= Tracing =

== Introduction ==

This document describes the tracing infrastructure in QEMU and how to use it

for debugging, profiling, and observing execution.

== Quickstart ==

1. Build with the 'simple' trace backend:

./configure --enable-trace-backends=simple

make

2. Create a file with the events you want to trace:

echo bdrv_aio_readv > /tmp/events

echo bdrv_aio_writev >> /tmp/events

3. Run the virtual machine to produce a trace file:

qemu -trace events=/tmp/events ... # your normal QEMU invocation

4. Pretty-print the binary trace file:

./scripts/simpletrace.py trace-events-all trace-* # Override * with QEMU <pid>

== Trace events ==

=== Sub-directory setup ===

Each directory in the source tree can declare a set of static trace events

in a local "trace-events" file. All directories which contain "trace-events"

files must be listed in the "trace-events-subdirs" make variable in the top

level Makefile.objs. During build, the "trace-events" file in each listed

subdirectory will be processed by the "tracetool" script to generate code for

the trace events.

The individual "trace-events" files are merged into a "trace-events-all" file,

which is also installed into "/usr/share/qemu" with the name "trace-events".

This merged file is to be used by the "simpletrace.py" script to later analyse

traces in the simpletrace data format.

In the sub-directory the following files will be automatically generated

- trace.c - the trace event state declarations

- trace.h - the trace event enums and probe functions

- trace-dtrace.h - DTrace event probe specification

- trace-dtrace.dtrace - DTrace event probe helper declaration

- trace-dtrace.o - binary DTrace provider (generated by dtrace)

- trace-ust.h - UST event probe helper declarations

Source files in the sub-directory should #include the local 'trace.h' file,

without any sub-directory path prefix. eg io/channel-buffer.c would do

#include "trace.h"

To access the 'io/trace.h' file. While it is possible to include a trace.h

file from outside a source files' own sub-directory, this is discouraged in

general. It is strongly preferred that all events be declared directly in

the sub-directory that uses them. The only exception is where there are some

shared trace events defined in the top level directory trace-events file.

The top level directory generates trace files with a filename prefix of

"trace-root" instead of just "trace". This is to avoid ambiguity between

a trace.h in the current directory, vs the top level directory.

=== Using trace events ===

Trace events are invoked directly from source code like this:

#include "trace.h" /* needed for trace event prototype */

void *qemu_vmalloc(size_t size)

{

void *ptr;

size_t align = QEMU_VMALLOC_ALIGN;

if (size < align) {

align = getpagesize();

}

ptr = qemu_memalign(align, size);

trace_qemu_vmalloc(size, ptr);

return ptr;

}

=== Declaring trace events ===

The "tracetool" script produces the trace.h header file which is included by

every source file that uses trace events. Since many source files include

trace.h, it uses a minimum of types and other header files included to keep the

namespace clean and compile times and dependencies down.

Trace events should use types as follows:

* Use stdint.h types for fixed-size types. Most offsets and guest memory

addresses are best represented with uint32_t or uint64_t. Use fixed-size

types over primitive types whose size may change depending on the host

(32-bit versus 64-bit) so trace events don't truncate values or break

the build.

* Use void * for pointers to structs or for arrays. The trace.h header

cannot include all user-defined struct declarations and it is therefore

necessary to use void * for pointers to structs.

* For everything else, use primitive scalar types (char, int, long) with the

appropriate signedness.

Format strings should reflect the types defined in the trace event. Take

special care to use PRId64 and PRIu64 for int64_t and uint64_t types,

respectively. This ensures portability between 32- and 64-bit platforms.

Each event declaration will start with the event name, then its arguments,

finally a format string for pretty-printing. For example:

qemu_vmalloc(size_t size, void *ptr) "size %zu ptr %p"

qemu_vfree(void *ptr) "ptr %p"

=== Hints for adding new trace events ===

1. Trace state changes in the code. Interesting points in the code usually

involve a state change like starting, stopping, allocating, freeing. State

changes are good trace events because they can be used to understand the

execution of the system.

2. Trace guest operations. Guest I/O accesses like reading device registers

are good trace events because they can be used to understand guest

interactions.

3. Use correlator fields so the context of an individual line of trace output

can be understood. For example, trace the pointer returned by malloc and

used as an argument to free. This way mallocs and frees can be matched up.

Trace events with no context are not very useful.

4. Name trace events after their function. If there are multiple trace events

in one function, append a unique distinguisher at the end of the name.

== Generic interface and monitor commands ==

You can programmatically query and control the state of trace events through a

backend-agnostic interface provided by the header "trace/control.h".

Note that some of the backends do not provide an implementation for some parts

of this interface, in which case QEMU will just print a warning (please refer to

header "trace/control.h" to see which routines are backend-dependent).

The state of events can also be queried and modified through monitor commands:

* info trace-events

View available trace events and their state. State 1 means enabled, state 0

means disabled.

* trace-event NAME on|off

Enable/disable a given trace event or a group of events (using wildcards).

The "-trace events=<file>" command line argument can be used to enable the

events listed in <file> from the very beginning of the program. This file must

contain one event name per line.

If a line in the "-trace events=<file>" file begins with a '-', the trace event

will be disabled instead of enabled. This is useful when a wildcard was used

to enable an entire family of events but one noisy event needs to be disabled.

Wildcard matching is supported in both the monitor command "trace-event" and the

events list file. That means you can enable/disable the events having a common

prefix in a batch. For example, virtio-blk trace events could be enabled using

the following monitor command:

trace-event virtio_blk_* on

== Trace backends ==

The "tracetool" script automates tedious trace event code generation and also

keeps the trace event declarations independent of the trace backend. The trace

events are not tightly coupled to a specific trace backend, such as LTTng or

SystemTap. Support for trace backends can be added by extending the "tracetool"

script.

The trace backends are chosen at configure time:

./configure --enable-trace-backends=simple

For a list of supported trace backends, try ./configure --help or see below.

If multiple backends are enabled, the trace is sent to them all.

If no backends are explicitly selected, configure will default to the

"log" backend.

The following subsections describe the supported trace backends.

=== Nop ===

The "nop" backend generates empty trace event functions so that the compiler

can optimize out trace events completely. This imposes no performance

penalty.

Note that regardless of the selected trace backend, events with the "disable"

property will be generated with the "nop" backend.

=== Log ===

The "log" backend sends trace events directly to standard error. This

effectively turns trace events into debug printfs.

This is the simplest backend and can be used together with existing code that

uses DPRINTF().

=== Simpletrace ===

The "simple" backend supports common use cases and comes as part of the QEMU

source tree. It may not be as powerful as platform-specific or third-party

trace backends but it is portable. This is the recommended trace backend

unless you have specific needs for more advanced backends.

=== Ftrace ===

The "ftrace" backend writes trace data to ftrace marker. This effectively

sends trace events to ftrace ring buffer, and you can compare qemu trace

data and kernel(especially kvm.ko when using KVM) trace data.

if you use KVM, enable kvm events in ftrace:

# echo 1 > /sys/kernel/debug/tracing/events/kvm/enable

After running qemu by root user, you can get the trace:

# cat /sys/kernel/debug/tracing/trace

Restriction: "ftrace" backend is restricted to Linux only.

=== Syslog ===

The "syslog" backend sends trace events using the POSIX syslog API. The log

is opened specifying the LOG_DAEMON facility and LOG_PID option (so events

are tagged with the pid of the particular QEMU process that generated

them). All events are logged at LOG_INFO level.

NOTE: syslog may squash duplicate consecutive trace events and apply rate

limiting.

Restriction: "syslog" backend is restricted to POSIX compliant OS.

==== Monitor commands ====

* trace-file on|off|flush|set <path>

Enable/disable/flush the trace file or set the trace file name.

==== Analyzing trace files ====

The "simple" backend produces binary trace files that can be formatted with the

simpletrace.py script. The script takes the "trace-events-all" file and the

binary trace:

./scripts/simpletrace.py trace-events-all trace-12345

You must ensure that the same "trace-events-all" file was used to build QEMU,

otherwise trace event declarations may have changed and output will not be

consistent.

=== LTTng Userspace Tracer ===

The "ust" backend uses the LTTng Userspace Tracer library. There are no

monitor commands built into QEMU, instead UST utilities should be used to list,

enable/disable, and dump traces.

Package lttng-tools is required for userspace tracing. You must ensure that the

current user belongs to the "tracing" group, or manually launch the

lttng-sessiond daemon for the current user prior to running any instance of

QEMU.

While running an instrumented QEMU, LTTng should be able to list all available

events:

lttng list -u

Create tracing session:

lttng create mysession

Enable events:

lttng enable-event qemu:g_malloc -u

Where the events can either be a comma-separated list of events, or "-a" to

enable all tracepoint events. Start and stop tracing as needed:

lttng start

lttng stop

View the trace:

lttng view

Destroy tracing session:

lttng destroy

Babeltrace can be used at any later time to view the trace:

babeltrace $HOME/lttng-traces/mysession-<date>-<time>

=== SystemTap ===

The "dtrace" backend uses DTrace sdt probes but has only been tested with

SystemTap. When SystemTap support is detected a .stp file with wrapper probes

is generated to make use in scripts more convenient. This step can also be

performed manually after a build in order to change the binary name in the .stp

probes:

scripts/tracetool.py --backends=dtrace --format=stap \

--binary path/to/qemu-binary \

--target-type system \

--target-name x86_64 \

<trace-events-all >qemu.stp

== Trace event properties ==

Each event in the "trace-events-all" file can be prefixed with a space-separated

list of zero or more of the following event properties.

=== "disable" ===

If a specific trace event is going to be invoked a huge number of times, this

might have a noticeable performance impact even when the event is

programmatically disabled.

In this case you should declare such event with the "disable" property. This

will effectively disable the event at compile time (by using the "nop" backend),

thus having no performance impact at all on regular builds (i.e., unless you

edit the "trace-events-all" file).

In addition, there might be cases where relatively complex computations must be

performed to generate values that are only used as arguments for a trace

function. In these cases you can use the macro 'TRACE_${EVENT_NAME}_ENABLED' to

guard such computations and avoid its compilation when the event is disabled:

#include "trace.h" /* needed for trace event prototype */

void *qemu_vmalloc(size_t size)

{

void *ptr;

size_t align = QEMU_VMALLOC_ALIGN;

if (size < align) {

align = getpagesize();

}

ptr = qemu_memalign(align, size);

if (TRACE_QEMU_VMALLOC_ENABLED) { /* preprocessor macro */

void *complex;

/* some complex computations to produce the 'complex' value */

trace_qemu_vmalloc(size, ptr, complex);

}

return ptr;

}

You can check both if the event has been disabled and is dynamically enabled at

the same time using the 'trace_event_get_state' routine (see header

"trace/control.h" for more information).

=== "tcg" ===

Guest code generated by TCG can be traced by defining an event with the "tcg"

event property. Internally, this property generates two events:

"<eventname>_trans" to trace the event at translation time, and

"<eventname>_exec" to trace the event at execution time.

Instead of using these two events, you should instead use the function

"trace_<eventname>_tcg" during translation (TCG code generation). This function

will automatically call "trace_<eventname>_trans", and will generate the

necessary TCG code to call "trace_<eventname>_exec" during guest code execution.

Events with the "tcg" property can be declared in the "trace-events" file with a

mix of native and TCG types, and "trace_<eventname>_tcg" will gracefully forward

them to the "<eventname>_trans" and "<eventname>_exec" events. Since TCG values

are not known at translation time, these are ignored by the "<eventname>_trans"

event. Because of this, the entry in the "trace-events" file needs two printing

formats (separated by a comma):

tcg foo(uint8_t a1, TCGv_i32 a2) "a1=%d", "a1=%d a2=%d"

For example:

#include "trace-tcg.h"

void some_disassembly_func (...)

{

uint8_t a1 = ...;

TCGv_i32 a2 = ...;

trace_foo_tcg(a1, a2);

}

This will immediately call:

void trace_foo_trans(uint8_t a1);

and will generate the TCG code to call:

void trace_foo(uint8_t a1, uint32_t a2);

=== "vcpu" ===

Identifies events that trace vCPU-specific information. It implicitly adds a

"CPUState*" argument, and extends the tracing print format to show the vCPU

information. If used together with the "tcg" property, it adds a second

"TCGv_env" argument that must point to the per-target global TCG register that

points to the vCPU when guest code is executed (usually the "cpu_env" variable).

The following example events:

foo(uint32_t a) "a=%x"

vcpu bar(uint32_t a) "a=%x"

tcg vcpu baz(uint32_t a) "a=%x", "a=%x"

Can be used as:

#include "trace-tcg.h"

CPUArchState *env;

TCGv_ptr cpu_env;

void some_disassembly_func(...)

{

/* trace emitted at this point */

trace_foo(0xd1);

/* trace emitted at this point */

trace_bar(ENV_GET_CPU(env), 0xd2);

/* trace emitted at this point (env) and when guest code is executed (cpu_env) */

trace_baz_tcg(ENV_GET_CPU(env), cpu_env, 0xd3);

}

If the translating vCPU has address 0xc1 and code is later executed by vCPU

0xc2, this would be an example output:

// at guest code translation

foo a=0xd1

bar cpu=0xc1 a=0xd2

baz_trans cpu=0xc1 a=0xd3

// at guest code execution

baz_exec cpu=0xc2 a=0xd3