
748mode too if the workload creates new processes.
749
750Note also, that to use tracepoints, read-access to debugfs is required. So if
751debugfs is not mounted or the user does not have read-access, it will again not
752be possible to decode Intel PT in per-cpu mode.
753
754
755sched_switch tracepoint
756~~~~~~~~~~~~~~~~~~~~~~~
757
758The sched_switch tracepoint is used to provide side-band data for Intel PT
759decoding in kernels where the PERF_RECORD_SWITCH metadata event isn't
760available.
761
762The sched_switch events are automatically added. e.g. the second event shown
763below:
764
765 $ perf record -vv -e intel_pt//u uname
766 ------------------------------------------------------------
767 perf_event_attr:
768 type 6
769 size 112
770 config 0x400
771 { sample_period, sample_freq } 1
772 sample_type IP|TID|TIME|CPU|IDENTIFIER
773 read_format ID
774 disabled 1
775 inherit 1
776 exclude_kernel 1
777 exclude_hv 1
778 enable_on_exec 1
779 sample_id_all 1
780 ------------------------------------------------------------
781 sys_perf_event_open: pid 31104 cpu 0 group_fd -1 flags 0x8
782 sys_perf_event_open: pid 31104 cpu 1 group_fd -1 flags 0x8
783 sys_perf_event_open: pid 31104 cpu 2 group_fd -1 flags 0x8
784 sys_perf_event_open: pid 31104 cpu 3 group_fd -1 flags 0x8
785 ------------------------------------------------------------
786 perf_event_attr:
787 type 2
788 size 112
789 config 0x108
790 { sample_period, sample_freq } 1
791 sample_type IP|TID|TIME|CPU|PERIOD|RAW|IDENTIFIER
792 read_format ID
793 inherit 1
794 sample_id_all 1
795 exclude_guest 1
796 ------------------------------------------------------------
797 sys_perf_event_open: pid -1 cpu 0 group_fd -1 flags 0x8
798 sys_perf_event_open: pid -1 cpu 1 group_fd -1 flags 0x8
799 sys_perf_event_open: pid -1 cpu 2 group_fd -1 flags 0x8
800 sys_perf_event_open: pid -1 cpu 3 group_fd -1 flags 0x8
801 ------------------------------------------------------------
802 perf_event_attr:
803 type 1
804 size 112
805 config 0x9
806 { sample_period, sample_freq } 1
807 sample_type IP|TID|TIME|IDENTIFIER
808 read_format ID
809 disabled 1
810 inherit 1
811 exclude_kernel 1
812 exclude_hv 1
813 mmap 1
814 comm 1
815 enable_on_exec 1
816 task 1
817 sample_id_all 1
818 mmap2 1
819 comm_exec 1
820 ------------------------------------------------------------
821 sys_perf_event_open: pid 31104 cpu 0 group_fd -1 flags 0x8
822 sys_perf_event_open: pid 31104 cpu 1 group_fd -1 flags 0x8
823 sys_perf_event_open: pid 31104 cpu 2 group_fd -1 flags 0x8
824 sys_perf_event_open: pid 31104 cpu 3 group_fd -1 flags 0x8
825 mmap size 528384B
826 AUX area mmap length 4194304
827 perf event ring buffer mmapped per cpu
828 Synthesizing auxtrace information
829 Linux
830 [ perf record: Woken up 1 times to write data ]
831 [ perf record: Captured and wrote 0.042 MB perf.data ]
832
833Note, the sched_switch event is only added if the user is permitted to use it
834and only in per-cpu mode.
835
836Note also, the sched_switch event is only added if TSC packets are requested.
837That is because, in the absence of timing information, the sched_switch events
838cannot be matched against the Intel PT trace.
839
840
841perf script
842-----------
843
844By default, perf script will decode trace data found in the perf.data file.
845This can be further controlled by new option --itrace.
846
847
848New --itrace option
849~~~~~~~~~~~~~~~~~~~
850
851Having no option is the same as
852
853 --itrace
854
855which, in turn, is the same as
856
857 --itrace=cepwx
858
859The letters are:
860
861 i synthesize "instructions" events
862 b synthesize "branches" events
863 x synthesize "transactions" events
864 w synthesize "ptwrite" events
865 p synthesize "power" events (incl. PSB events)
866 c synthesize branches events (calls only)
867 r synthesize branches events (returns only)
868 e synthesize tracing error events
869 d create a debug log
870 g synthesize a call chain (use with i or x)
871 G synthesize a call chain on existing event records
872 l synthesize last branch entries (use with i or x)
873 L synthesize last branch entries on existing event records
874 s skip initial number of events
875 q quicker (less detailed) decoding
876 Z prefer to ignore timestamps (so-called "timeless" decoding)
877
878"Instructions" events look like they were recorded by "perf record -e
879instructions".
880
881"Branches" events look like they were recorded by "perf record -e branches". "c"
882and "r" can be combined to get calls and returns.
883
884"Transactions" events correspond to the start or end of transactions. The
885'flags' field can be used in perf script to determine whether the event is a
886tranasaction start, commit or abort.
887
888Note that "instructions", "branches" and "transactions" events depend on code
889flow packets which can be disabled by using the config term "branch=0". Refer
890to the config terms section above.
891
892"ptwrite" events record the payload of the ptwrite instruction and whether
893"fup_on_ptw" was used. "ptwrite" events depend on PTWRITE packets which are
894recorded only if the "ptw" config term was used. Refer to the config terms
895section above. perf script "synth" field displays "ptwrite" information like
896this: "ip: 0 payload: 0x123456789abcdef0" where "ip" is 1 if "fup_on_ptw" was
897used.
898
899"Power" events correspond to power event packets and CBR (core-to-bus ratio)
900packets. While CBR packets are always recorded when tracing is enabled, power
901event packets are recorded only if the "pwr_evt" config term was used. Refer to
902the config terms section above. The power events record information about
903C-state changes, whereas CBR is indicative of CPU frequency. perf script
904"event,synth" fields display information like this:
905 cbr: cbr: 22 freq: 2189 MHz (200%)
906 mwait: hints: 0x60 extensions: 0x1
907 pwre: hw: 0 cstate: 2 sub-cstate: 0
908 exstop: ip: 1
909 pwrx: deepest cstate: 2 last cstate: 2 wake reason: 0x4
910Where:
911 "cbr" includes the frequency and the percentage of maximum non-turbo
912 "mwait" shows mwait hints and extensions
913 "pwre" shows C-state transitions (to a C-state deeper than C0) and
914 whether initiated by hardware
915 "exstop" indicates execution stopped and whether the IP was recorded
916 exactly,
917 "pwrx" indicates return to C0
918For more details refer to the Intel 64 and IA-32 Architectures Software
919Developer Manuals.
920
921PSB events show when a PSB+ occurred and also the byte-offset in the trace.
922Emitting a PSB+ can cause a CPU a slight delay. When doing timing analysis
923of code with Intel PT, it is useful to know if a timing bubble was caused
924by Intel PT or not.
925
926Error events show where the decoder lost the trace. Error events
927are quite important. Users must know if what they are seeing is a complete
928picture or not. The "e" option may be followed by flags which affect what errors
929will or will not be reported. Each flag must be preceded by either '+' or '-'.
930The flags supported by Intel PT are:
931 -o Suppress overflow errors
932 -l Suppress trace data lost errors
933For example, for errors but not overflow or data lost errors:
934
935 --itrace=e-o-l
936
937The "d" option will cause the creation of a file "intel_pt.log" containing all
938decoded packets and instructions. Note that this option slows down the decoder
939and that the resulting file may be very large. The "d" option may be followed
940by flags which affect what debug messages will or will not be logged. Each flag
941must be preceded by either '+' or '-'. The flags support by Intel PT are:
942 -a Suppress logging of perf events
943 +a Log all perf events
944By default, logged perf events are filtered by any specified time ranges, but
945flag +a overrides that.
946
947In addition, the period of the "instructions" event can be specified. e.g.
948
949 --itrace=i10us
950
951sets the period to 10us i.e. one instruction sample is synthesized for each 10
952microseconds of trace. Alternatives to "us" are "ms" (milliseconds),
953"ns" (nanoseconds), "t" (TSC ticks) or "i" (instructions).
954
955"ms", "us" and "ns" are converted to TSC ticks.
956
957The timing information included with Intel PT does not give the time of every
958instruction. Consequently, for the purpose of sampling, the decoder estimates
959the time since the last timing packet based on 1 tick per instruction. The time
960on the sample is *not* adjusted and reflects the last known value of TSC.
961
962For Intel PT, the default period is 100us.
963
964Setting it to a zero period means "as often as possible".
965
966In the case of Intel PT that is the same as a period of 1 and a unit of
967'instructions' (i.e. --itrace=i1i).
968
969Also the call chain size (default 16, max. 1024) for instructions or
970transactions events can be specified. e.g.
971
972 --itrace=ig32
973 --itrace=xg32
974
975Also the number of last branch entries (default 64, max. 1024) for instructions or
976transactions events can be specified. e.g.
977
978 --itrace=il10
979 --itrace=xl10
980
981Note that last branch entries are cleared for each sample, so there is no overlap
982from one sample to the next.
983
984The G and L options are designed in particular for sample mode, and work much
985like g and l but add call chain and branch stack to the other selected events
986instead of synthesized events. For example, to record branch-misses events for
987'ls' and then add a call chain derived from the Intel PT trace:
988
989 perf record --aux-sample -e '{intel_pt//u,branch-misses:u}' -- ls
990 perf report --itrace=Ge
991
992Although in fact G is a default for perf report, so that is the same as just:
993
994 perf report
995
996One caveat with the G and L options is that they work poorly with "Large PEBS".
997Large PEBS means PEBS records will be accumulated by hardware and the written
998into the event buffer in one go. That reduces interrupts, but can give very
999late timestamps. Because the Intel PT trace is synchronized by timestamps,
1000the PEBS events do not match the trace. Currently, Large PEBS is used only in
1001certain circumstances:
1002 - hardware supports it
1003 - PEBS is used
1004 - event period is specified, instead of frequency
1005 - the sample type is limited to the following flags:
1006 PERF_SAMPLE_IP | PERF_SAMPLE_TID | PERF_SAMPLE_ADDR |
1007 PERF_SAMPLE_ID | PERF_SAMPLE_CPU | PERF_SAMPLE_STREAM_ID |
1008 PERF_SAMPLE_DATA_SRC | PERF_SAMPLE_IDENTIFIER |
1009 PERF_SAMPLE_TRANSACTION | PERF_SAMPLE_PHYS_ADDR |
1010 PERF_SAMPLE_REGS_INTR | PERF_SAMPLE_REGS_USER |
1011 PERF_SAMPLE_PERIOD (and sometimes) | PERF_SAMPLE_TIME
1012Because Intel PT sample mode uses a different sample type to the list above,
1013Large PEBS is not used with Intel PT sample mode. To avoid Large PEBS in other
1014cases, avoid specifying the event period i.e. avoid the 'perf record' -c option,
1015--count option, or 'period' config term.
1016
1017To disable trace decoding entirely, use the option --no-itrace.
1018
1019It is also possible to skip events generated (instructions, branches, transactions)
1020at the beginning. This is useful to ignore initialization code.
1021
1022 --itrace=i0nss1000000
1023
1024skips the first million instructions.
1025
1026The q option changes the way the trace is decoded. The decoding is much faster
1027but much less detailed. Specifically, with the q option, the decoder does not
1028decode TNT packets, and does not walk object code, but gets the ip from FUP and
1029TIP packets. The q option can be used with the b and i options but the period
1030is not used. The q option decodes more quickly, but is useful only if the
1031control flow of interest is represented or indicated by FUP, TIP, TIP.PGE, or
1032TIP.PGD packets (refer below). However the q option could be used to find time
1033ranges that could then be decoded fully using the --time option.
1034
1035What will *not* be decoded with the (single) q option:
1036
1037 - direct calls and jmps
1038 - conditional branches
1039 - non-branch instructions
1040
1041What *will* be decoded with the (single) q option:
1042
1043 - asynchronous branches such as interrupts
1044 - indirect branches
1045 - function return target address *if* the noretcomp config term (refer
1046 config terms section) was used
1047 - start of (control-flow) tracing
1048 - end of (control-flow) tracing, if it is not out of context
1049 - power events, ptwrite, transaction start and abort
1050 - instruction pointer associated with PSB packets
1051
1052Note the q option does not specify what events will be synthesized e.g. the p
1053option must be used also to show power events.
1054
1055Repeating the q option (double-q i.e. qq) results in even faster decoding and even
1056less detail. The decoder decodes only extended PSB (PSB+) packets, getting the
1057instruction pointer if there is a FUP packet within PSB+ (i.e. between PSB and
1058PSBEND). Note PSB packets occur regularly in the trace based on the psb_period
1059config term (refer config terms section). There will be a FUP packet if the
1060PSB+ occurs while control flow is being traced.
1061
1062What will *not* be decoded with the qq option:
1063
1064 - everything except instruction pointer associated with PSB packets
1065
1066What *will* be decoded with the qq option:
1067
1068 - instruction pointer associated with PSB packets
1069
1070The Z option is equivalent to having recorded a trace without TSC
1071(i.e. config term tsc=0). It can be useful to avoid timestamp issues when
1072decoding a trace of a virtual machine.
1073
1074
1075dump option
1076~~~~~~~~~~~
1077
1078perf script has an option (-D) to "dump" the events i.e. display the binary
1079data.
1080
1081When -D is used, Intel PT packets are displayed. The packet decoder does not
1082pay attention to PSB packets, but just decodes the bytes - so the packets seen
1083by the actual decoder may not be identical in places where the data is corrupt.
1084One example of that would be when the buffer-switching interrupt has been too
1085slow, and the buffer has been filled completely. In that case, the last packet
1086in the buffer might be truncated and immediately followed by a PSB as the trace
1087continues in the next buffer.
1088
1089To disable the display of Intel PT packets, combine the -D option with
1090--no-itrace.
1091
1092
1093perf report
1094-----------
1095
1096By default, perf report will decode trace data found in the perf.data file.
1097This can be further controlled by new option --itrace exactly the same as
1098perf script, with the exception that the default is --itrace=igxe.
1099
1100
1101perf inject
1102-----------
1103
1104perf inject also accepts the --itrace option in which case tracing data is
1105removed and replaced with the synthesized events. e.g.
1106
1107 perf inject --itrace -i perf.data -o perf.data.new
1108
1109Below is an example of using Intel PT with autofdo. It requires autofdo
1110(https://github.com/google/autofdo) and gcc version 5. The bubble
1111sort example is from the AutoFDO tutorial (https://gcc.gnu.org/wiki/AutoFDO/Tutorial)
1112amended to take the number of elements as a parameter.
1113
1114 $ gcc-5 -O3 sort.c -o sort_optimized
1115 $ ./sort_optimized 30000
1116 Bubble sorting array of 30000 elements
1117 2254 ms
1118
1119 $ cat ~/.perfconfig
1120 [intel-pt]
1121 mispred-all = on
1122
1123 $ perf record -e intel_pt//u ./sort 3000
1124 Bubble sorting array of 3000 elements
1125 58 ms
1126 [ perf record: Woken up 2 times to write data ]
1127 [ perf record: Captured and wrote 3.939 MB perf.data ]
1128 $ perf inject -i perf.data -o inj --itrace=i100usle --strip
1129 $ ./create_gcov --binary=./sort --profile=inj --gcov=sort.gcov -gcov_version=1
1130 $ gcc-5 -O3 -fauto-profile=sort.gcov sort.c -o sort_autofdo
1131 $ ./sort_autofdo 30000
1132 Bubble sorting array of 30000 elements
1133 2155 ms
1134
1135Note there is currently no advantage to using Intel PT instead of LBR, but
1136that may change in the future if greater use is made of the data.
1137
1138
1139PEBS via Intel PT
1140-----------------
1141
1142Some hardware has the feature to redirect PEBS records to the Intel PT trace.
1143Recording is selected by using the aux-output config term e.g.
1144
1145 perf record -c 10000 -e '{intel_pt/branch=0/,cycles/aux-output/ppp}' uname
1146
1147Note that currently, software only supports redirecting at most one PEBS event.
1148
1149To display PEBS events from the Intel PT trace, use the itrace 'o' option e.g.
1150
1151 perf script --itrace=oe
1152
1153XED
1154---
1155
1156include::build-xed.txt[]
1157
1158
1159Tracing Virtual Machines
1160------------------------
1161
1162Currently, only kernel tracing is supported and only with either "timeless" decoding
1163(i.e. no TSC timestamps) or VM Time Correlation. VM Time Correlation is an extra step
1164using 'perf inject' and requires unchanging VMX TSC Offset and no VMX TSC Scaling.
1165
1166Other limitations and caveats
1167
1168 VMX controls may suppress packets needed for decoding resulting in decoding errors
1169 VMX controls may block the perf NMI to the host potentially resulting in lost trace data
1170 Guest kernel self-modifying code (e.g. jump labels or JIT-compiled eBPF) will result in decoding errors
1171 Guest thread information is unknown
1172 Guest VCPU is unknown but may be able to be inferred from the host thread
1173 Callchains are not supported
1174
1175Example using "timeless" decoding
1176
1177Start VM
1178
1179 $ sudo virsh start kubuntu20.04
1180 Domain kubuntu20.04 started
1181
1182Mount the guest file system. Note sshfs needs -o direct_io to enable reading of proc files. root access is needed to read /proc/kcore.
1183
1184 $ mkdir vm0
1185 $ sshfs -o direct_io root@vm0:/ vm0
1186
1187Copy the guest /proc/kallsyms, /proc/modules and /proc/kcore
1188
1189 $ perf buildid-cache -v --kcore vm0/proc/kcore
1190 kcore added to build-id cache directory /home/user/.debug/[kernel.kcore]/9600f316a53a0f54278885e8d9710538ec5f6a08/2021021807494306
1191 $ KALLSYMS=/home/user/.debug/[kernel.kcore]/9600f316a53a0f54278885e8d9710538ec5f6a08/2021021807494306/kallsyms
1192
1193Find the VM process
1194
1195 $ ps -eLl | grep 'KVM\|PID'
1196 F S UID PID PPID LWP C PRI NI ADDR SZ WCHAN TTY TIME CMD
1197 3 S 64055 1430 1 1440 1 80 0 - 1921718 - ? 00:02:47 CPU 0/KVM
1198 3 S 64055 1430 1 1441 1 80 0 - 1921718 - ? 00:02:41 CPU 1/KVM
1199 3 S 64055 1430 1 1442 1 80 0 - 1921718 - ? 00:02:38 CPU 2/KVM
1200 3 S 64055 1430 1 1443 2 80 0 - 1921718 - ? 00:03:18 CPU 3/KVM
1201
1202Start an open-ended perf record, tracing the VM process, do something on the VM, and then ctrl-C to stop.
1203TSC is not supported and tsc=0 must be specified. That means mtc is useless, so add mtc=0.
1204However, IPC can still be determined, hence cyc=1 can be added.
1205Only kernel decoding is supported, so 'k' must be specified.
1206Intel PT traces both the host and the guest so --guest and --host need to be specified.
1207Without timestamps, --per-thread must be specified to distinguish threads.
1208
1209 $ sudo perf kvm --guest --host --guestkallsyms $KALLSYMS record --kcore -e intel_pt/tsc=0,mtc=0,cyc=1/k -p 1430 --per-thread
1210 ^C
1211 [ perf record: Woken up 1 times to write data ]
1212 [ perf record: Captured and wrote 5.829 MB ]
1213
1214perf script can be used to provide an instruction trace
1215
1216 $ perf script --guestkallsyms $KALLSYMS --insn-trace --xed -F+ipc | grep -C10 vmresume | head -21
1217 CPU 0/KVM 1440 ffffffff82133cdd __vmx_vcpu_run+0x3d ([kernel.kallsyms]) movq 0x48(%rax), %r9
1218 CPU 0/KVM 1440 ffffffff82133ce1 __vmx_vcpu_run+0x41 ([kernel.kallsyms]) movq 0x50(%rax), %r10
1219 CPU 0/KVM 1440 ffffffff82133ce5 __vmx_vcpu_run+0x45 ([kernel.kallsyms]) movq 0x58(%rax), %r11
1220 CPU 0/KVM 1440 ffffffff82133ce9 __vmx_vcpu_run+0x49 ([kernel.kallsyms]) movq 0x60(%rax), %r12
1221 CPU 0/KVM 1440 ffffffff82133ced __vmx_vcpu_run+0x4d ([kernel.kallsyms]) movq 0x68(%rax), %r13
1222 CPU 0/KVM 1440 ffffffff82133cf1 __vmx_vcpu_run+0x51 ([kernel.kallsyms]) movq 0x70(%rax), %r14
1223 CPU 0/KVM 1440 ffffffff82133cf5 __vmx_vcpu_run+0x55 ([kernel.kallsyms]) movq 0x78(%rax), %r15
1224 CPU 0/KVM 1440 ffffffff82133cf9 __vmx_vcpu_run+0x59 ([kernel.kallsyms]) movq (%rax), %rax
1225 CPU 0/KVM 1440 ffffffff82133cfc __vmx_vcpu_run+0x5c ([kernel.kallsyms]) callq 0xffffffff82133c40
1226 CPU 0/KVM 1440 ffffffff82133c40 vmx_vmenter+0x0 ([kernel.kallsyms]) jz 0xffffffff82133c46
1227 CPU 0/KVM 1440 ffffffff82133c42 vmx_vmenter+0x2 ([kernel.kallsyms]) vmresume IPC: 0.11 (50/445)
1228 :1440 1440 ffffffffbb678b06 native_write_msr+0x6 ([guest.kernel.kallsyms]) nopl %eax, (%rax,%rax,1)
1229 :1440 1440 ffffffffbb678b0b native_write_msr+0xb ([guest.kernel.kallsyms]) retq IPC: 0.04 (2/41)
1230 :1440 1440 ffffffffbb666646 lapic_next_deadline+0x26 ([guest.kernel.kallsyms]) data16 nop
1231 :1440 1440 ffffffffbb666648 lapic_next_deadline+0x28 ([guest.kernel.kallsyms]) xor %eax, %eax
1232 :1440 1440 ffffffffbb66664a lapic_next_deadline+0x2a ([guest.kernel.kallsyms]) popq %rbp
1233 :1440 1440 ffffffffbb66664b lapic_next_deadline+0x2b ([guest.kernel.kallsyms]) retq IPC: 0.16 (4/25)
1234 :1440 1440 ffffffffbb74607f clockevents_program_event+0x8f ([guest.kernel.kallsyms]) test %eax, %eax
1235 :1440 1440 ffffffffbb746081 clockevents_program_event+0x91 ([guest.kernel.kallsyms]) jz 0xffffffffbb74603c IPC: 0.06 (2/30)
1236 :1440 1440 ffffffffbb74603c clockevents_program_event+0x4c ([guest.kernel.kallsyms]) popq %rbx
1237 :1440 1440 ffffffffbb74603d clockevents_program_event+0x4d ([guest.kernel.kallsyms]) popq %r12
1238
1239Example using VM Time Correlation
1240
1241Start VM
1242
1243 $ sudo virsh start kubuntu20.04
1244 Domain kubuntu20.04 started
1245
1246Mount the guest file system. Note sshfs needs -o direct_io to enable reading of proc files. root access is needed to read /proc/kcore.
1247
1248 $ mkdir -p vm0
1249 $ sshfs -o direct_io root@vm0:/ vm0
1250
1251Copy the guest /proc/kallsyms, /proc/modules and /proc/kcore
1252
1253 $ perf buildid-cache -v --kcore vm0/proc/kcore
1254 same kcore found in /home/user/.debug/[kernel.kcore]/cc9c55a98c5e4ec0aeda69302554aabed5cd6491/2021021312450777
1255 $ KALLSYMS=/home/user/.debug/\[kernel.kcore\]/cc9c55a98c5e4ec0aeda69302554aabed5cd6491/2021021312450777/kallsyms
1256
1257Find the VM process
1258
1259 $ ps -eLl | grep 'KVM\|PID'
1260 F S UID PID PPID LWP C PRI NI ADDR SZ WCHAN TTY TIME CMD
1261 3 S 64055 16998 1 17005 13 80 0 - 1818189 - ? 00:00:16 CPU 0/KVM
1262 3 S 64055 16998 1 17006 4 80 0 - 1818189 - ? 00:00:05 CPU 1/KVM
1263 3 S 64055 16998 1 17007 3 80 0 - 1818189 - ? 00:00:04 CPU 2/KVM
1264 3 S 64055 16998 1 17008 4 80 0 - 1818189 - ? 00:00:05 CPU 3/KVM
1265
1266Start an open-ended perf record, tracing the VM process, do something on the VM, and then ctrl-C to stop.
1267IPC can be determined, hence cyc=1 can be added.
1268Only kernel decoding is supported, so 'k' must be specified.
1269Intel PT traces both the host and the guest so --guest and --host need to be specified.
1270
1271 $ sudo perf kvm --guest --host --guestkallsyms $KALLSYMS record --kcore -e intel_pt/cyc=1/k -p 16998
1272 ^C[ perf record: Woken up 1 times to write data ]
1273 [ perf record: Captured and wrote 9.041 MB perf.data.kvm ]
1274
1275Now 'perf inject' can be used to determine the VMX TCS Offset. Note, Intel PT TSC packets are
1276only 7-bytes, so the TSC Offset might differ from the actual value in the 8th byte. That will
1277have no effect i.e. the resulting timestamps will be correct anyway.
1278
1279 $ perf inject -i perf.data.kvm --vm-time-correlation=dry-run
1280 ERROR: Unknown TSC Offset for VMCS 0x1bff6a
1281 VMCS: 0x1bff6a TSC Offset 0xffffe42722c64c41
1282 ERROR: Unknown TSC Offset for VMCS 0x1cbc08
1283 VMCS: 0x1cbc08 TSC Offset 0xffffe42722c64c41
1284 ERROR: Unknown TSC Offset for VMCS 0x1c3ce8
1285 VMCS: 0x1c3ce8 TSC Offset 0xffffe42722c64c41
1286 ERROR: Unknown TSC Offset for VMCS 0x1cbce9
1287 VMCS: 0x1cbce9 TSC Offset 0xffffe42722c64c41
1288
1289Each virtual CPU has a different Virtual Machine Control Structure (VMCS)
1290shown above with the calculated TSC Offset. For an unchanging TSC Offset
1291they should all be the same for the same virtual machine.
1292
1293Now that the TSC Offset is known, it can be provided to 'perf inject'
1294
1295 $ perf inject -i perf.data.kvm --vm-time-correlation="dry-run 0xffffe42722c64c41"
1296
1297Note the options for 'perf inject' --vm-time-correlation are:
1298
1299 [ dry-run ] [ <TSC Offset> [ : <VMCS> [ , <VMCS> ]... ] ]...
1300
1301So it is possible to specify different TSC Offsets for different VMCS.
1302The option "dry-run" will cause the file to be processed but without updating it.
1303Note it is also possible to get a intel_pt.log file by adding option --itrace=d
1304
1305There were no errors so, do it for real
1306
1307 $ perf inject -i perf.data.kvm --vm-time-correlation=0xffffe42722c64c41 --force
1308
1309'perf script' can be used to see if there are any decoder errors
1310
1311 $ perf script -i perf.data.kvm --guestkallsyms $KALLSYMS --itrace=e-o
1312
1313There were none.
1314
1315'perf script' can be used to provide an instruction trace showing timestamps
1316
1317 $ perf script -i perf.data.kvm --guestkallsyms $KALLSYMS --insn-trace --xed -F+ipc | grep -C10 vmresume | head -21
1318 CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133cdd __vmx_vcpu_run+0x3d ([kernel.kallsyms]) movq 0x48(%rax), %r9
1319 CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133ce1 __vmx_vcpu_run+0x41 ([kernel.kallsyms]) movq 0x50(%rax), %r10
1320 CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133ce5 __vmx_vcpu_run+0x45 ([kernel.kallsyms]) movq 0x58(%rax), %r11
1321 CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133ce9 __vmx_vcpu_run+0x49 ([kernel.kallsyms]) movq 0x60(%rax), %r12
1322 CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133ced __vmx_vcpu_run+0x4d ([kernel.kallsyms]) movq 0x68(%rax), %r13
1323 CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133cf1 __vmx_vcpu_run+0x51 ([kernel.kallsyms]) movq 0x70(%rax), %r14
1324 CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133cf5 __vmx_vcpu_run+0x55 ([kernel.kallsyms]) movq 0x78(%rax), %r15
1325 CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133cf9 __vmx_vcpu_run+0x59 ([kernel.kallsyms]) movq (%rax), %rax
1326 CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133cfc __vmx_vcpu_run+0x5c ([kernel.kallsyms]) callq 0xffffffff82133c40
1327 CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133c40 vmx_vmenter+0x0 ([kernel.kallsyms]) jz 0xffffffff82133c46
1328 CPU 1/KVM 17006 [001] 11500.262866075: ffffffff82133c42 vmx_vmenter+0x2 ([kernel.kallsyms]) vmresume IPC: 0.05 (40/769)
1329 :17006 17006 [001] 11500.262869216: ffffffff82200cb0 asm_sysvec_apic_timer_interrupt+0x0 ([guest.kernel.kallsyms]) clac
1330 :17006 17006 [001] 11500.262869216: ffffffff82200cb3 asm_sysvec_apic_timer_interrupt+0x3 ([guest.kernel.kallsyms]) pushq $0xffffffffffffffff
1331 :17006 17006 [001] 11500.262869216: ffffffff82200cb5 asm_sysvec_apic_timer_interrupt+0x5 ([guest.kernel.kallsyms]) callq 0xffffffff82201160
1332 :17006 17006 [001] 11500.262869216: ffffffff82201160 error_entry+0x0 ([guest.kernel.kallsyms]) cld
1333 :17006 17006 [001] 11500.262869216: ffffffff82201161 error_entry+0x1 ([guest.kernel.kallsyms]) pushq %rsi
1334 :17006 17006 [001] 11500.262869216: ffffffff82201162 error_entry+0x2 ([guest.kernel.kallsyms]) movq 0x8(%rsp), %rsi
1335 :17006 17006 [001] 11500.262869216: ffffffff82201167 error_entry+0x7 ([guest.kernel.kallsyms]) movq %rdi, 0x8(%rsp)
1336 :17006 17006 [001] 11500.262869216: ffffffff8220116c error_entry+0xc ([guest.kernel.kallsyms]) pushq %rdx
1337 :17006 17006 [001] 11500.262869216: ffffffff8220116d error_entry+0xd ([guest.kernel.kallsyms]) pushq %rcx
1338 :17006 17006 [001] 11500.262869216: ffffffff8220116e error_entry+0xe ([guest.kernel.kallsyms]) pushq %rax
1339
1340
1341
1342SEE ALSO
1343--------
1344
1345linkperf:perf-record[1], linkperf:perf-script[1], linkperf:perf-report[1],
1346linkperf:perf-inject[1]