Tracing

Introduction

This document describes the tracing infrastructure in QEMU and how to use it for debugging, profiling, and observing execution.

Quickstart

Enable tracing of memory_region_ops_read and memory_region_ops_write events:

$ qemu --trace "memory_region_ops_*" ...
...
719585@1608130130.441188:memory_region_ops_read cpu 0 mr 0x562fdfbb3820 addr 0x3cc value 0x67 size 1
719585@1608130130.441190:memory_region_ops_write cpu 0 mr 0x562fdfbd2f00 addr 0x3d4 value 0x70e size 2

This output comes from the “log” trace backend that is enabled by default when ./configure --enable-trace-backends=BACKENDS was not explicitly specified.

Multiple patterns can be specified by repeating the --trace option:

$ qemu --trace "kvm_*" --trace "virtio_*" ...

When patterns are used frequently it is more convenient to store them in a file to avoid long command-line options:

$ echo "memory_region_ops_*" >/tmp/events
$ echo "kvm_*" >>/tmp/events
$ qemu --trace events=/tmp/events ...

Trace events

Sub-directory setup

Each directory in the source tree can declare a set of trace events in a local “trace-events” file. All directories which contain “trace-events” files must be listed in the “trace_events_subdirs” variable in the top level meson.build file. 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”. This merged file is to be used by the “simpletrace.py” script to later analyse traces in the simpletrace data format.

The following files are automatically generated in <builddir>/trace/ during the build:

  • trace-<subdir>.c - the trace event state declarations

  • trace-<subdir>.h - the trace event enums and probe functions

  • trace-dtrace-<subdir>.h - DTrace event probe specification

  • trace-dtrace-<subdir>.dtrace - DTrace event probe helper declaration

  • trace-dtrace-<subdir>.o - binary DTrace provider (generated by dtrace)

  • trace-ust-<subdir>.h - UST event probe helper declarations

Here <subdir> is the sub-directory path with ‘/’ replaced by ‘_’. For example, “accel/kvm” becomes “accel_kvm” and the final filename for “trace-<subdir>.c” becomes “trace-accel_kvm.c”.

Source files in the source tree do not directly include generated files in “<builddir>/trace/”. Instead they #include the local “trace.h” file, without any sub-directory path prefix. eg io/channel-buffer.c would do:

#include "trace.h"

The “io/trace.h” file must be created manually with an #include of the corresponding “trace/trace-<subdir>.h” file that will be generated in the builddir:

$ echo '#include "trace/trace-io.h"' >io/trace.h

While it is possible to include a trace.h file from outside a source file’s 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/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.

  • Avoid floating point types (float and double) because SystemTap does not support them. In most cases it is possible to round to an integer type instead. This may require scaling the value first by multiplying it by 1000 or the like when digits after the decimal point need to be preserved.

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. Format strings must not end with a newline character. It is the responsibility of backends to adapt line ending for proper logging.

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,dtrace

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().

The -msg timestamp=on|off command-line option controls whether or not to print the tid/timestamp prefix for each trace event.

Simpletrace

The “simple” backend writes binary trace logs to a file from a thread, making it lower overhead than the “log” backend. A Python API is available for writing offline trace file analysis scripts. It may not be as powerful as platform-specific or third-party trace backends but it is portable and has no special library dependencies.

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.

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.

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 \
                     --group=all \
                     trace-events-all \
                     qemu.stp

To facilitate simple usage of systemtap where there merely needs to be printf logging of certain probes, a helper script “qemu-trace-stap” is provided. Consult its manual page for guidance on its usage.

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 ‘trace_event_get_state_backends()’ to guard such computations, so they are skipped if the event has been either compile-time disabled or run-time disabled. If the event is compile-time disabled, this check will have no performance impact.

#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_event_get_state_backends(TRACE_QEMU_VMALLOC)) {
        void *complex;
        /* some complex computations to produce the 'complex' value */
        trace_qemu_vmalloc(size, ptr, complex);
    }
    return ptr;
}