Re: [PATCH v2 0/4] [RfC] fix tracing for modules

[Stefan Hajnoczi]

On Tue, Nov 24, 2020 at 05:02:51PM +0100, Gerd Hoffmann wrote:
> First version that actually works.  Only qxl covered for this RfC, other
> modules will follow once the basics are hashed out.
> 
> More context:
>   https://bugzilla.redhat.com/show_bug.cgi?id=1898700
>   https://bugzilla.redhat.com/show_bug.cgi?id=1869642
> 
> take care,
>   Gerd
> 
> Gerd Hoffmann (4):
>   meson: add trace_events_config[]
>   meson: move up hw subdir (specifically before trace subdir)
>   meson: add module_trace & module_trace_src
>   meson: move qxl trace events to separate file

Awesome, thank you for working on this!

I noticed an issue with simpletrace: the trace file does not contain
qxl_* TRACE_RECORD_TYPE_MAPPING records when ./configure
--enable-modules is used. This happens because st_write_event_mapping()
is called before the qxl module calls trace_event_register_group().

(The mapping records describe the integer ID to string name mapping used
in a simpletrace file.)

You can check this using "grep -a qxl_ trace-$LAST_QEMU_PID" after
running qemu --device qxl built with ./configure --enable-modules
--enable-trace-backend=simple.

Remove --enable-modules and the file will contain the qxl_ trace events.

This means the trace file is broken because the simpletrace records
cannot be mapped to a trace event name.

One way to solve this is by modifying trace_event_register_group() to
call into trace/simple.c (maybe with a TraceEventIter initialized to
iterate over the newly registered trace events group?).

Alternatively, simpletrace.c might be able to emit
TRACE_RECORD_TYPE_MAPPING on demand instead of ahead of time.

Stefan

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