Re: [Qemu-devel] [PATCH for-2.8 v1 60/60] trace: update docs to reflect new code generation approach

[Daniel P. Berrange]

On Wed, Aug 10, 2016 at 04:33:47PM +0200, Lluís Vilanova wrote:
> Daniel P Berrange writes:
> 
> > Describe use of per-subdir trace events files and how it impacts
> > code generation.
> 
> > Signed-off-by: Daniel P. Berrange <address@hidden>
> > ---
> >  docs/tracing.txt | 61 
> > +++++++++++++++++++++++++++++++++++++++++++++++---------
> >  1 file changed, 52 insertions(+), 9 deletions(-)
> 
> > diff --git a/docs/tracing.txt b/docs/tracing.txt
> > index 29f2f9a..b7ba179 100644
> > --- a/docs/tracing.txt
> > +++ b/docs/tracing.txt
> > @@ -27,18 +27,51 @@ for debugging, profiling, and observing execution.
>  
> >  == Trace events ==
>  
> > +=== Sub-directory setup ===
> > +
> >  Each directory in the source tree can declare a set of static trace events
> > -in a "trace-events" file. Each trace event declaration names the event, its
> > -arguments, and the format string which can be used for pretty-printing:
> > +in a "trace-events" file. The first (non-comment) statement in the file
> > +must be "@id_offset(NN)" where NN is an integer that is unique among all
> > +"trace-events" files in the QEMU source tree. The IDs can be arbitrarily
> > +chosen, but extra fun the current "trace-events" files use values from the
> > +"powerful number" integer sequence :-)
> 
> Can we get rid of the "@id_offset(NN)"? If we're generating a trace-events-all
> all the same, we can use that to generate all the per-directory file *and* 
> still
> keep the simpler flat event ids.

One thing that occurred to me is that we could take a hash of the
group name. Since we have few enough sub-dirs / groups, and a large
enough ID space, it ought to be possible to get a hash which is
unique. The trace tool can double check that all groups get a unique
hash

> For example, first we concatenate /trace-events, /io/trace-events and
> /hw/usb/trace-events into /trace-events-all to produce:
> 
>    @group core
>    @dir /
>    # contents of /trace-events
>    @group io
>    @dir /io
>    # contents of /io/trace-events
>    @group hw_usb
>    @dir /hw/usb
>    # contents of /hw/usb/trace-events
> 
> Then we can use /trace-events-all to generate all the per-directory trace.h
> files while ensuring a perfectly flat event identifier space (a single
> generated-events.c is used).
> 
> This should minimize changes to tracing code and avoid further restrictions
> (like the "top-level vcpu" one).

That means when any individual trace file is changed, we re-generate the
world, which is exactly what this series is designed to avoid.

Regards,
Daniel
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