5 This document describes the tracing infrastructure in QEMU and how to use it
6 for debugging, profiling, and observing execution.
10 1. Build with the 'simple' trace backend:
12 ./configure --trace-backend=simple
15 2. Create a file with the events you want to trace:
17 echo bdrv_aio_readv > /tmp/events
18 echo bdrv_aio_writev >> /tmp/events
20 3. Run the virtual machine to produce a trace file:
22 qemu -trace events=/tmp/events ... # your normal QEMU invocation
24 4. Pretty-print the binary trace file:
26 ./simpletrace.py trace-events trace-*
30 There is a set of static trace events declared in the "trace-events" source
31 file. Each trace event declaration names the event, its arguments, and the
32 format string which can be used for pretty-printing:
34 qemu_malloc(size_t size, void *ptr) "size %zu ptr %p"
35 qemu_free(void *ptr) "ptr %p"
37 The "trace-events" file is processed by the "tracetool" script during build to
38 generate code for the trace events. Trace events are invoked directly from
39 source code like this:
41 #include "trace.h" /* needed for trace event prototype */
43 void *qemu_malloc(size_t size)
46 if (!size && !allow_zero_malloc()) {
49 ptr = oom_check(malloc(size ? size : 1));
50 trace_qemu_malloc(size, ptr); /* <-- trace event */
54 === Declaring trace events ===
56 The "tracetool" script produces the trace.h header file which is included by
57 every source file that uses trace events. Since many source files include
58 trace.h, it uses a minimum of types and other header files included to keep the
59 namespace clean and compile times and dependencies down.
61 Trace events should use types as follows:
63 * Use stdint.h types for fixed-size types. Most offsets and guest memory
64 addresses are best represented with uint32_t or uint64_t. Use fixed-size
65 types over primitive types whose size may change depending on the host
66 (32-bit versus 64-bit) so trace events don't truncate values or break
69 * Use void * for pointers to structs or for arrays. The trace.h header
70 cannot include all user-defined struct declarations and it is therefore
71 necessary to use void * for pointers to structs.
73 * For everything else, use primitive scalar types (char, int, long) with the
74 appropriate signedness.
76 Format strings should reflect the types defined in the trace event. Take
77 special care to use PRId64 and PRIu64 for int64_t and uint64_t types,
78 respectively. This ensures portability between 32- and 64-bit platforms.
80 === Hints for adding new trace events ===
82 1. Trace state changes in the code. Interesting points in the code usually
83 involve a state change like starting, stopping, allocating, freeing. State
84 changes are good trace events because they can be used to understand the
85 execution of the system.
87 2. Trace guest operations. Guest I/O accesses like reading device registers
88 are good trace events because they can be used to understand guest
91 3. Use correlator fields so the context of an individual line of trace output
92 can be understood. For example, trace the pointer returned by malloc and
93 used as an argument to free. This way mallocs and frees can be matched up.
94 Trace events with no context are not very useful.
96 4. Name trace events after their function. If there are multiple trace events
97 in one function, append a unique distinguisher at the end of the name.
99 5. If specific trace events are going to be called a huge number of times, this
100 might have a noticeable performance impact even when the trace events are
101 programmatically disabled. In this case you should declare the trace event
102 with the "disable" property, which will effectively disable it at compile
103 time (using the "nop" backend).
105 == Generic interface and monitor commands ==
107 You can programmatically query and control the dynamic state of trace events
108 through a backend-agnostic interface:
112 * trace_event_set_state
113 Enables or disables trace events at runtime inside QEMU.
114 The function returns "true" if the state of the event has been successfully
115 changed, or "false" otherwise:
117 #include "trace/control.h"
119 trace_event_set_state("virtio_irq", true); /* enable */
121 trace_event_set_state("virtio_irq", false); /* disable */
123 Note that some of the backends do not provide an implementation for this
124 interface, in which case QEMU will just print a warning.
126 This functionality is also provided through monitor commands:
129 View available trace events and their state. State 1 means enabled, state 0
132 * trace-event NAME on|off
133 Enable/disable a given trace event.
135 The "-trace events=<file>" command line argument can be used to enable the
136 events listed in <file> from the very beginning of the program. This file must
137 contain one event name per line.
141 The "tracetool" script automates tedious trace event code generation and also
142 keeps the trace event declarations independent of the trace backend. The trace
143 events are not tightly coupled to a specific trace backend, such as LTTng or
144 SystemTap. Support for trace backends can be added by extending the "tracetool"
147 The trace backend is chosen at configure time and only one trace backend can
148 be built into the binary:
150 ./configure --trace-backend=simple
152 For a list of supported trace backends, try ./configure --help or see below.
154 The following subsections describe the supported trace backends.
158 The "nop" backend generates empty trace event functions so that the compiler
159 can optimize out trace events completely. This is the default and imposes no
162 Note that regardless of the selected trace backend, events with the "disable"
163 property will be generated with the "nop" backend.
167 The "stderr" backend sends trace events directly to standard error. This
168 effectively turns trace events into debug printfs.
170 This is the simplest backend and can be used together with existing code that
175 The "simple" backend supports common use cases and comes as part of the QEMU
176 source tree. It may not be as powerful as platform-specific or third-party
177 trace backends but it is portable. This is the recommended trace backend
178 unless you have specific needs for more advanced backends.
180 The "simple" backend currently does not capture string arguments, it simply
181 records the char* pointer value instead of the string that is pointed to.
183 ==== Monitor commands ====
186 Display the contents of trace buffer. This command dumps the trace buffer
187 with simple formatting. For full pretty-printing, use the simpletrace.py
188 script on a binary trace file.
190 The trace buffer is written into until full. The full trace buffer is
191 flushed and emptied. This means the 'info trace' will display few or no
192 entries if the buffer has just been flushed.
194 * trace-file on|off|flush|set <path>
195 Enable/disable/flush the trace file or set the trace file name.
197 ==== Analyzing trace files ====
199 The "simple" backend produces binary trace files that can be formatted with the
200 simpletrace.py script. The script takes the "trace-events" file and the binary
203 ./simpletrace.py trace-events trace-12345
205 You must ensure that the same "trace-events" file was used to build QEMU,
206 otherwise trace event declarations may have changed and output will not be
209 === LTTng Userspace Tracer ===
211 The "ust" backend uses the LTTng Userspace Tracer library. There are no
212 monitor commands built into QEMU, instead UST utilities should be used to list,
213 enable/disable, and dump traces.
217 The "dtrace" backend uses DTrace sdt probes but has only been tested with
218 SystemTap. When SystemTap support is detected a .stp file with wrapper probes
219 is generated to make use in scripts more convenient. This step can also be
220 performed manually after a build in order to change the binary name in the .stp
223 scripts/tracetool --dtrace --stap \
224 --binary path/to/qemu-binary \
225 --target-type system \
226 --target-arch x86_64 \
227 <trace-events >qemu.stp