docs/tracing.txt

   1 = Tracing =
   2
   3 == Introduction ==
   4
   5 This document describes the tracing infrastructure in QEMU and how to use it
   6 for debugging, profiling, and observing execution.
   7
   8 == Quickstart ==
   9
  10 1. Build with the 'simple' trace backend:
  11
  12     ./configure --trace-backend=simple
  13     make
  14
  15 2. Enable trace events you are interested in:
  16
  17     $EDITOR trace-events  # remove "disable" from events you want
  18
  19 3. Run the virtual machine to produce a trace file:
  20
  21     qemu ... # your normal QEMU invocation
  22
  23 4. Pretty-print the binary trace file:
  24
  25     ./simpletrace.py trace-events trace-*
  26
  27 == Trace events ==
  28
  29 There is a set of static trace events declared in the trace-events source
  30 file.  Each trace event declaration names the event, its arguments, and the
  31 format string which can be used for pretty-printing:
  32
  33     qemu_malloc(size_t size, void *ptr) "size %zu ptr %p"
  34     qemu_free(void *ptr) "ptr %p"
  35
  36 The trace-events file is processed by the tracetool script during build to
  37 generate code for the trace events.  Trace events are invoked directly from
  38 source code like this:
  39
  40     #include "trace.h"  /* needed for trace event prototype */
  41
  42     void *qemu_malloc(size_t size)
  43     {
  44         void *ptr;
  45         if (!size && !allow_zero_malloc()) {
  46             abort();
  47         }
  48         ptr = oom_check(malloc(size ? size : 1));
  49         trace_qemu_malloc(size, ptr);  /* <-- trace event */
  50         return ptr;
  51     }
  52
  53 === Declaring trace events ===
  54
  55 The tracetool script produces the trace.h header file which is included by
  56 every source file that uses trace events.  Since many source files include
  57 trace.h, it uses a minimum of types and other header files included to keep
  58 the namespace clean and compile times and dependencies down.
  59
  60 Trace events should use types as follows:
  61
  62  * Use stdint.h types for fixed-size types.  Most offsets and guest memory
  63    addresses are best represented with uint32_t or uint64_t.  Use fixed-size
  64    types over primitive types whose size may change depending on the host
  65    (32-bit versus 64-bit) so trace events don't truncate values or break
  66    the build.
  67
  68  * Use void * for pointers to structs or for arrays.  The trace.h header
  69    cannot include all user-defined struct declarations and it is therefore
  70    necessary to use void * for pointers to structs.
  71
  72  * For everything else, use primitive scalar types (char, int, long) with the
  73    appropriate signedness.
  74
  75 Format strings should reflect the types defined in the trace event.  Take
  76 special care to use PRId64 and PRIu64 for int64_t and uint64_t types,
  77 respectively.  This ensures portability between 32- and 64-bit platforms.  Note
  78 that format strings must begin and end with double quotes.  When using
  79 portability macros, ensure they are preceded and followed by double quotes:
  80 "value %"PRIx64"".
  81
  82 === Hints for adding new trace events ===
  83
  84 1. Trace state changes in the code.  Interesting points in the code usually
  85    involve a state change like starting, stopping, allocating, freeing.  State
  86    changes are good trace events because they can be used to understand the
  87    execution of the system.
  88
  89 2. Trace guest operations.  Guest I/O accesses like reading device registers
  90    are good trace events because they can be used to understand guest
  91    interactions.
  92
  93 3. Use correlator fields so the context of an individual line of trace output
  94    can be understood.  For example, trace the pointer returned by malloc and
  95    used as an argument to free.  This way mallocs and frees can be matched up.
  96    Trace events with no context are not very useful.
  97
  98 4. Name trace events after their function.  If there are multiple trace events
  99    in one function, append a unique distinguisher at the end of the name.
 100
 101 5. Declare trace events with the "disable" keyword.  Some trace events can
 102    produce a lot of output and users are typically only interested in a subset
 103    of trace events.  Marking trace events disabled by default saves the user
 104    from having to manually disable noisy trace events.
 105
 106 == Trace backends ==
 107
 108 The tracetool script automates tedious trace event code generation and also
 109 keeps the trace event declarations independent of the trace backend.  The trace
 110 events are not tightly coupled to a specific trace backend, such as LTTng or
 111 SystemTap.  Support for trace backends can be added by extending the tracetool
 112 script.
 113
 114 The trace backend is chosen at configure time and only one trace backend can
 115 be built into the binary:
 116
 117     ./configure --trace-backend=simple
 118
 119 For a list of supported trace backends, try ./configure --help or see below.
 120
 121 The following subsections describe the supported trace backends.
 122
 123 === Nop ===
 124
 125 The "nop" backend generates empty trace event functions so that the compiler
 126 can optimize out trace events completely.  This is the default and imposes no
 127 performance penalty.
 128
 129 === Stderr ===
 130
 131 The "stderr" backend sends trace events directly to standard error.  This
 132 effectively turns trace events into debug printfs.
 133
 134 This is the simplest backend and can be used together with existing code that
 135 uses DPRINTF().
 136
 137 === Simpletrace ===
 138
 139 The "simple" backend supports common use cases and comes as part of the QEMU
 140 source tree.  It may not be as powerful as platform-specific or third-party
 141 trace backends but it is portable.  This is the recommended trace backend
 142 unless you have specific needs for more advanced backends.
 143
 144 ==== Monitor commands ====
 145
 146 * info trace
 147   Display the contents of trace buffer.  This command dumps the trace buffer
 148   with simple formatting.  For full pretty-printing, use the simpletrace.py
 149   script on a binary trace file.
 150
 151   The trace buffer is written into until full.  The full trace buffer is
 152   flushed and emptied.  This means the 'info trace' will display few or no
 153   entries if the buffer has just been flushed.
 154
 155 * info trace-events
 156   View available trace events and their state.  State 1 means enabled, state 0
 157   means disabled.
 158
 159 * trace-event NAME on|off
 160   Enable/disable a given trace event.
 161
 162 * trace-file on|off|flush|set <path>
 163   Enable/disable/flush the trace file or set the trace file name.
 164
 165 ==== Enabling/disabling trace events programmatically ====
 166
 167 The st_change_trace_event_state() function can be used to enable or disable trace
 168 events at runtime inside QEMU:
 169
 170     #include "trace.h"
 171
 172     st_change_trace_event_state("virtio_irq", true); /* enable */
 173     [...]
 174     st_change_trace_event_state("virtio_irq", false); /* disable */
 175
 176 ==== Analyzing trace files ====
 177
 178 The "simple" backend produces binary trace files that can be formatted with the
 179 simpletrace.py script.  The script takes the trace-events file and the binary
 180 trace:
 181
 182     ./simpletrace.py trace-events trace-12345
 183
 184 You must ensure that the same trace-events file was used to build QEMU,
 185 otherwise trace event declarations may have changed and output will not be
 186 consistent.
 187
 188 === LTTng Userspace Tracer ===
 189
 190 The "ust" backend uses the LTTng Userspace Tracer library.  There are no
 191 monitor commands built into QEMU, instead UST utilities should be used to list,
 192 enable/disable, and dump traces.
 193
 194 === SystemTap ===
 195
 196 The "dtrace" backend uses DTrace sdt probes but has only been tested with
 197 SystemTap.  When SystemTap support is detected a .stp file with wrapper probes
 198 is generated to make use in scripts more convenient.  This step can also be
 199 performed manually after a build in order to change the binary name in the .stp
 200 probes:
 201
 202     scripts/tracetool --dtrace --stap \
 203                       --binary path/to/qemu-binary \
 204                       --target-type system \
 205                       --target-arch x86_64 \
 206                       <trace-events >qemu.stp