Re: [Qemu-devel] [PATCH] trace: Clarify DTrace/SystemTap help message

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 8/15/19 4:45 PM, Stefan Hajnoczi wrote:
> On Thu, Aug 15, 2019 at 02:02:47PM +0200, Philippe Mathieu-Daudé wrote:
>> Most tracing backends are implemented within QEMU, except the
>> DTrace/SystemTap backends.
>>
>> One side effect is when running 'qemu -trace help', an incomplete
>> list of trace events is displayed when using the DTrace/SystemTap
>> backends.
>>
>> This is partly due to trace events registered as modules with
>> trace_init(), and since the events are not used within QEMU,
>> the linker optimize and remove the unused modules (which is
>> OK in this particular case).
>> Currently only the events compiled in trace-root.o and in the
>> last trace.o member of libqemuutil.a are linked, resulting in
>> an incomplete list of events.
>>
>> To avoid confusion, improve the help message, recommending to
>> use the proper systemtap script to display the events list.
>>
>> Before:
>>
>>   $ lm32-softmmu/qemu-system-lm32 -trace help 2>&1 | wc -l
>>   70
>>
>> After:
>>
>>   $ lm32-softmmu/qemu-system-lm32 -trace help
>>   Run 'qemu-trace-stap list qemu-system-lm32' to print a list
>>   of names of trace points with the DTrace/SystemTap backends.
>>
>>   $ qemu-trace-stap list qemu-system-lm32 | wc -l
>>   1136
>>
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>> ---
>>  trace/control.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/trace/control.c b/trace/control.c
>> index 43fb7868db..bc2fe0859d 100644
>> --- a/trace/control.c
>> +++ b/trace/control.c
>> @@ -159,12 +159,19 @@ TraceEvent *trace_event_iter_next(TraceEventIter *iter)
>>  
>>  void trace_list_events(void)
>>  {
>> +#ifdef CONFIG_TRACE_DTRACE
>> +    fprintf(stderr, "Run 'qemu-trace-stap list %s' to print a list\n"
>> +                    "of names of trace points with the DTrace/SystemTap"
>> +                    " backends.\n",
>> +                    error_get_progname());
>> +#else
>>      TraceEventIter iter;
>>      TraceEvent *ev;
>>      trace_event_iter_init(&iter, NULL);
>>      while ((ev = trace_event_iter_next(&iter)) != NULL) {
>>          fprintf(stderr, "%s\n", trace_event_get_name(ev));
>>      }
>> +#endif
> 
> Multiple trace backends can be built into QEMU.  In that case the list

I did not know, that explains the final 's' to the
--enable-trace-backends option.

The "== Trace backends ==" of docs/devel/tracing.txt is not clear about
this.

> might be complete and the user may not be using stap at all.  Perhaps
> the message should be turned into a warning instead and the list should
> still be printed:
> 
>   This list of trace events may be incompletel.  Run 'qemu-trace-stap
>   list %s' to print a list of names of trace events with the
>   DTrace/SystemTap backends.

OK, thanks!

Phil.