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 the
  58 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    Pointers (including char *) cannot be dereferenced easily (or at all) in
  73    some trace backends.  If pointers are used, ensure they are meaningful by
  74    themselves and do not assume the data they point to will be traced.  Do
  75    not pass in string arguments.
  76
  77  * For everything else, use primitive scalar types (char, int, long) with the
  78    appropriate signedness.
  79
  80 Format strings should reflect the types defined in the trace event.  Take
  81 special care to use PRId64 and PRIu64 for int64_t and uint64_t types,
  82 respectively.  This ensures portability between 32- and 64-bit platforms.  Note
  83 that format strings must begin and end with double quotes.  When using
  84 portability macros, ensure they are preceded and followed by double quotes:
  85 "value %"PRIx64"".
  86
  87 === Hints for adding new trace events ===
  88
  89 1. Trace state changes in the code.  Interesting points in the code usually
  90    involve a state change like starting, stopping, allocating, freeing.  State
  91    changes are good trace events because they can be used to understand the
  92    execution of the system.
  93
  94 2. Trace guest operations.  Guest I/O accesses like reading device registers
  95    are good trace events because they can be used to understand guest
  96    interactions.
  97
  98 3. Use correlator fields so the context of an individual line of trace output
  99    can be understood.  For example, trace the pointer returned by malloc and
 100    used as an argument to free.  This way mallocs and frees can be matched up.
 101    Trace events with no context are not very useful.
 102
 103 4. Name trace events after their function.  If there are multiple trace events
 104    in one function, append a unique distinguisher at the end of the name.
 105
 106 5. Declare trace events with the "disable" keyword.  Some trace events can
 107    produce a lot of output and users are typically only interested in a subset
 108    of trace events.  Marking trace events disabled by default saves the user
 109    from having to manually disable noisy trace events.
 110
 111 == Trace backends ==
 112
 113 The "tracetool" script automates tedious trace event code generation and also
 114 keeps the trace event declarations independent of the trace backend.  The trace
 115 events are not tightly coupled to a specific trace backend, such as LTTng or
 116 SystemTap.  Support for trace backends can be added by extending the "tracetool"
 117 script.
 118
 119 The trace backend is chosen at configure time and only one trace backend can
 120 be built into the binary:
 121
 122     ./configure --trace-backend=simple
 123
 124 For a list of supported trace backends, try ./configure --help or see below.
 125
 126 The following subsections describe the supported trace backends.
 127
 128 === Nop ===
 129
 130 The "nop" backend generates empty trace event functions so that the compiler
 131 can optimize out trace events completely.  This is the default and imposes no
 132 performance penalty.
 133
 134 === Stderr ===
 135
 136 The "stderr" backend sends trace events directly to standard error.  This
 137 effectively turns trace events into debug printfs.
 138
 139 This is the simplest backend and can be used together with existing code that
 140 uses DPRINTF().
 141
 142 === Simpletrace ===
 143
 144 The "simple" backend supports common use cases and comes as part of the QEMU
 145 source tree.  It may not be as powerful as platform-specific or third-party
 146 trace backends but it is portable.  This is the recommended trace backend
 147 unless you have specific needs for more advanced backends.
 148
 149 ==== Monitor commands ====
 150
 151 * info trace
 152   Display the contents of trace buffer.  This command dumps the trace buffer
 153   with simple formatting.  For full pretty-printing, use the simpletrace.py
 154   script on a binary trace file.
 155
 156   The trace buffer is written into until full.  The full trace buffer is
 157   flushed and emptied.  This means the 'info trace' will display few or no
 158   entries if the buffer has just been flushed.
 159
 160 * info trace-events
 161   View available trace events and their state.  State 1 means enabled, state 0
 162   means disabled.
 163
 164 * trace-event NAME on|off
 165   Enable/disable a given trace event.
 166
 167 * trace-file on|off|flush|set <path>
 168   Enable/disable/flush the trace file or set the trace file name.
 169
 170 ==== Enabling/disabling trace events programmatically ====
 171
 172 The st_change_trace_event_state() function can be used to enable or disable trace
 173 events at runtime inside QEMU:
 174
 175     #include "trace.h"
 176
 177     st_change_trace_event_state("virtio_irq", true); /* enable */
 178     [...]
 179     st_change_trace_event_state("virtio_irq", false); /* disable */
 180
 181 ==== Analyzing trace files ====
 182
 183 The "simple" backend produces binary trace files that can be formatted with the
 184 simpletrace.py script.  The script takes the "trace-events" file and the binary
 185 trace:
 186
 187     ./simpletrace.py trace-events trace-12345
 188
 189 You must ensure that the same "trace-events" file was used to build QEMU,
 190 otherwise trace event declarations may have changed and output will not be
 191 consistent.
 192
 193 === LTTng Userspace Tracer ===
 194
 195 The "ust" backend uses the LTTng Userspace Tracer library.  There are no
 196 monitor commands built into QEMU, instead UST utilities should be used to list,
 197 enable/disable, and dump traces.
 198
 199 === SystemTap ===
 200
 201 The "dtrace" backend uses DTrace sdt probes but has only been tested with
 202 SystemTap.  When SystemTap support is detected a .stp file with wrapper probes
 203 is generated to make use in scripts more convenient.  This step can also be
 204 performed manually after a build in order to change the binary name in the .stp
 205 probes:
 206
 207     scripts/tracetool --dtrace --stap \
 208                       --binary path/to/qemu-binary \
 209                       --target-type system \
 210                       --target-arch x86_64 \
 211                       <trace-events >qemu.stp