Re: [Qemu-devel] [PATCH v3 13/18] trace: dynamically allocate event IDs at runtime

[Daniel P. Berrange]

On Mon, Sep 19, 2016 at 08:05:12PM +0200, Lluís Vilanova wrote:
> Daniel P Berrange writes:
> 
> > Instead of having the code generator assign event IDs and
> > event VCPU IDs, assign them when the events are registered
> > at runtime. This will allow us allow code to be generated
> > from individual trace-events without having to figure out
> > globally unique numbering at build time.
> 
> > Signed-off-by: Daniel P. Berrange <address@hidden>
> > ---
> >  scripts/tracetool/format/events_c.py | 10 ++--------
> >  scripts/tracetool/format/events_h.py |  7 -------
> >  trace/control.c                      | 11 ++++++++++-
> >  3 files changed, 12 insertions(+), 16 deletions(-)
> 
> > diff --git a/scripts/tracetool/format/events_c.py 
> > b/scripts/tracetool/format/events_c.py
> > index 7f00b50..1b5790d 100644
> > --- a/scripts/tracetool/format/events_c.py
> > +++ b/scripts/tracetool/format/events_c.py
> > @@ -28,25 +28,19 @@ def generate(events, backend):
> >      for e in events:
> >          out('uint16_t %s;' % e.api(e.QEMU_DSTATE))
>  
> > -    next_id = 0
> > -    next_vcpu_id = 0
> >      for e in events:
> > -        id = next_id
> > -        next_id += 1
> >          if "vcpu" in e.properties:
> > -            vcpu_id = next_vcpu_id
> > -            next_vcpu_id += 1
> > +            vcpu_id = 0
> >          else:
> >              vcpu_id = "TRACE_VCPU_EVENT_NONE"
> >          out('TraceEvent %(event)s = {',
> > -            '  .id = %(id)s,',
> > +            '  .id = 0,',
> 
> Better to use some invalid value, like in TRACE_VCPU_EVENT_NONE.

I don't think it adds anything todo that - TRACE_VCPU_EVENT_NONE is
special because it is a persistent value for lifetime of QEMU
that will be used by runtime comparisons. In contrast this is
just a placeholder that gets replaced by the real value immediately
at startup in trace_event_register_group().  We never even check
the statically set value of 'id' - we just unconditionally replace
it with the real value.

Regards,
Daniel
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