Archipelago » qemu

= 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-backend=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:

    ./simpletrace.py trace-events trace-*

== Trace events ==

There is a set of static trace events declared in the "trace-events" source

file.  Each trace event declaration names the event, its arguments, and the

format string which can be used for pretty-printing:

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

    qemu_vfree(void *ptr) "ptr %p"

The "trace-events" file is processed by the "tracetool" script during build to

generate code for the 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.

=== 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 dynamic state of trace events

through a backend-agnostic interface:

* trace_print_events

* trace_event_set_state

  Enables or disables trace events at runtime inside QEMU.

  The function returns "true" if the state of the event has been successfully

  changed, or "false" otherwise:

    #include "trace/control.h"

    trace_event_set_state("virtio_irq", true); /* enable */

    [...]

    trace_event_set_state("virtio_irq", false); /* disable */

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

interface, in which case QEMU will just print a warning.

This functionality is also provided 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 having common prefix

  through wildcard.

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.

A basic 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:

  trace-event virtio_blk_* on

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.

== 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 backend is chosen at configure time and only one trace backend can

be built into the binary:

    ./configure --trace-backend=simple

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

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 is the default and imposes no

performance penalty.

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

property will be generated with the "nop" backend.

=== Stderr ===

The "stderr" 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.

The "simple" backend currently does not capture string arguments, it simply

records the char* pointer value instead of the string that is pointed to.

==== 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" file and the binary

trace:

    ./simpletrace.py trace-events trace-12345

You must ensure that the same "trace-events" 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.

=== 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 --dtrace --stap \

                      --binary path/to/qemu-binary \

                      --target-type system \

                      --target-arch x86_64 \

                      <trace-events >qemu.stp

== Trace event properties ==

Each event in the "trace-events" 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" 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;

    }