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perf-intel-pt(1)
================
NAME
----
perf-intel-pt - Support for Intel Processor Trace within perf tools
SYNOPSIS
--------
[verse]
'perf record' -e intel_pt//
DESCRIPTION
-----------
Intel Processor Trace (Intel PT) is an extension of Intel Architecture that
collects information about software execution such as control flow, execution
modes and timings and formats it into highly compressed binary packets.
Technical details are documented in the Intel 64 and IA-32 Architectures
Software Developer Manuals, Chapter 36 Intel Processor Trace.
Intel PT is first supported in Intel Core M and 5th generation Intel Core
processors that are based on the Intel micro-architecture code name Broadwell.
Trace data is collected by 'perf record' and stored within the perf.data file.
See below for options to 'perf record'.
Trace data must be 'decoded' which involves walking the object code and matching
the trace data packets. For example a TNT packet only tells whether a
conditional branch was taken or not taken, so to make use of that packet the
decoder must know precisely which instruction was being executed.
Decoding is done on-the-fly. The decoder outputs samples in the same format as
samples output by perf hardware events, for example as though the "instructions"
or "branches" events had been recorded. Presently 3 tools support this:
'perf script', 'perf report' and 'perf inject'. See below for more information
on using those tools.
The main distinguishing feature of Intel PT is that the decoder can determine
the exact flow of software execution. Intel PT can be used to understand why
and how did software get to a certain point, or behave a certain way. The
software does not have to be recompiled, so Intel PT works with debug or release
builds, however the executed images are needed - which makes use in JIT-compiled
environments, or with self-modified code, a challenge. Also symbols need to be
provided to make sense of addresses.
A limitation of Intel PT is that it produces huge amounts of trace data
(hundreds of megabytes per second per core) which takes a long time to decode,
for example two or three orders of magnitude longer than it took to collect.
Another limitation is the performance impact of tracing, something that will
vary depending on the use-case and architecture.
Quickstart
----------
It is important to start small. That is because it is easy to capture vastly
more data than can possibly be processed.
The simplest thing to do with Intel PT is userspace profiling of small programs.
Data is captured with 'perf record' e.g. to trace 'ls' userspace-only:
perf record -e intel_pt//u ls
And profiled with 'perf report' e.g.
perf report
To also trace kernel space presents a problem, namely kernel self-modifying
code. A fairly good kernel image is available in /proc/kcore but to get an
accurate image a copy of /proc/kcore needs to be made under the same conditions
as the data capture. 'perf record' can make a copy of /proc/kcore if the option
--kcore is used, but access to /proc/kcore is restricted e.g.
sudo perf record -o pt_ls --kcore -e intel_pt// -- ls
which will create a directory named 'pt_ls' and put the perf.data file (named
simply 'data') and copies of /proc/kcore, /proc/kallsyms and /proc/modules into
it. The other tools understand the directory format, so to use 'perf report'
becomes:
sudo perf report -i pt_ls
Because samples are synthesized after-the-fact, the sampling period can be
selected for reporting. e.g. sample every microsecond
sudo perf report pt_ls --itrace=i1usge
See the sections below for more information about the --itrace option.
Beware the smaller the period, the more samples that are produced, and the
longer it takes to process them.
Also note that the coarseness of Intel PT timing information will start to
distort the statistical value of the sampling as the sampling period becomes
smaller.
To represent software control flow, "branches" samples are produced. By default
a branch sample is synthesized for every single branch. To get an idea what
data is available you can use the 'perf script' tool with all itrace sampling
options, which will list all the samples.
perf record -e intel_pt//u ls
perf script --itrace=ibxwpe
An interesting field that is not printed by default is 'flags' which can be
displayed as follows:
perf script --itrace=ibxwpe -F+flags
The flags are "bcrosyiABExghDt" which stand for branch, call, return, conditional,
system, asynchronous, interrupt, transaction abort, trace begin, trace end,
in transaction, VM-entry, VM-exit, interrupt disabled, and interrupt disable
toggle respectively.
perf script also supports higher level ways to dump instruction traces:
perf script --insn-trace --xed
Dump all instructions. This requires installing the xed tool (see XED below)
Dumping all instructions in a long trace can be fairly slow. It is usually better
to start with higher level decoding, like
perf script --call-trace
or
perf script --call-ret-trace
and then select a time range of interest. The time range can then be examined
in detail with
perf script --time starttime,stoptime --insn-trace --xed
While examining the trace it's also useful to filter on specific CPUs using
the -C option
perf script --time starttime,stoptime --insn-trace --xed -C 1
Dump all instructions in time range on CPU 1.
Another interesting field that is not printed by default is 'ipc' which can be
displayed as follows:
perf script --itrace=be -F+ipc
There are two ways that instructions-per-cycle (IPC) can be calculated depending
on the recording.
If the 'cyc' config term (see config terms section below) was used, then IPC is
calculated using the cycle count from CYC packets, otherwise MTC packets are
used - refer to the 'mtc' config term. When MTC is used, however, the values
are less accurate because the timing is less accurate.
Because Intel PT does not update the cycle count on every branch or instruction,
the values will often be zero. When there are values, they will be the number
of instructions and number of cycles since the last update, and thus represent
the average IPC since the last IPC for that event type. Note IPC for "branches"
events is calculated separately from IPC for "instructions" events.
Even with the 'cyc' config term, it is possible to produce IPC information for
every change of timestamp, but at the expense of accuracy. That is selected by
specifying the itrace 'A' option. Due to the granularity of timestamps, the
actual number of cycles increases even though the cycles reported does not.
The number of instructions is known, but if IPC is reported, cycles can be too
low and so IPC is too high. Note that inaccuracy decreases as the period of
sampling increases i.e. if the number of cycles is too low by a small amount,
that becomes less significant if the number of cycles is large. It may also be
useful to use the 'A' option in conjunction with dlfilter-show-cycles.so to
provide higher granularity cycle information.
Also note that the IPC instruction count may or may not include the current
instruction. If the cycle count is associated with an asynchronous branch
(e.g. page fault or interrupt), then the instruction count does not include the
current instruction, otherwise it does. That is consistent with whether or not
that instruction has retired when the cycle count is updated.
Another note, in the case of "branches" events, non-taken branches are not
presently sampled, so IPC values for them do not appear e.g. a CYC packet with a
TNT packet that starts with a non-taken branch. To see every possible IPC
value, "instructions" events can be used e.g. --itrace=i0ns
While it is possible to create scripts to analyze the data, an alternative
approach is available to export the data to a sqlite or postgresql database.
Refer to script export-to-sqlite.py or export-to-postgresql.py for more details,
and to script exported-sql-viewer.py for an example of using the database.
There is also script intel-pt-events.py which provides an example of how to
unpack the raw data for power events and PTWRITE. The script also displays
branches, and supports 2 additional modes selected by option:
--insn-trace - instruction trace
--src-trace - source trace
As mentioned above, it is easy to capture too much data. One way to limit the
data captured is to use 'snapshot' mode which is explained further below.
Refer to 'new snapshot option' and 'Intel PT modes of operation' further below.
Another problem that will be experienced is decoder errors. They can be caused
by inability to access the executed image, self-modified or JIT-ed code, or the
inability to match side-band information (such as context switches and mmaps)
which results in the decoder not knowing what code was executed.
There is also the problem of perf not being able to copy the data fast enough,
resulting in data lost because the buffer was full. See 'Buffer handling' below
for more details.
perf record
-----------
new event
~~~~~~~~~
The Intel PT kernel driver creates a new PMU for Intel PT. PMU events are
selected by providing the PMU name followed by the "config" separated by slashes.
An enhancement has been made to allow default "config" e.g. the option
-e intel_pt//
will use a default config value. Currently that is the same as
-e intel_pt/tsc,noretcomp=0/
which is the same as
-e intel_pt/tsc=1,noretcomp=0/
Note there are now new config terms - see section 'config terms' further below.
The config terms are listed in /sys/devices/intel_pt/format. They are bit
fields within the config member of the struct perf_event_attr which is
passed to the kernel by the perf_event_open system call. They correspond to bit
fields in the IA32_RTIT_CTL MSR. Here is a list of them and their definitions:
$ grep -H . /sys/bus/event_source/devices/intel_pt/format/*
/sys/bus/event_source/devices/intel_pt/format/cyc:config:1
/sys/bus/event_source/devices/intel_pt/format/cyc_thresh:config:19-22
/sys/bus/event_source/devices/intel_pt/format/mtc:config:9
/sys/bus/event_source/devices/intel_pt/format/mtc_period:config:14-17
/sys/bus/event_source/devices/intel_pt/format/noretcomp:config:11
/sys/bus/event_source/devices/intel_pt/format/psb_period:config:24-27
/sys/bus/event_source/devices/intel_pt/format/tsc:config:10
Note that the default config must be overridden for each term i.e.
-e intel_pt/noretcomp=0/
is the same as:
-e intel_pt/tsc=1,noretcomp=0/
So, to disable TSC packets use:
-e intel_pt/tsc=0/
It is also possible to specify the config value explicitly:
-e intel_pt/config=0x400/
Note that, as with all events, the event is suffixed with event modifiers:
u userspace
k kernel
h hypervisor
G guest
H host
p precise ip
'h', 'G' and 'H' are for virtualization which are not used by Intel PT.
'p' is also not relevant to Intel PT. So only options 'u' and 'k' are
meaningful for Intel PT.
perf_event_attr is displayed if the -vv option is used e.g.
------------------------------------------------------------
perf_event_attr:
type 6
size 112
config 0x400
{ sample_period, sample_freq } 1
sample_type IP|TID|TIME|CPU|IDENTIFIER
read_format ID
disabled 1
inherit 1
exclude_kernel 1
exclude_hv 1
enable_on_exec 1
sample_id_all 1
------------------------------------------------------------
sys_perf_event_open: pid 31104 cpu 0 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 1 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 2 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 3 group_fd -1 flags 0x8
------------------------------------------------------------
config terms
~~~~~~~~~~~~
The June 2015 version of Intel 64 and IA-32 Architectures Software Developer
Manuals, Chapter 36 Intel Processor Trace, defined new Intel PT features.
Some of the features are reflect in new config terms. All the config terms are
described below.
tsc Always supported. Produces TSC timestamp packets to provide
timing information. In some cases it is possible to decode
without timing information, for example a per-thread context
that does not overlap executable memory maps.
The default config selects tsc (i.e. tsc=1).
noretcomp Always supported. Disables "return compression" so a TIP packet
is produced when a function returns. Causes more packets to be
produced but might make decoding more reliable.
The default config does not select noretcomp (i.e. noretcomp=0).
psb_period Allows the frequency of PSB packets to be specified.
The PSB packet is a synchronization packet that provides a
starting point for decoding or recovery from errors.
Support for psb_period is indicated by:
/sys/bus/event_source/devices/intel_pt/caps/psb_cyc
which contains "1" if the feature is supported and "0"
otherwise.
Valid values are given by:
/sys/bus/event_source/devices/intel_pt/caps/psb_periods
which contains a hexadecimal value, the bits of which represent
valid values e.g. bit 2 set means value 2 is valid.
The psb_period value is converted to the approximate number of
trace bytes between PSB packets as:
2 ^ (value + 11)
e.g. value 3 means 16KiB bytes between PSBs
If an invalid value is entered, the error message
will give a list of valid values e.g.
$ perf record -e intel_pt/psb_period=15/u uname
Invalid psb_period for intel_pt. Valid values are: 0-5
If MTC packets are selected, the default config selects a value
of 3 (i.e. psb_period=3) or the nearest lower value that is
supported (0 is always supported). Otherwise the default is 0.
If decoding is expected to be reliable and the buffer is large
then a large PSB period can be used.
Because a TSC packet is produced with PSB, the PSB period can
also affect the granularity to timing information in the absence
of MTC or CYC.
mtc Produces MTC timing packets.
MTC packets provide finer grain timestamp information than TSC
packets. MTC packets record time using the hardware crystal
clock (CTC) which is related to TSC packets using a TMA packet.
Support for this feature is indicated by:
/sys/bus/event_source/devices/intel_pt/caps/mtc
which contains "1" if the feature is supported and
"0" otherwise.
The frequency of MTC packets can also be specified - see
mtc_period below.
mtc_period Specifies how frequently MTC packets are produced - see mtc
above for how to determine if MTC packets are supported.
Valid values are given by:
/sys/bus/event_source/devices/intel_pt/caps/mtc_periods
which contains a hexadecimal value, the bits of which represent
valid values e.g. bit 2 set means value 2 is valid.
The mtc_period value is converted to the MTC frequency as:
CTC-frequency / (2 ^ value)
e.g. value 3 means one eighth of CTC-frequency
Where CTC is the hardware crystal clock, the frequency of which
can be related to TSC via values provided in cpuid leaf 0x15.
If an invalid value is entered, the error message
will give a list of valid values e.g.
$ perf record -e intel_pt/mtc_period=15/u uname
Invalid mtc_period for intel_pt. Valid values are: 0,3,6,9
The default value is 3 or the nearest lower value
that is supported (0 is always supported).
cyc Produces CYC timing packets.
CYC packets provide even finer grain timestamp information than
MTC and TSC packets. A CYC packet contains the number of CPU
cycles since the last CYC packet. Unlike MTC and TSC packets,
CYC packets are only sent when another packet is also sent.
Support for this feature is indicated by:
/sys/bus/event_source/devices/intel_pt/caps/psb_cyc
which contains "1" if the feature is supported and
"0" otherwise.
The number of CYC packets produced can be reduced by specifying
a threshold - see cyc_thresh below.
cyc_thresh Specifies how frequently CYC packets are produced - see cyc
above for how to determine if CYC packets are supported.
Valid cyc_thresh values are given by:
/sys/bus/event_source/devices/intel_pt/caps/cycle_thresholds
which contains a hexadecimal value, the bits of which represent
valid values e.g. bit 2 set means value 2 is valid.
The cyc_thresh value represents the minimum number of CPU cycles
that must have passed before a CYC packet can be sent. The
number of CPU cycles is:
2 ^ (value - 1)
e.g. value 4 means 8 CPU cycles must pass before a CYC packet
can be sent. Note a CYC packet is still only sent when another
packet is sent, not at, e.g. every 8 CPU cycles.
If an invalid value is entered, the error message
will give a list of valid values e.g.
$ perf record -e intel_pt/cyc,cyc_thresh=15/u uname
Invalid cyc_thresh for intel_pt. Valid values are: 0-12
CYC packets are not requested by default.
pt Specifies pass-through which enables the 'branch' config term.
The default config selects 'pt' if it is available, so a user will
never need to specify this term.
branch Enable branch tracing. Branch tracing is enabled by default so to
disable branch tracing use 'branch=0'.
The default config selects 'branch' if it is available.
ptw Enable PTWRITE packets which are produced when a ptwrite instruction
is executed.
Support for this feature is indicated by:
/sys/bus/event_source/devices/intel_pt/caps/ptwrite
which contains "1" if the feature is supported and
"0" otherwise.
As an alternative, refer to "Emulated PTWRITE" further below.
fup_on_ptw Enable a FUP packet to follow the PTWRITE packet. The FUP packet
provides the address of the ptwrite instruction. In the absence of
fup_on_ptw, the decoder will use the address of the previous branch
if branch tracing is enabled, otherwise the address will be zero.
Note that fup_on_ptw will work even when branch tracing is disabled.
pwr_evt Enable power events. The power events provide information about
changes to the CPU C-state.
Support for this feature is indicated by:
/sys/bus/event_source/devices/intel_pt/caps/power_event_trace
which contains "1" if the feature is supported and
"0" otherwise.
event Enable Event Trace. The events provide information about asynchronous
events.
Support for this feature is indicated by:
/sys/bus/event_source/devices/intel_pt/caps/event_trace
which contains "1" if the feature is supported and
"0" otherwise.
notnt Disable TNT packets. Without TNT packets, it is not possible to walk
executable code to reconstruct control flow, however FUP, TIP, TIP.PGE
and TIP.PGD packets still indicate asynchronous control flow, and (if
return compression is disabled - see noretcomp) return statements.
The advantage of eliminating TNT packets is reducing the size of the
trace and corresponding tracing overhead.
Support for this feature is indicated by:
/sys/bus/event_source/devices/intel_pt/caps/tnt_disable
which contains "1" if the feature is supported and
"0" otherwise.
AUX area sampling option
~~~~~~~~~~~~~~~~~~~~~~~~
To select Intel PT "sampling" the AUX area sampling option can be used:
--aux-sample
Optionally it can be followed by the sample size in bytes e.g.
--aux-sample=8192
In addition, the Intel PT event to sample must be defined e.g.
-e intel_pt//u
Samples on other events will be created containing Intel PT data e.g. the
following will create Intel PT samples on the branch-misses event, note the
events must be grouped using {}:
perf record --aux-sample -e '{intel_pt//u,branch-misses:u}'
An alternative to '--aux-sample' is to add the config term 'aux-sample-size' to
events. In this case, the grouping is implied e.g.
perf record -e intel_pt//u -e branch-misses/aux-sample-size=8192/u
is the same as:
perf record -e '{intel_pt//u,branch-misses/aux-sample-size=8192/u}'
but allows for also using an address filter e.g.:
perf record -e intel_pt//u --filter 'filter * @/bin/ls' -e branch-misses/aux-sample-size=8192/u -- ls
It is important to select a sample size that is big enough to contain at least
one PSB packet. If not a warning will be displayed:
Intel PT sample size (%zu) may be too small for PSB period (%zu)
The calculation used for that is: if sample_size <= psb_period + 256 display the
warning. When sampling is used, psb_period defaults to 0 (2KiB).
The default sample size is 4KiB.
The sample size is passed in aux_sample_size in struct perf_event_attr. The
sample size is limited by the maximum event size which is 64KiB. It is
difficult to know how big the event might be without the trace sample attached,
but the tool validates that the sample size is not greater than 60KiB.
new snapshot option
~~~~~~~~~~~~~~~~~~~
The difference between full trace and snapshot from the kernel's perspective is
that in full trace we don't overwrite trace data that the user hasn't collected
yet (and indicated that by advancing aux_tail), whereas in snapshot mode we let
the trace run and overwrite older data in the buffer so that whenever something
interesting happens, we can stop it and grab a snapshot of what was going on
around that interesting moment.
To select snapshot mode a new option has been added:
-S
Optionally it can be followed by the snapshot size e.g.
-S0x100000
The default snapshot size is the auxtrace mmap size. If neither auxtrace mmap size
nor snapshot size is specified, then the default is 4MiB for privileged users
(or if /proc/sys/kernel/perf_event_paranoid < 0), 128KiB for unprivileged users.
If an unprivileged user does not specify mmap pages, the mmap pages will be
reduced as described in the 'new auxtrace mmap size option' section below.
The snapshot size is displayed if the option -vv is used e.g.
Intel PT snapshot size: %zu
new auxtrace mmap size option
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Intel PT buffer size is specified by an addition to the -m option e.g.
-m,16
selects a buffer size of 16 pages i.e. 64KiB.
Note that the existing functionality of -m is unchanged. The auxtrace mmap size
is specified by the optional addition of a comma and the value.
The default auxtrace mmap size for Intel PT is 4MiB/page_size for privileged users
(or if /proc/sys/kernel/perf_event_paranoid < 0), 128KiB for unprivileged users.
If an unprivileged user does not specify mmap pages, the mmap pages will be
reduced from the default 512KiB/page_size to 256KiB/page_size, otherwise the
user is likely to get an error as they exceed their mlock limit (Max locked
memory as shown in /proc/self/limits). Note that perf does not count the first
512KiB (actually /proc/sys/kernel/perf_event_mlock_kb minus 1 page) per cpu
against the mlock limit so an unprivileged user is allowed 512KiB per cpu plus
their mlock limit (which defaults to 64KiB but is not multiplied by the number
of cpus).
In full-trace mode, powers of two are allowed for buffer size, with a minimum
size of 2 pages. In snapshot mode or sampling mode, it is the same but the
minimum size is 1 page.
The mmap size and auxtrace mmap size are displayed if the -vv option is used e.g.
mmap length 528384
auxtrace mmap length 4198400
Intel PT modes of operation
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Intel PT can be used in 3 modes:
full-trace mode
sample mode
snapshot mode
Full-trace mode traces continuously e.g.
perf record -e intel_pt//u uname
Sample mode attaches a Intel PT sample to other events e.g.
perf record --aux-sample -e intel_pt//u -e branch-misses:u
Snapshot mode captures the available data when a signal is sent or "snapshot"
control command is issued. e.g. using a signal
perf record -v -e intel_pt//u -S ./loopy 1000000000 &
[1] 11435
kill -USR2 11435
Recording AUX area tracing snapshot
Note that the signal sent is SIGUSR2.
Note that "Recording AUX area tracing snapshot" is displayed because the -v
option is used.
The advantage of using "snapshot" control command is that the access is
controlled by access to a FIFO e.g.
$ mkfifo perf.control
$ mkfifo perf.ack
$ cat perf.ack &
[1] 15235
$ sudo ~/bin/perf record --control fifo:perf.control,perf.ack -S -e intel_pt//u -- sleep 60 &
[2] 15243
$ ps -e | grep perf
15244 pts/1 00:00:00 perf
$ kill -USR2 15244
bash: kill: (15244) - Operation not permitted
$ echo snapshot > perf.control
ack
The 3 Intel PT modes of operation cannot be used together.
Buffer handling
~~~~~~~~~~~~~~~
There may be buffer limitations (i.e. single ToPa entry) which means that actual
buffer sizes are limited to powers of 2 up to 4MiB (MAX_ORDER). In order to
provide other sizes, and in particular an arbitrarily large size, multiple
buffers are logically concatenated. However an interrupt must be used to switch
between buffers. That has two potential problems:
a) the interrupt may not be handled in time so that the current buffer
becomes full and some trace data is lost.
b) the interrupts may slow the system and affect the performance
results.
If trace data is lost, the driver sets 'truncated' in the PERF_RECORD_AUX event
which the tools report as an error.
In full-trace mode, the driver waits for data to be copied out before allowing
the (logical) buffer to wrap-around. If data is not copied out quickly enough,
again 'truncated' is set in the PERF_RECORD_AUX event. If the driver has to
wait, the intel_pt event gets disabled. Because it is difficult to know when
that happens, perf tools always re-enable the intel_pt event after copying out
data.
Intel PT and build ids
~~~~~~~~~~~~~~~~~~~~~~
By default "perf record" post-processes the event stream to find all build ids
for executables for all addresses sampled. Deliberately, Intel PT is not
decoded for that purpose (it would take too long). Instead the build ids for
all executables encountered (due to mmap, comm or task events) are included
in the perf.data file.
To see buildids included in the perf.data file use the command:
perf buildid-list
If the perf.data file contains Intel PT data, that is the same as:
perf buildid-list --with-hits
Snapshot mode and event disabling
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In order to make a snapshot, the intel_pt event is disabled using an IOCTL,
namely PERF_EVENT_IOC_DISABLE. However doing that can also disable the
collection of side-band information. In order to prevent that, a dummy
software event has been introduced that permits tracking events (like mmaps) to
continue to be recorded while intel_pt is disabled. That is important to ensure
there is complete side-band information to allow the decoding of subsequent
snapshots.
A test has been created for that. To find the test:
perf test list
...
23: Test using a dummy software event to keep tracking
To run the test:
perf test 23
23: Test using a dummy software event to keep tracking : Ok
perf record modes (nothing new here)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
perf record essentially operates in one of three modes:
per thread
per cpu
workload only
"per thread" mode is selected by -t or by --per-thread (with -p or -u or just a
workload).
"per cpu" is selected by -C or -a.
"workload only" mode is selected by not using the other options but providing a
command to run (i.e. the workload).
In per-thread mode an exact list of threads is traced. There is no inheritance.
Each thread has its own event buffer.
In per-cpu mode all processes (or processes from the selected cgroup i.e. -G
option, or processes selected with -p or -u) are traced. Each cpu has its own
buffer. Inheritance is allowed.
In workload-only mode, the workload is traced but with per-cpu buffers.
Inheritance is allowed. Note that you can now trace a workload in per-thread
mode by using the --per-thread option.
Privileged vs non-privileged users
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Unless /proc/sys/kernel/perf_event_paranoid is set to -1, unprivileged users
have memory limits imposed upon them. That affects what buffer sizes they can
have as outlined above.
The v4.2 kernel introduced support for a context switch metadata event,
PERF_RECORD_SWITCH, which allows unprivileged users to see when their processes
are scheduled out and in, just not by whom, which is left for the
PERF_RECORD_SWITCH_CPU_WIDE, that is only accessible in system wide context,
which in turn requires CAP_PERFMON or CAP_SYS_ADMIN.
Please see the 45ac1403f564 ("perf: Add PERF_RECORD_SWITCH to indicate context
switches") commit, that introduces these metadata events for further info.
When working with kernels < v4.2, the following considerations must be taken,
as the sched:sched_switch tracepoints will be used to receive such information:
Unless /proc/sys/kernel/perf_event_paranoid is set to -1, unprivileged users are
not permitted to use tracepoints which means there is insufficient side-band
information to decode Intel PT in per-cpu mode, and potentially workload-only
mode too if the workload creates new processes.
Note also, that to use tracepoints, read-access to debugfs is required. So if
debugfs is not mounted or the user does not have read-access, it will again not
be possible to decode Intel PT in per-cpu mode.
sched_switch tracepoint
~~~~~~~~~~~~~~~~~~~~~~~
The sched_switch tracepoint is used to provide side-band data for Intel PT
decoding in kernels where the PERF_RECORD_SWITCH metadata event isn't
available.
The sched_switch events are automatically added. e.g. the second event shown
below:
$ perf record -vv -e intel_pt//u uname
------------------------------------------------------------
perf_event_attr:
type 6
size 112
config 0x400
{ sample_period, sample_freq } 1
sample_type IP|TID|TIME|CPU|IDENTIFIER
read_format ID
disabled 1
inherit 1
exclude_kernel 1
exclude_hv 1
enable_on_exec 1
sample_id_all 1
------------------------------------------------------------
sys_perf_event_open: pid 31104 cpu 0 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 1 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 2 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 3 group_fd -1 flags 0x8
------------------------------------------------------------
perf_event_attr:
type 2
size 112
config 0x108
{ sample_period, sample_freq } 1
sample_type IP|TID|TIME|CPU|PERIOD|RAW|IDENTIFIER
read_format ID
inherit 1
sample_id_all 1
exclude_guest 1
------------------------------------------------------------
sys_perf_event_open: pid -1 cpu 0 group_fd -1 flags 0x8
sys_perf_event_open: pid -1 cpu 1 group_fd -1 flags 0x8
sys_perf_event_open: pid -1 cpu 2 group_fd -1 flags 0x8
sys_perf_event_open: pid -1 cpu 3 group_fd -1 flags 0x8
------------------------------------------------------------
perf_event_attr:
type 1
size 112
config 0x9
{ sample_period, sample_freq } 1
sample_type IP|TID|TIME|IDENTIFIER
read_format ID
disabled 1
inherit 1
exclude_kernel 1
exclude_hv 1
mmap 1
comm 1
enable_on_exec 1
task 1
sample_id_all 1
mmap2 1
comm_exec 1
------------------------------------------------------------
sys_perf_event_open: pid 31104 cpu 0 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 1 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 2 group_fd -1 flags 0x8
sys_perf_event_open: pid 31104 cpu 3 group_fd -1 flags 0x8
mmap size 528384B
AUX area mmap length 4194304
perf event ring buffer mmapped per cpu
Synthesizing auxtrace information
Linux
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.042 MB perf.data ]
Note, the sched_switch event is only added if the user is permitted to use it
and only in per-cpu mode.
Note also, the sched_switch event is only added if TSC packets are requested.
That is because, in the absence of timing information, the sched_switch events
cannot be matched against the Intel PT trace.
perf script
-----------
By default, perf script will decode trace data found in the perf.data file.
This can be further controlled by new option --itrace.
New --itrace option
~~~~~~~~~~~~~~~~~~~
Having no option is the same as
--itrace
which, in turn, is the same as
--itrace=cepwx
The letters are:
i synthesize "instructions" events
b synthesize "branches" events
x synthesize "transactions" events
w synthesize "ptwrite" events
p synthesize "power" events (incl. PSB events)
c synthesize branches events (calls only)
r synthesize branches events (returns only)
o synthesize PEBS-via-PT events
I synthesize Event Trace events
e synthesize tracing error events
d create a debug log
g synthesize a call chain (use with i or x)
G synthesize a call chain on existing event records
l synthesize last branch entries (use with i or x)
L synthesize last branch entries on existing event records
s skip initial number of events
q quicker (less detailed) decoding
A approximate IPC
Z prefer to ignore timestamps (so-called "timeless" decoding)
"Instructions" events look like they were recorded by "perf record -e
instructions".
"Branches" events look like they were recorded by "perf record -e branches". "c"
and "r" can be combined to get calls and returns.
"Transactions" events correspond to the start or end of transactions. The
'flags' field can be used in perf script to determine whether the event is a
transaction start, commit or abort.
Note that "instructions", "branches" and "transactions" events depend on code
flow packets which can be disabled by using the config term "branch=0". Refer
to the config terms section above.
"ptwrite" events record the payload of the ptwrite instruction and whether
"fup_on_ptw" was used. "ptwrite" events depend on PTWRITE packets which are
recorded only if the "ptw" config term was used. Refer to the config terms
section above. perf script "synth" field displays "ptwrite" information like
this: "ip: 0 payload: 0x123456789abcdef0" where "ip" is 1 if "fup_on_ptw" was
used.
"Power" events correspond to power event packets and CBR (core-to-bus ratio)
packets. While CBR packets are always recorded when tracing is enabled, power
event packets are recorded only if the "pwr_evt" config term was used. Refer to
the config terms section above. The power events record information about
C-state changes, whereas CBR is indicative of CPU frequency. perf script
"event,synth" fields display information like this:
cbr: cbr: 22 freq: 2189 MHz (200%)
mwait: hints: 0x60 extensions: 0x1
pwre: hw: 0 cstate: 2 sub-cstate: 0
exstop: ip: 1
pwrx: deepest cstate: 2 last cstate: 2 wake reason: 0x4
Where:
"cbr" includes the frequency and the percentage of maximum non-turbo
"mwait" shows mwait hints and extensions
"pwre" shows C-state transitions (to a C-state deeper than C0) and
whether initiated by hardware
"exstop" indicates execution stopped and whether the IP was recorded
exactly,
"pwrx" indicates return to C0
For more details refer to the Intel 64 and IA-32 Architectures Software
Developer Manuals.
PSB events show when a PSB+ occurred and also the byte-offset in the trace.
Emitting a PSB+ can cause a CPU a slight delay. When doing timing analysis
of code with Intel PT, it is useful to know if a timing bubble was caused
by Intel PT or not.
Error events show where the decoder lost the trace. Error events
are quite important. Users must know if what they are seeing is a complete
picture or not. The "e" option may be followed by flags which affect what errors
will or will not be reported. Each flag must be preceded by either '+' or '-'.
The flags supported by Intel PT are:
-o Suppress overflow errors
-l Suppress trace data lost errors
For example, for errors but not overflow or data lost errors:
--itrace=e-o-l
The "d" option will cause the creation of a file "intel_pt.log" containing all
decoded packets and instructions. Note that this option slows down the decoder
and that the resulting file may be very large. The "d" option may be followed
by flags which affect what debug messages will or will not be logged. Each flag
must be preceded by either '+' or '-'. The flags support by Intel PT are:
-a Suppress logging of perf events
+a Log all perf events
+e Output only on decoding errors (size configurable)
+o Output to stdout instead of "intel_pt.log"
By default, logged perf events are filtered by any specified time ranges, but
flag +a overrides that. The +e flag can be useful for analyzing errors. By
default, the log size in that case is 16384 bytes, but can be altered by
linkperf:perf-config[1] e.g. perf config itrace.debug-log-buffer-size=30000
In addition, the period of the "instructions" event can be specified. e.g.
--itrace=i10us
sets the period to 10us i.e. one instruction sample is synthesized for each 10
microseconds of trace. Alternatives to "us" are "ms" (milliseconds),
"ns" (nanoseconds), "t" (TSC ticks) or "i" (instructions).
"ms", "us" and "ns" are converted to TSC ticks.
The timing information included with Intel PT does not give the time of every
instruction. Consequently, for the purpose of sampling, the decoder estimates
the time since the last timing packet based on 1 tick per instruction. The time
on the sample is *not* adjusted and reflects the last known value of TSC.
For Intel PT, the default period is 100us.
Setting it to a zero period means "as often as possible".
In the case of Intel PT that is the same as a period of 1 and a unit of
'instructions' (i.e. --itrace=i1i).
Also the call chain size (default 16, max. 1024) for instructions or
transactions events can be specified. e.g.
--itrace=ig32
--itrace=xg32
Also the number of last branch entries (default 64, max. 1024) for instructions or
transactions events can be specified. e.g.
--itrace=il10
--itrace=xl10
Note that last branch entries are cleared for each sample, so there is no overlap
from one sample to the next.
The G and L options are designed in particular for sample mode, and work much
like g and l but add call chain and branch stack to the other selected events
instead of synthesized events. For example, to record branch-misses events for
'ls' and then add a call chain derived from the Intel PT trace:
perf record --aux-sample -e '{intel_pt//u,branch-misses:u}' -- ls
perf report --itrace=Ge
Although in fact G is a default for perf report, so that is the same as just:
perf report
One caveat with the G and L options is that they work poorly with "Large PEBS".
Large PEBS means PEBS records will be accumulated by hardware and the written
into the event buffer in one go. That reduces interrupts, but can give very
late timestamps. Because the Intel PT trace is synchronized by timestamps,
the PEBS events do not match the trace. Currently, Large PEBS is used only in
certain circumstances:
- hardware supports it
- PEBS is used
- event period is specified, instead of frequency
- the sample type is limited to the following flags:
PERF_SAMPLE_IP | PERF_SAMPLE_TID | PERF_SAMPLE_ADDR |
PERF_SAMPLE_ID | PERF_SAMPLE_CPU | PERF_SAMPLE_STREAM_ID |
PERF_SAMPLE_DATA_SRC | PERF_SAMPLE_IDENTIFIER |
PERF_SAMPLE_TRANSACTION | PERF_SAMPLE_PHYS_ADDR |
PERF_SAMPLE_REGS_INTR | PERF_SAMPLE_REGS_USER |
PERF_SAMPLE_PERIOD (and sometimes) | PERF_SAMPLE_TIME
Because Intel PT sample mode uses a different sample type to the list above,
Large PEBS is not used with Intel PT sample mode. To avoid Large PEBS in other
cases, avoid specifying the event period i.e. avoid the 'perf record' -c option,
--count option, or 'period' config term.
To disable trace decoding entirely, use the option --no-itrace.
It is also possible to skip events generated (instructions, branches, transactions)
at the beginning. This is useful to ignore initialization code.
--itrace=i0nss1000000
skips the first million instructions.
The q option changes the way the trace is decoded. The decoding is much faster
but much less detailed. Specifically, with the q option, the decoder does not
decode TNT packets, and does not walk object code, but gets the ip from FUP and
TIP packets. The q option can be used with the b and i options but the period
is not used. The q option decodes more quickly, but is useful only if the
control flow of interest is represented or indicated by FUP, TIP, TIP.PGE, or
TIP.PGD packets (refer below). However the q option could be used to find time
ranges that could then be decoded fully using the --time option.
What will *not* be decoded with the (single) q option:
- direct calls and jmps
- conditional branches
- non-branch instructions
What *will* be decoded with the (single) q option:
- asynchronous branches such as interrupts
- indirect branches
- function return target address *if* the noretcomp config term (refer
config terms section) was used
- start of (control-flow) tracing
- end of (control-flow) tracing, if it is not out of context
- power events, ptwrite, transaction start and abort
- instruction pointer associated with PSB packets
Note the q option does not specify what events will be synthesized e.g. the p
option must be used also to show power events.
Repeating the q option (double-q i.e. qq) results in even faster decoding and even
less detail. The decoder decodes only extended PSB (PSB+) packets, getting the
instruction pointer if there is a FUP packet within PSB+ (i.e. between PSB and
PSBEND). Note PSB packets occur regularly in the trace based on the psb_period
config term (refer config terms section). There will be a FUP packet if the
PSB+ occurs while control flow is being traced.
What will *not* be decoded with the qq option:
- everything except instruction pointer associated with PSB packets
What *will* be decoded with the qq option:
- instruction pointer associated with PSB packets
The Z option is equivalent to having recorded a trace without TSC
(i.e. config term tsc=0). It can be useful to avoid timestamp issues when
decoding a trace of a virtual machine.
dlfilter-show-cycles.so
~~~~~~~~~~~~~~~~~~~~~~~
Cycles can be displayed using dlfilter-show-cycles.so in which case the itrace A
option can be useful to provide higher granularity cycle information:
perf script --itrace=A --call-trace --dlfilter dlfilter-show-cycles.so
To see a list of dlfilters:
perf script -v --list-dlfilters
See also linkperf:perf-dlfilters[1]
dump option
~~~~~~~~~~~
perf script has an option (-D) to "dump" the events i.e. display the binary
data.
When -D is used, Intel PT packets are displayed. The packet decoder does not
pay attention to PSB packets, but just decodes the bytes - so the packets seen
by the actual decoder may not be identical in places where the data is corrupt.
One example of that would be when the buffer-switching interrupt has been too
slow, and the buffer has been filled completely. In that case, the last packet
in the buffer might be truncated and immediately followed by a PSB as the trace
continues in the next buffer.
To disable the display of Intel PT packets, combine the -D option with
--no-itrace.
perf report
-----------
By default, perf report will decode trace data found in the perf.data file.
This can be further controlled by new option --itrace exactly the same as
perf script, with the exception that the default is --itrace=igxe.
perf inject
-----------
perf inject also accepts the --itrace option in which case tracing data is
removed and replaced with the synthesized events. e.g.
perf inject --itrace -i perf.data -o perf.data.new
Below is an example of using Intel PT with autofdo. It requires autofdo
(https://github.com/google/autofdo) and gcc version 5. The bubble
sort example is from the AutoFDO tutorial (https://gcc.gnu.org/wiki/AutoFDO/Tutorial)
amended to take the number of elements as a parameter.
$ gcc-5 -O3 sort.c -o sort_optimized
$ ./sort_optimized 30000
Bubble sorting array of 30000 elements
2254 ms
$ cat ~/.perfconfig
[intel-pt]
mispred-all = on
$ perf record -e intel_pt//u ./sort 3000
Bubble sorting array of 3000 elements
58 ms
[ perf record: Woken up 2 times to write data ]
[ perf record: Captured and wrote 3.939 MB perf.data ]
$ perf inject -i perf.data -o inj --itrace=i100usle --strip
$ ./create_gcov --binary=./sort --profile=inj --gcov=sort.gcov -gcov_version=1
$ gcc-5 -O3 -fauto-profile=sort.gcov sort.c -o sort_autofdo
$ ./sort_autofdo 30000
Bubble sorting array of 30000 elements
2155 ms
Note there is currently no advantage to using Intel PT instead of LBR, but
that may change in the future if greater use is made of the data.
PEBS via Intel PT
-----------------
Some hardware has the feature to redirect PEBS records to the Intel PT trace.
Recording is selected by using the aux-output config term e.g.
perf record -c 10000 -e '{intel_pt/branch=0/,cycles/aux-output/ppp}' uname
Originally, software only supported redirecting at most one PEBS event because it
was not able to differentiate one event from another. To overcome that, more recent
kernels and perf tools add support for the PERF_RECORD_AUX_OUTPUT_HW_ID side-band event.
To check for the presence of that event in a PEBS-via-PT trace:
perf script -D --no-itrace | grep PERF_RECORD_AUX_OUTPUT_HW_ID
To display PEBS events from the Intel PT trace, use the itrace 'o' option e.g.
perf script --itrace=oe
XED
---
include::build-xed.txt[]
Tracing Virtual Machines (kernel only)
--------------------------------------
Currently, kernel tracing is supported with either "timeless" decoding
(i.e. no TSC timestamps) or VM Time Correlation. VM Time Correlation is an extra step
using 'perf inject' and requires unchanging VMX TSC Offset and no VMX TSC Scaling.
Other limitations and caveats
VMX controls may suppress packets needed for decoding resulting in decoding errors
VMX controls may block the perf NMI to the host potentially resulting in lost trace data
Guest kernel self-modifying code (e.g. jump labels or JIT-compiled eBPF) will result in decoding errors
Guest thread information is unknown
Guest VCPU is unknown but may be able to be inferred from the host thread
Callchains are not supported
Example using "timeless" decoding
Start VM
$ sudo virsh start kubuntu20.04
Domain kubuntu20.04 started
Mount the guest file system. Note sshfs needs -o direct_io to enable reading of proc files. root access is needed to read /proc/kcore.
$ mkdir vm0
$ sshfs -o direct_io root@vm0:/ vm0
Copy the guest /proc/kallsyms, /proc/modules and /proc/kcore
$ perf buildid-cache -v --kcore vm0/proc/kcore
kcore added to build-id cache directory /home/user/.debug/[kernel.kcore]/9600f316a53a0f54278885e8d9710538ec5f6a08/2021021807494306
$ KALLSYMS=/home/user/.debug/[kernel.kcore]/9600f316a53a0f54278885e8d9710538ec5f6a08/2021021807494306/kallsyms
Find the VM process
$ ps -eLl | grep 'KVM\|PID'
F S UID PID PPID LWP C PRI NI ADDR SZ WCHAN TTY TIME CMD
3 S 64055 1430 1 1440 1 80 0 - 1921718 - ? 00:02:47 CPU 0/KVM
3 S 64055 1430 1 1441 1 80 0 - 1921718 - ? 00:02:41 CPU 1/KVM
3 S 64055 1430 1 1442 1 80 0 - 1921718 - ? 00:02:38 CPU 2/KVM
3 S 64055 1430 1 1443 2 80 0 - 1921718 - ? 00:03:18 CPU 3/KVM
Start an open-ended perf record, tracing the VM process, do something on the VM, and then ctrl-C to stop.
TSC is not supported and tsc=0 must be specified. That means mtc is useless, so add mtc=0.
However, IPC can still be determined, hence cyc=1 can be added.
Only kernel decoding is supported, so 'k' must be specified.
Intel PT traces both the host and the guest so --guest and --host need to be specified.
Without timestamps, --per-thread must be specified to distinguish threads.
$ sudo perf kvm --guest --host --guestkallsyms $KALLSYMS record --kcore -e intel_pt/tsc=0,mtc=0,cyc=1/k -p 1430 --per-thread
^C
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 5.829 MB ]
perf script can be used to provide an instruction trace
$ perf script --guestkallsyms $KALLSYMS --insn-trace --xed -F+ipc | grep -C10 vmresume | head -21
CPU 0/KVM 1440 ffffffff82133cdd __vmx_vcpu_run+0x3d ([kernel.kallsyms]) movq 0x48(%rax), %r9
CPU 0/KVM 1440 ffffffff82133ce1 __vmx_vcpu_run+0x41 ([kernel.kallsyms]) movq 0x50(%rax), %r10
CPU 0/KVM 1440 ffffffff82133ce5 __vmx_vcpu_run+0x45 ([kernel.kallsyms]) movq 0x58(%rax), %r11
CPU 0/KVM 1440 ffffffff82133ce9 __vmx_vcpu_run+0x49 ([kernel.kallsyms]) movq 0x60(%rax), %r12
CPU 0/KVM 1440 ffffffff82133ced __vmx_vcpu_run+0x4d ([kernel.kallsyms]) movq 0x68(%rax), %r13
CPU 0/KVM 1440 ffffffff82133cf1 __vmx_vcpu_run+0x51 ([kernel.kallsyms]) movq 0x70(%rax), %r14
CPU 0/KVM 1440 ffffffff82133cf5 __vmx_vcpu_run+0x55 ([kernel.kallsyms]) movq 0x78(%rax), %r15
CPU 0/KVM 1440 ffffffff82133cf9 __vmx_vcpu_run+0x59 ([kernel.kallsyms]) movq (%rax), %rax
CPU 0/KVM 1440 ffffffff82133cfc __vmx_vcpu_run+0x5c ([kernel.kallsyms]) callq 0xffffffff82133c40
CPU 0/KVM 1440 ffffffff82133c40 vmx_vmenter+0x0 ([kernel.kallsyms]) jz 0xffffffff82133c46
CPU 0/KVM 1440 ffffffff82133c42 vmx_vmenter+0x2 ([kernel.kallsyms]) vmresume IPC: 0.11 (50/445)
:1440 1440 ffffffffbb678b06 native_write_msr+0x6 ([guest.kernel.kallsyms]) nopl %eax, (%rax,%rax,1)
:1440 1440 ffffffffbb678b0b native_write_msr+0xb ([guest.kernel.kallsyms]) retq IPC: 0.04 (2/41)
:1440 1440 ffffffffbb666646 lapic_next_deadline+0x26 ([guest.kernel.kallsyms]) data16 nop
:1440 1440 ffffffffbb666648 lapic_next_deadline+0x28 ([guest.kernel.kallsyms]) xor %eax, %eax
:1440 1440 ffffffffbb66664a lapic_next_deadline+0x2a ([guest.kernel.kallsyms]) popq %rbp
:1440 1440 ffffffffbb66664b lapic_next_deadline+0x2b ([guest.kernel.kallsyms]) retq IPC: 0.16 (4/25)
:1440 1440 ffffffffbb74607f clockevents_program_event+0x8f ([guest.kernel.kallsyms]) test %eax, %eax
:1440 1440 ffffffffbb746081 clockevents_program_event+0x91 ([guest.kernel.kallsyms]) jz 0xffffffffbb74603c IPC: 0.06 (2/30)
:1440 1440 ffffffffbb74603c clockevents_program_event+0x4c ([guest.kernel.kallsyms]) popq %rbx
:1440 1440 ffffffffbb74603d clockevents_program_event+0x4d ([guest.kernel.kallsyms]) popq %r12
Example using VM Time Correlation
Start VM
$ sudo virsh start kubuntu20.04
Domain kubuntu20.04 started
Mount the guest file system. Note sshfs needs -o direct_io to enable reading of proc files. root access is needed to read /proc/kcore.
$ mkdir -p vm0
$ sshfs -o direct_io root@vm0:/ vm0
Copy the guest /proc/kallsyms, /proc/modules and /proc/kcore
$ perf buildid-cache -v --kcore vm0/proc/kcore
same kcore found in /home/user/.debug/[kernel.kcore]/cc9c55a98c5e4ec0aeda69302554aabed5cd6491/2021021312450777
$ KALLSYMS=/home/user/.debug/\[kernel.kcore\]/cc9c55a98c5e4ec0aeda69302554aabed5cd6491/2021021312450777/kallsyms
Find the VM process
$ ps -eLl | grep 'KVM\|PID'
F S UID PID PPID LWP C PRI NI ADDR SZ WCHAN TTY TIME CMD
3 S 64055 16998 1 17005 13 80 0 - 1818189 - ? 00:00:16 CPU 0/KVM
3 S 64055 16998 1 17006 4 80 0 - 1818189 - ? 00:00:05 CPU 1/KVM
3 S 64055 16998 1 17007 3 80 0 - 1818189 - ? 00:00:04 CPU 2/KVM
3 S 64055 16998 1 17008 4 80 0 - 1818189 - ? 00:00:05 CPU 3/KVM
Start an open-ended perf record, tracing the VM process, do something on the VM, and then ctrl-C to stop.
IPC can be determined, hence cyc=1 can be added.
Only kernel decoding is supported, so 'k' must be specified.
Intel PT traces both the host and the guest so --guest and --host need to be specified.
$ sudo perf kvm --guest --host --guestkallsyms $KALLSYMS record --kcore -e intel_pt/cyc=1/k -p 16998
^C[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 9.041 MB perf.data.kvm ]
Now 'perf inject' can be used to determine the VMX TCS Offset. Note, Intel PT TSC packets are
only 7-bytes, so the TSC Offset might differ from the actual value in the 8th byte. That will
have no effect i.e. the resulting timestamps will be correct anyway.
$ perf inject -i perf.data.kvm --vm-time-correlation=dry-run
ERROR: Unknown TSC Offset for VMCS 0x1bff6a
VMCS: 0x1bff6a TSC Offset 0xffffe42722c64c41
ERROR: Unknown TSC Offset for VMCS 0x1cbc08
VMCS: 0x1cbc08 TSC Offset 0xffffe42722c64c41
ERROR: Unknown TSC Offset for VMCS 0x1c3ce8
VMCS: 0x1c3ce8 TSC Offset 0xffffe42722c64c41
ERROR: Unknown TSC Offset for VMCS 0x1cbce9
VMCS: 0x1cbce9 TSC Offset 0xffffe42722c64c41
Each virtual CPU has a different Virtual Machine Control Structure (VMCS)
shown above with the calculated TSC Offset. For an unchanging TSC Offset
they should all be the same for the same virtual machine.
Now that the TSC Offset is known, it can be provided to 'perf inject'
$ perf inject -i perf.data.kvm --vm-time-correlation="dry-run 0xffffe42722c64c41"
Note the options for 'perf inject' --vm-time-correlation are:
[ dry-run ] [ <TSC Offset> [ : <VMCS> [ , <VMCS> ]... ] ]...
So it is possible to specify different TSC Offsets for different VMCS.
The option "dry-run" will cause the file to be processed but without updating it.
Note it is also possible to get a intel_pt.log file by adding option --itrace=d
There were no errors so, do it for real
$ perf inject -i perf.data.kvm --vm-time-correlation=0xffffe42722c64c41 --force
'perf script' can be used to see if there are any decoder errors
$ perf script -i perf.data.kvm --guestkallsyms $KALLSYMS --itrace=e-o
There were none.
'perf script' can be used to provide an instruction trace showing timestamps
$ perf script -i perf.data.kvm --guestkallsyms $KALLSYMS --insn-trace --xed -F+ipc | grep -C10 vmresume | head -21
CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133cdd __vmx_vcpu_run+0x3d ([kernel.kallsyms]) movq 0x48(%rax), %r9
CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133ce1 __vmx_vcpu_run+0x41 ([kernel.kallsyms]) movq 0x50(%rax), %r10
CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133ce5 __vmx_vcpu_run+0x45 ([kernel.kallsyms]) movq 0x58(%rax), %r11
CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133ce9 __vmx_vcpu_run+0x49 ([kernel.kallsyms]) movq 0x60(%rax), %r12
CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133ced __vmx_vcpu_run+0x4d ([kernel.kallsyms]) movq 0x68(%rax), %r13
CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133cf1 __vmx_vcpu_run+0x51 ([kernel.kallsyms]) movq 0x70(%rax), %r14
CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133cf5 __vmx_vcpu_run+0x55 ([kernel.kallsyms]) movq 0x78(%rax), %r15
CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133cf9 __vmx_vcpu_run+0x59 ([kernel.kallsyms]) movq (%rax), %rax
CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133cfc __vmx_vcpu_run+0x5c ([kernel.kallsyms]) callq 0xffffffff82133c40
CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133c40 vmx_vmenter+0x0 ([kernel.kallsyms]) jz 0xffffffff82133c46
CPU 1/KVM 17006 [001] 11500.262866075: ffffffff82133c42 vmx_vmenter+0x2 ([kernel.kallsyms]) vmresume IPC: 0.05 (40/769)
:17006 17006 [001] 11500.262869216: ffffffff82200cb0 asm_sysvec_apic_timer_interrupt+0x0 ([guest.kernel.kallsyms]) clac
:17006 17006 [001] 11500.262869216: ffffffff82200cb3 asm_sysvec_apic_timer_interrupt+0x3 ([guest.kernel.kallsyms]) pushq $0xffffffffffffffff
:17006 17006 [001] 11500.262869216: ffffffff82200cb5 asm_sysvec_apic_timer_interrupt+0x5 ([guest.kernel.kallsyms]) callq 0xffffffff82201160
:17006 17006 [001] 11500.262869216: ffffffff82201160 error_entry+0x0 ([guest.kernel.kallsyms]) cld
:17006 17006 [001] 11500.262869216: ffffffff82201161 error_entry+0x1 ([guest.kernel.kallsyms]) pushq %rsi
:17006 17006 [001] 11500.262869216: ffffffff82201162 error_entry+0x2 ([guest.kernel.kallsyms]) movq 0x8(%rsp), %rsi
:17006 17006 [001] 11500.262869216: ffffffff82201167 error_entry+0x7 ([guest.kernel.kallsyms]) movq %rdi, 0x8(%rsp)
:17006 17006 [001] 11500.262869216: ffffffff8220116c error_entry+0xc ([guest.kernel.kallsyms]) pushq %rdx
:17006 17006 [001] 11500.262869216: ffffffff8220116d error_entry+0xd ([guest.kernel.kallsyms]) pushq %rcx
:17006 17006 [001] 11500.262869216: ffffffff8220116e error_entry+0xe ([guest.kernel.kallsyms]) pushq %rax
Tracing Virtual Machines (including user space)
-----------------------------------------------
It is possible to use perf record to record sideband events within a virtual machine, so that an Intel PT trace on the host can be decoded.
Sideband events from the guest perf.data file can be injected into the host perf.data file using perf inject.
Here is an example of the steps needed:
On the guest machine:
Check that no-kvmclock kernel command line option was used to boot:
Note, this is essential to enable time correlation between host and guest machines.
$ cat /proc/cmdline
BOOT_IMAGE=/boot/vmlinuz-5.10.0-16-amd64 root=UUID=cb49c910-e573-47e0-bce7-79e293df8e1d ro no-kvmclock
There is no BPF support at present so, if possible, disable JIT compiling:
$ echo 0 | sudo tee /proc/sys/net/core/bpf_jit_enable
0
Start perf record to collect sideband events:
$ sudo perf record -o guest-sideband-testing-guest-perf.data --sample-identifier --buildid-all --switch-events --kcore -a -e dummy
On the host machine:
Start perf record to collect Intel PT trace:
Note, the host trace will get very big, very fast, so the steps from starting to stopping the host trace really need to be done so that they happen in the shortest time possible.
$ sudo perf record -o guest-sideband-testing-host-perf.data -m,64M --kcore -a -e intel_pt/cyc/
On the guest machine:
Run a small test case, just 'uname' in this example:
$ uname
Linux
On the host machine:
Stop the Intel PT trace:
^C
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 76.122 MB guest-sideband-testing-host-perf.data ]
On the guest machine:
Stop the Intel PT trace:
^C
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 1.247 MB guest-sideband-testing-guest-perf.data ]
And then copy guest-sideband-testing-guest-perf.data to the host (not shown here).
On the host machine:
With the 2 perf.data recordings, and with their ownership changed to the user.
Identify the TSC Offset:
$ perf inject -i guest-sideband-testing-host-perf.data --vm-time-correlation=dry-run
VMCS: 0x103fc6 TSC Offset 0xfffffa6ae070cb20
VMCS: 0x103ff2 TSC Offset 0xfffffa6ae070cb20
VMCS: 0x10fdaa TSC Offset 0xfffffa6ae070cb20
VMCS: 0x24d57c TSC Offset 0xfffffa6ae070cb20
Correct Intel PT TSC timestamps for the guest machine:
$ perf inject -i guest-sideband-testing-host-perf.data --vm-time-correlation=0xfffffa6ae070cb20 --force
Identify the guest machine PID:
$ perf script -i guest-sideband-testing-host-perf.data --no-itrace --show-task-events | grep KVM
CPU 0/KVM 0 [000] 0.000000: PERF_RECORD_COMM: CPU 0/KVM:13376/13381
CPU 1/KVM 0 [000] 0.000000: PERF_RECORD_COMM: CPU 1/KVM:13376/13382
CPU 2/KVM 0 [000] 0.000000: PERF_RECORD_COMM: CPU 2/KVM:13376/13383
CPU 3/KVM 0 [000] 0.000000: PERF_RECORD_COMM: CPU 3/KVM:13376/13384
Note, the QEMU option -name debug-threads=on is needed so that thread names
can be used to determine which thread is running which VCPU as above. libvirt seems to use this by default.
Create a guestmount, assuming the guest machine is 'vm_to_test':
$ mkdir -p ~/guestmount/13376
$ sshfs -o direct_io vm_to_test:/ ~/guestmount/13376
Inject the guest perf.data file into the host perf.data file:
Note, due to the guestmount option, guest object files and debug files will be copied into the build ID cache from the guest machine, with the notable exception of VDSO.
If needed, VDSO can be copied manually in a fashion similar to that used by the perf-archive script.
$ perf inject -i guest-sideband-testing-host-perf.data -o inj --guestmount ~/guestmount --guest-data=guest-sideband-testing-guest-perf.data,13376,0xfffffa6ae070cb20
Show an excerpt from the result. In this case the CPU and time range have been to chosen to show interaction between guest and host when 'uname' is starting to run on the guest machine:
Notes:
- the CPU displayed, [002] in this case, is always the host CPU
- events happening in the virtual machine start with VM:13376 VCPU:003, which shows the hypervisor PID 13376 and the VCPU number
- only calls and errors are displayed i.e. --itrace=ce
- branches entering and exiting the virtual machine are split, and show as 2 branches to/from "0 [unknown] ([unknown])"
$ perf script -i inj --itrace=ce -F+machine_pid,+vcpu,+addr,+pid,+tid,-period --ns --time 7919.408803365,7919.408804631 -C 2
CPU 3/KVM 13376/13384 [002] 7919.408803365: branches: ffffffffc0f8ebe0 vmx_vcpu_enter_exit+0xc0 ([kernel.kallsyms]) => ffffffffc0f8edc0 __vmx_vcpu_run+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803365: branches: ffffffffc0f8edd5 __vmx_vcpu_run+0x15 ([kernel.kallsyms]) => ffffffffc0f8eca0 vmx_update_host_rsp+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803365: branches: ffffffffc0f8ee1b __vmx_vcpu_run+0x5b ([kernel.kallsyms]) => ffffffffc0f8ed60 vmx_vmenter+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803461: branches: ffffffffc0f8ed62 vmx_vmenter+0x2 ([kernel.kallsyms]) => 0 [unknown] ([unknown])
VM:13376 VCPU:003 uname 3404/3404 [002] 7919.408803461: branches: 0 [unknown] ([unknown]) => 7f851c9b5a5c init_cacheinfo+0x3ac (/usr/lib/x86_64-linux-gnu/libc-2.31.so)
VM:13376 VCPU:003 uname 3404/3404 [002] 7919.408803567: branches: 7f851c9b5a5a init_cacheinfo+0x3aa (/usr/lib/x86_64-linux-gnu/libc-2.31.so) => 0 [unknown] ([unknown])
CPU 3/KVM 13376/13384 [002] 7919.408803567: branches: 0 [unknown] ([unknown]) => ffffffffc0f8ed80 vmx_vmexit+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803596: branches: ffffffffc0f6619a vmx_vcpu_run+0x26a ([kernel.kallsyms]) => ffffffffb2255c60 x86_virt_spec_ctrl+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803801: branches: ffffffffc0f66445 vmx_vcpu_run+0x515 ([kernel.kallsyms]) => ffffffffb2290b30 native_write_msr+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803850: branches: ffffffffc0f661f8 vmx_vcpu_run+0x2c8 ([kernel.kallsyms]) => ffffffffc1092300 kvm_load_host_xsave_state+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803850: branches: ffffffffc1092327 kvm_load_host_xsave_state+0x27 ([kernel.kallsyms]) => ffffffffc1092220 kvm_load_host_xsave_state.part.0+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803862: branches: ffffffffc0f662cf vmx_vcpu_run+0x39f ([kernel.kallsyms]) => ffffffffc0f63f90 vmx_recover_nmi_blocking+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803862: branches: ffffffffc0f662e9 vmx_vcpu_run+0x3b9 ([kernel.kallsyms]) => ffffffffc0f619a0 __vmx_complete_interrupts+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803872: branches: ffffffffc109cfb2 vcpu_enter_guest+0x752 ([kernel.kallsyms]) => ffffffffc0f5f570 vmx_handle_exit_irqoff+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803881: branches: ffffffffc109d028 vcpu_enter_guest+0x7c8 ([kernel.kallsyms]) => ffffffffb234f900 __srcu_read_lock+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803897: branches: ffffffffc109d06f vcpu_enter_guest+0x80f ([kernel.kallsyms]) => ffffffffc0f72e30 vmx_handle_exit+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803897: branches: ffffffffc0f72e3d vmx_handle_exit+0xd ([kernel.kallsyms]) => ffffffffc0f727c0 __vmx_handle_exit+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803897: branches: ffffffffc0f72b15 __vmx_handle_exit+0x355 ([kernel.kallsyms]) => ffffffffc0f60ae0 vmx_flush_pml_buffer+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803903: branches: ffffffffc0f72994 __vmx_handle_exit+0x1d4 ([kernel.kallsyms]) => ffffffffc10b7090 kvm_emulate_cpuid+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803903: branches: ffffffffc10b70f1 kvm_emulate_cpuid+0x61 ([kernel.kallsyms]) => ffffffffc10b6e10 kvm_cpuid+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803941: branches: ffffffffc10b7125 kvm_emulate_cpuid+0x95 ([kernel.kallsyms]) => ffffffffc1093110 kvm_skip_emulated_instruction+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803941: branches: ffffffffc109311f kvm_skip_emulated_instruction+0xf ([kernel.kallsyms]) => ffffffffc0f5e180 vmx_get_rflags+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803951: branches: ffffffffc109312a kvm_skip_emulated_instruction+0x1a ([kernel.kallsyms]) => ffffffffc0f5fd30 vmx_skip_emulated_instruction+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803951: branches: ffffffffc0f5fd79 vmx_skip_emulated_instruction+0x49 ([kernel.kallsyms]) => ffffffffc0f5fb50 skip_emulated_instruction+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803956: branches: ffffffffc0f5fc68 skip_emulated_instruction+0x118 ([kernel.kallsyms]) => ffffffffc0f6a940 vmx_cache_reg+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803964: branches: ffffffffc0f5fc11 skip_emulated_instruction+0xc1 ([kernel.kallsyms]) => ffffffffc0f5f9e0 vmx_set_interrupt_shadow+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803980: branches: ffffffffc109f8b1 vcpu_run+0x71 ([kernel.kallsyms]) => ffffffffc10ad2f0 kvm_cpu_has_pending_timer+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803980: branches: ffffffffc10ad2fb kvm_cpu_has_pending_timer+0xb ([kernel.kallsyms]) => ffffffffc10b0490 apic_has_pending_timer+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803991: branches: ffffffffc109f899 vcpu_run+0x59 ([kernel.kallsyms]) => ffffffffc109c860 vcpu_enter_guest+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803993: branches: ffffffffc109cd4c vcpu_enter_guest+0x4ec ([kernel.kallsyms]) => ffffffffc0f69140 vmx_prepare_switch_to_guest+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803996: branches: ffffffffc109cd7d vcpu_enter_guest+0x51d ([kernel.kallsyms]) => ffffffffb234f930 __srcu_read_unlock+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803996: branches: ffffffffc109cd9c vcpu_enter_guest+0x53c ([kernel.kallsyms]) => ffffffffc0f609b0 vmx_sync_pir_to_irr+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408803996: branches: ffffffffc0f60a6d vmx_sync_pir_to_irr+0xbd ([kernel.kallsyms]) => ffffffffc10adc20 kvm_lapic_find_highest_irr+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804010: branches: ffffffffc0f60abd vmx_sync_pir_to_irr+0x10d ([kernel.kallsyms]) => ffffffffc0f60820 vmx_set_rvi+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804019: branches: ffffffffc109ceca vcpu_enter_guest+0x66a ([kernel.kallsyms]) => ffffffffb2249840 fpregs_assert_state_consistent+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804021: branches: ffffffffc109cf10 vcpu_enter_guest+0x6b0 ([kernel.kallsyms]) => ffffffffc0f65f30 vmx_vcpu_run+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804024: branches: ffffffffc0f6603b vmx_vcpu_run+0x10b ([kernel.kallsyms]) => ffffffffb229bed0 __get_current_cr3_fast+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804024: branches: ffffffffc0f66055 vmx_vcpu_run+0x125 ([kernel.kallsyms]) => ffffffffb2253050 cr4_read_shadow+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804030: branches: ffffffffc0f6608d vmx_vcpu_run+0x15d ([kernel.kallsyms]) => ffffffffc10921e0 kvm_load_guest_xsave_state+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804030: branches: ffffffffc1092207 kvm_load_guest_xsave_state+0x27 ([kernel.kallsyms]) => ffffffffc1092110 kvm_load_guest_xsave_state.part.0+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804032: branches: ffffffffc0f660c6 vmx_vcpu_run+0x196 ([kernel.kallsyms]) => ffffffffb22061a0 perf_guest_get_msrs+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804032: branches: ffffffffb22061a9 perf_guest_get_msrs+0x9 ([kernel.kallsyms]) => ffffffffb220cda0 intel_guest_get_msrs+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804039: branches: ffffffffc0f66109 vmx_vcpu_run+0x1d9 ([kernel.kallsyms]) => ffffffffc0f652c0 clear_atomic_switch_msr+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804040: branches: ffffffffc0f66119 vmx_vcpu_run+0x1e9 ([kernel.kallsyms]) => ffffffffc0f73f60 intel_pmu_lbr_is_enabled+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804042: branches: ffffffffc0f73f81 intel_pmu_lbr_is_enabled+0x21 ([kernel.kallsyms]) => ffffffffc10b68e0 kvm_find_cpuid_entry+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804045: branches: ffffffffc0f66454 vmx_vcpu_run+0x524 ([kernel.kallsyms]) => ffffffffc0f61ff0 vmx_update_hv_timer+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804057: branches: ffffffffc0f66142 vmx_vcpu_run+0x212 ([kernel.kallsyms]) => ffffffffc10af100 kvm_wait_lapic_expire+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804057: branches: ffffffffc0f66156 vmx_vcpu_run+0x226 ([kernel.kallsyms]) => ffffffffb2255c60 x86_virt_spec_ctrl+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804057: branches: ffffffffc0f66161 vmx_vcpu_run+0x231 ([kernel.kallsyms]) => ffffffffc0f8eb20 vmx_vcpu_enter_exit+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804057: branches: ffffffffc0f8eb44 vmx_vcpu_enter_exit+0x24 ([kernel.kallsyms]) => ffffffffb2353e10 rcu_note_context_switch+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804057: branches: ffffffffb2353e1c rcu_note_context_switch+0xc ([kernel.kallsyms]) => ffffffffb2353db0 rcu_qs+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804066: branches: ffffffffc0f8ebe0 vmx_vcpu_enter_exit+0xc0 ([kernel.kallsyms]) => ffffffffc0f8edc0 __vmx_vcpu_run+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804066: branches: ffffffffc0f8edd5 __vmx_vcpu_run+0x15 ([kernel.kallsyms]) => ffffffffc0f8eca0 vmx_update_host_rsp+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804066: branches: ffffffffc0f8ee1b __vmx_vcpu_run+0x5b ([kernel.kallsyms]) => ffffffffc0f8ed60 vmx_vmenter+0x0 ([kernel.kallsyms])
CPU 3/KVM 13376/13384 [002] 7919.408804162: branches: ffffffffc0f8ed62 vmx_vmenter+0x2 ([kernel.kallsyms]) => 0 [unknown] ([unknown])
VM:13376 VCPU:003 uname 3404/3404 [002] 7919.408804162: branches: 0 [unknown] ([unknown]) => 7f851c9b5a5c init_cacheinfo+0x3ac (/usr/lib/x86_64-linux-gnu/libc-2.31.so)
VM:13376 VCPU:003 uname 3404/3404 [002] 7919.408804273: branches: 7f851cb7c0e4 _dl_init+0x74 (/usr/lib/x86_64-linux-gnu/ld-2.31.so) => 7f851cb7bf50 call_init.part.0+0x0 (/usr/lib/x86_64-linux-gnu/ld-2.31.so)
VM:13376 VCPU:003 uname 3404/3404 [002] 7919.408804526: branches: 55e0c00136f0 _start+0x0 (/usr/bin/uname) => ffffffff83200ac0 asm_exc_page_fault+0x0 ([kernel.kallsyms])
VM:13376 VCPU:003 uname 3404/3404 [002] 7919.408804526: branches: ffffffff83200ac3 asm_exc_page_fault+0x3 ([kernel.kallsyms]) => ffffffff83201290 error_entry+0x0 ([kernel.kallsyms])
VM:13376 VCPU:003 uname 3404/3404 [002] 7919.408804534: branches: ffffffff832012fa error_entry+0x6a ([kernel.kallsyms]) => ffffffff830b59a0 sync_regs+0x0 ([kernel.kallsyms])
VM:13376 VCPU:003 uname 3404/3404 [002] 7919.408804631: branches: ffffffff83200ad9 asm_exc_page_fault+0x19 ([kernel.kallsyms]) => ffffffff830b8210 exc_page_fault+0x0 ([kernel.kallsyms])
VM:13376 VCPU:003 uname 3404/3404 [002] 7919.408804631: branches: ffffffff830b82a4 exc_page_fault+0x94 ([kernel.kallsyms]) => ffffffff830b80e0 __kvm_handle_async_pf+0x0 ([kernel.kallsyms])
VM:13376 VCPU:003 uname 3404/3404 [002] 7919.408804631: branches: ffffffff830b80ed __kvm_handle_async_pf+0xd ([kernel.kallsyms]) => ffffffff830b80c0 kvm_read_and_reset_apf_flags+0x0 ([kernel.kallsyms])
Tracing Virtual Machines - Guest Code
-------------------------------------
A common case for KVM test programs is that the test program acts as the
hypervisor, creating, running and destroying the virtual machine, and
providing the guest object code from its own object code. In this case,
the VM is not running an OS, but only the functions loaded into it by the
hypervisor test program, and conveniently, loaded at the same virtual
addresses. To support that, option "--guest-code" has been added to perf script
and perf kvm report.
Here is an example tracing a test program from the kernel's KVM selftests:
# perf record --kcore -e intel_pt/cyc/ -- tools/testing/selftests/kselftest_install/kvm/tsc_msrs_test
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.280 MB perf.data ]
# perf script --guest-code --itrace=bep --ns -F-period,+addr,+flags
[SNIP]
tsc_msrs_test 18436 [007] 10897.962087733: branches: call ffffffffc13b2ff5 __vmx_vcpu_run+0x15 (vmlinux) => ffffffffc13b2f50 vmx_update_host_rsp+0x0 (vmlinux)
tsc_msrs_test 18436 [007] 10897.962087733: branches: return ffffffffc13b2f5d vmx_update_host_rsp+0xd (vmlinux) => ffffffffc13b2ffa __vmx_vcpu_run+0x1a (vmlinux)
tsc_msrs_test 18436 [007] 10897.962087733: branches: call ffffffffc13b303b __vmx_vcpu_run+0x5b (vmlinux) => ffffffffc13b2f80 vmx_vmenter+0x0 (vmlinux)
tsc_msrs_test 18436 [007] 10897.962087836: branches: vmentry ffffffffc13b2f82 vmx_vmenter+0x2 (vmlinux) => 0 [unknown] ([unknown])
[guest/18436] 18436 [007] 10897.962087836: branches: vmentry 0 [unknown] ([unknown]) => 402c81 guest_code+0x131 (/home/user/git/work/tools/testing/selftests/kselftest_install/kvm/tsc_msrs_test)
[guest/18436] 18436 [007] 10897.962087836: branches: call 402c81 guest_code+0x131 (/home/user/git/work/tools/testing/selftests/kselftest_install/kvm/tsc_msrs_test) => 40dba0 ucall+0x0 (/home/user/git/work/tools/testing/selftests/kselftest_install/kvm/tsc_msrs_test)
[guest/18436] 18436 [007] 10897.962088248: branches: vmexit 40dba0 ucall+0x0 (/home/user/git/work/tools/testing/selftests/kselftest_install/kvm/tsc_msrs_test) => 0 [unknown] ([unknown])
tsc_msrs_test 18436 [007] 10897.962088248: branches: vmexit 0 [unknown] ([unknown]) => ffffffffc13b2fa0 vmx_vmexit+0x0 (vmlinux)
tsc_msrs_test 18436 [007] 10897.962088248: branches: jmp ffffffffc13b2fa0 vmx_vmexit+0x0 (vmlinux) => ffffffffc13b2fd2 vmx_vmexit+0x32 (vmlinux)
tsc_msrs_test 18436 [007] 10897.962088256: branches: return ffffffffc13b2fd2 vmx_vmexit+0x32 (vmlinux) => ffffffffc13b3040 __vmx_vcpu_run+0x60 (vmlinux)
tsc_msrs_test 18436 [007] 10897.962088270: branches: return ffffffffc13b30b6 __vmx_vcpu_run+0xd6 (vmlinux) => ffffffffc13b2f2e vmx_vcpu_enter_exit+0x4e (vmlinux)
[SNIP]
tsc_msrs_test 18436 [007] 10897.962089321: branches: call ffffffffc13b2ff5 __vmx_vcpu_run+0x15 (vmlinux) => ffffffffc13b2f50 vmx_update_host_rsp+0x0 (vmlinux)
tsc_msrs_test 18436 [007] 10897.962089321: branches: return ffffffffc13b2f5d vmx_update_host_rsp+0xd (vmlinux) => ffffffffc13b2ffa __vmx_vcpu_run+0x1a (vmlinux)
tsc_msrs_test 18436 [007] 10897.962089321: branches: call ffffffffc13b303b __vmx_vcpu_run+0x5b (vmlinux) => ffffffffc13b2f80 vmx_vmenter+0x0 (vmlinux)
tsc_msrs_test 18436 [007] 10897.962089424: branches: vmentry ffffffffc13b2f82 vmx_vmenter+0x2 (vmlinux) => 0 [unknown] ([unknown])
[guest/18436] 18436 [007] 10897.962089424: branches: vmentry 0 [unknown] ([unknown]) => 40dba0 ucall+0x0 (/home/user/git/work/tools/testing/selftests/kselftest_install/kvm/tsc_msrs_test)
[guest/18436] 18436 [007] 10897.962089701: branches: jmp 40dc1b ucall+0x7b (/home/user/git/work/tools/testing/selftests/kselftest_install/kvm/tsc_msrs_test) => 40dc39 ucall+0x99 (/home/user/git/work/tools/testing/selftests/kselftest_install/kvm/tsc_msrs_test)
[guest/18436] 18436 [007] 10897.962089701: branches: jcc 40dc3c ucall+0x9c (/home/user/git/work/tools/testing/selftests/kselftest_install/kvm/tsc_msrs_test) => 40dc20 ucall+0x80 (/home/user/git/work/tools/testing/selftests/kselftest_install/kvm/tsc_msrs_test)
[guest/18436] 18436 [007] 10897.962089701: branches: jcc 40dc3c ucall+0x9c (/home/user/git/work/tools/testing/selftests/kselftest_install/kvm/tsc_msrs_test) => 40dc20 ucall+0x80 (/home/user/git/work/tools/testing/selftests/kselftest_install/kvm/tsc_msrs_test)
[guest/18436] 18436 [007] 10897.962089701: branches: jcc 40dc37 ucall+0x97 (/home/user/git/work/tools/testing/selftests/kselftest_install/kvm/tsc_msrs_test) => 40dc50 ucall+0xb0 (/home/user/git/work/tools/testing/selftests/kselftest_install/kvm/tsc_msrs_test)
[guest/18436] 18436 [007] 10897.962089878: branches: vmexit 40dc55 ucall+0xb5 (/home/user/git/work/tools/testing/selftests/kselftest_install/kvm/tsc_msrs_test) => 0 [unknown] ([unknown])
tsc_msrs_test 18436 [007] 10897.962089878: branches: vmexit 0 [unknown] ([unknown]) => ffffffffc13b2fa0 vmx_vmexit+0x0 (vmlinux)
tsc_msrs_test 18436 [007] 10897.962089878: branches: jmp ffffffffc13b2fa0 vmx_vmexit+0x0 (vmlinux) => ffffffffc13b2fd2 vmx_vmexit+0x32 (vmlinux)
tsc_msrs_test 18436 [007] 10897.962089887: branches: return ffffffffc13b2fd2 vmx_vmexit+0x32 (vmlinux) => ffffffffc13b3040 __vmx_vcpu_run+0x60 (vmlinux)
tsc_msrs_test 18436 [007] 10897.962089901: branches: return ffffffffc13b30b6 __vmx_vcpu_run+0xd6 (vmlinux) => ffffffffc13b2f2e vmx_vcpu_enter_exit+0x4e (vmlinux)
[SNIP]
# perf kvm --guest-code --guest --host report -i perf.data --stdio | head -20
# To display the perf.data header info, please use --header/--header-only options.
#
#
# Total Lost Samples: 0
#
# Samples: 12 of event 'instructions'
# Event count (approx.): 2274583
#
# Children Self Command Shared Object Symbol
# ........ ........ ............. .................... ...........................................
#
54.70% 0.00% tsc_msrs_test [kernel.vmlinux] [k] entry_SYSCALL_64_after_hwframe
|
---entry_SYSCALL_64_after_hwframe
do_syscall_64
|
|--29.44%--syscall_exit_to_user_mode
| exit_to_user_mode_prepare
| task_work_run
| __fput
Event Trace
-----------
Event Trace records information about asynchronous events, for example interrupts,
faults, VM exits and entries. The information is recorded in CFE and EVD packets,
and also the Interrupt Flag is recorded on the MODE.Exec packet. The CFE packet
contains a type field to identify one of the following:
1 INTR interrupt, fault, exception, NMI
2 IRET interrupt return
3 SMI system management interrupt
4 RSM resume from system management mode
5 SIPI startup interprocessor interrupt
6 INIT INIT signal
7 VMENTRY VM-Entry
8 VMEXIT VM-Entry
9 VMEXIT_INTR VM-Exit due to interrupt
10 SHUTDOWN Shutdown
For more details, refer to the Intel 64 and IA-32 Architectures Software
Developer Manuals (version 076 or later).
The capability to do Event Trace is indicated by the
/sys/bus/event_source/devices/intel_pt/caps/event_trace file.
Event trace is selected for recording using the "event" config term. e.g.
perf record -e intel_pt/event/u uname
Event trace events are output using the --itrace I option. e.g.
perf script --itrace=Ie
perf script displays events containing CFE type, vector and event data,
in the form:
evt: hw int (t) cfe: INTR IP: 1 vector: 3 PFA: 0x8877665544332211
The IP flag indicates if the event binds to an IP, which includes any case where
flow control packet generation is enabled, as well as when CFE packet IP bit is
set.
perf script displays events containing changes to the Interrupt Flag in the form:
iflag: t IFLAG: 1->0 via branch
where "via branch" indicates a branch (interrupt or return from interrupt) and
"non branch" indicates an instruction such as CFI, STI or POPF).
In addition, the current state of the interrupt flag is indicated by the presence
or absence of the "D" (interrupt disabled) perf script flag. If the interrupt
flag is changed, then the "t" flag is also included i.e.
no flag, interrupts enabled IF=1
t interrupts become disabled IF=1 -> IF=0
D interrupts are disabled IF=0
Dt interrupts become enabled IF=0 -> IF=1
The intel-pt-events.py script illustrates how to access Event Trace information
using a Python script.
TNT Disable
-----------
TNT packets are disabled using the "notnt" config term. e.g.
perf record -e intel_pt/notnt/u uname
In that case the --itrace q option is forced because walking executable code
to reconstruct the control flow is not possible.
Emulated PTWRITE
----------------
Later perf tools support a method to emulate the ptwrite instruction, which
can be useful if hardware does not support the ptwrite instruction.
Instead of using the ptwrite instruction, a function is used which produces
a trace that encodes the payload data into TNT packets. Here is an example
of the function:
#include <stdint.h>
void perf_emulate_ptwrite(uint64_t x)
__attribute__((externally_visible, noipa, no_instrument_function, naked));
#define PERF_EMULATE_PTWRITE_8_BITS \
"1: shl %rax\n" \
" jc 1f\n" \
"1: shl %rax\n" \
" jc 1f\n" \
"1: shl %rax\n" \
" jc 1f\n" \
"1: shl %rax\n" \
" jc 1f\n" \
"1: shl %rax\n" \
" jc 1f\n" \
"1: shl %rax\n" \
" jc 1f\n" \
"1: shl %rax\n" \
" jc 1f\n" \
"1: shl %rax\n" \
" jc 1f\n"
/* Undefined instruction */
#define PERF_EMULATE_PTWRITE_UD2 ".byte 0x0f, 0x0b\n"
#define PERF_EMULATE_PTWRITE_MAGIC PERF_EMULATE_PTWRITE_UD2 ".ascii \"perf,ptwrite \"\n"
void perf_emulate_ptwrite(uint64_t x __attribute__ ((__unused__)))
{
/* Assumes SysV ABI : x passed in rdi */
__asm__ volatile (
"jmp 1f\n"
PERF_EMULATE_PTWRITE_MAGIC
"1: mov %rdi, %rax\n"
PERF_EMULATE_PTWRITE_8_BITS
PERF_EMULATE_PTWRITE_8_BITS
PERF_EMULATE_PTWRITE_8_BITS
PERF_EMULATE_PTWRITE_8_BITS
PERF_EMULATE_PTWRITE_8_BITS
PERF_EMULATE_PTWRITE_8_BITS
PERF_EMULATE_PTWRITE_8_BITS
PERF_EMULATE_PTWRITE_8_BITS
"1: ret\n"
);
}
For example, a test program with the function above:
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include "perf_emulate_ptwrite.h"
int main(int argc, char *argv[])
{
uint64_t x = 0;
if (argc > 1)
x = strtoull(argv[1], NULL, 0);
perf_emulate_ptwrite(x);
return 0;
}
Can be compiled and traced:
$ gcc -Wall -Wextra -O3 -g -o eg_ptw eg_ptw.c
$ perf record -e intel_pt//u ./eg_ptw 0x1234567890abcdef
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.017 MB perf.data ]
$ perf script --itrace=ew
eg_ptw 19875 [007] 8061.235912: ptwrite: IP: 0 payload: 0x1234567890abcdef 55701249a196 perf_emulate_ptwrite+0x16 (/home/user/eg_ptw)
$
Pipe mode
---------
Pipe mode is a problem for Intel PT and possibly other auxtrace users.
It's not recommended to use a pipe as data output with Intel PT because
of the following reason.
Essentially the auxtrace buffers do not behave like the regular perf
event buffers. That is because the head and tail are updated by
software, but in the auxtrace case the data is written by hardware.
So the head and tail do not get updated as data is written.
In the Intel PT case, the head and tail are updated only when the trace
is disabled by software, for example:
- full-trace, system wide : when buffer passes watermark
- full-trace, not system-wide : when buffer passes watermark or
context switches
- snapshot mode : as above but also when a snapshot is made
- sample mode : as above but also when a sample is made
That means finished-round ordering doesn't work. An auxtrace buffer
can turn up that has data that extends back in time, possibly to the
very beginning of tracing.
For a perf.data file, that problem is solved by going through the trace
and queuing up the auxtrace buffers in advance.
For pipe mode, the order of events and timestamps can presumably
be messed up.
EXAMPLE
-------
Examples can be found on perf wiki page "Perf tools support for Intel® Processor Trace":
https://perf.wiki.kernel.org/index.php/Perf_tools_support_for_Intel%C2%AE_Processor_Trace
SEE ALSO
--------
linkperf:perf-record[1], linkperf:perf-script[1], linkperf:perf-report[1],
linkperf:perf-inject[1]
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