Re: [Qemu-block] [Qemu-devel] [PATCH v14 03/21] qapi: add trace events for visitor

[Daniel P. Berrange]

On Fri, Oct 07, 2016 at 03:59:07PM +0200, Markus Armbruster wrote:
> "Daniel P. Berrange" <address@hidden> writes:
> 
> > Allow tracing of the operation of visitors
> >
> > Signed-off-by: Daniel P. Berrange <address@hidden>
> > ---
> >  Makefile.objs          |  1 +
> >  qapi/qapi-visit-core.c | 27 +++++++++++++++++++++++++++
> >  qapi/trace-events      | 33 +++++++++++++++++++++++++++++++++
> >  3 files changed, 61 insertions(+)
> >  create mode 100644 qapi/trace-events
> >
> > diff --git a/Makefile.objs b/Makefile.objs
> > index a8e0224..b3e8aef 100644
> > --- a/Makefile.objs
> > +++ b/Makefile.objs
> > @@ -160,3 +160,4 @@ trace-events-y += target-s390x/trace-events
> >  trace-events-y += target-ppc/trace-events
> >  trace-events-y += qom/trace-events
> >  trace-events-y += linux-user/trace-events
> > +trace-events-y += qapi/trace-events
> > diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c
> > index 55f5876..bfcb276 100644
> > --- a/qapi/qapi-visit-core.c
> > +++ b/qapi/qapi-visit-core.c
> > @@ -19,10 +19,12 @@
> >  #include "qapi/qmp/qerror.h"
> >  #include "qapi/visitor.h"
> >  #include "qapi/visitor-impl.h"
> > +#include "trace.h"
> >  
> >  void visit_complete(Visitor *v, void *opaque)
> >  {
> >      assert(v->type != VISITOR_OUTPUT || v->complete);
> > +    trace_visit_complete(v, opaque);
> 
> Trace after or before checking the precondition?  Preferences, anyone?

I'm ambivalent, as the assert will crash you either way so
whether you get a trace event just before the crash seems
mostly irrelevant to me - the abort stack trace is what
you'll use to diagnose the crash.

> 
> >      if (v->complete) {
> >          v->complete(v, opaque);
> >      }
> > @@ -30,6 +32,7 @@ void visit_complete(Visitor *v, void *opaque)
> >  
> >  void visit_free(Visitor *v)
> >  {
> > +    trace_visit_free(v);
> >      if (v) {
> >          v->free(v);
> >      }
> > @@ -40,6 +43,7 @@ void visit_start_struct(Visitor *v, const char *name, 
> > void **obj,
> >  {
> >      Error *err = NULL;
> >  
> > +    trace_visit_start_struct(v, name, obj, size);
> >      if (obj) {
> >          assert(size);
> >          assert(!(v->type & VISITOR_OUTPUT) || *obj);
> > @@ -53,6 +57,7 @@ void visit_start_struct(Visitor *v, const char *name, 
> > void **obj,
> >  
> >  void visit_check_struct(Visitor *v, Error **errp)
> >  {
> > +    trace_visit_check_struct(v);
> >      if (v->check_struct) {
> >          v->check_struct(v, errp);
> >      }
> > @@ -60,6 +65,7 @@ void visit_check_struct(Visitor *v, Error **errp)
> >  
> >  void visit_end_struct(Visitor *v, void **obj)
> >  {
> > +    trace_visit_end_struct(v, obj);
> >      v->end_struct(v, obj);
> >  }
> >  
> > @@ -69,6 +75,7 @@ void visit_start_list(Visitor *v, const char *name, 
> > GenericList **list,
> >      Error *err = NULL;
> >  
> >      assert(!list || size >= sizeof(GenericList));
> > +    trace_visit_start_list(v, name, list, size);
> >      v->start_list(v, name, list, size, &err);
> >      if (list && (v->type & VISITOR_INPUT)) {
> >          assert(!(err && *list));
> > @@ -79,11 +86,13 @@ void visit_start_list(Visitor *v, const char *name, 
> > GenericList **list,
> >  GenericList *visit_next_list(Visitor *v, GenericList *tail, size_t size)
> >  {
> >      assert(tail && size >= sizeof(GenericList));
> > +    trace_visit_next_list(v, tail, size);
> >      return v->next_list(v, tail, size);
> >  }
> >  
> >  void visit_end_list(Visitor *v, void **obj)
> >  {
> > +    trace_visit_end_list(v, obj);
> >      v->end_list(v, obj);
> >  }
> >  
> > @@ -95,6 +104,7 @@ void visit_start_alternate(Visitor *v, const char *name,
> >  
> >      assert(obj && size >= sizeof(GenericAlternate));
> >      assert(!(v->type & VISITOR_OUTPUT) || *obj);
> > +    trace_visit_start_alternate(v, name, obj, size, promote_int);
> >      if (v->start_alternate) {
> >          v->start_alternate(v, name, obj, size, promote_int, &err);
> >      }
> > @@ -106,6 +116,7 @@ void visit_start_alternate(Visitor *v, const char *name,
> >  
> >  void visit_end_alternate(Visitor *v, void **obj)
> >  {
> > +    trace_visit_end_alternate(v, obj);
> >      if (v->end_alternate) {
> >          v->end_alternate(v, obj);
> >      }
> > @@ -113,6 +124,7 @@ void visit_end_alternate(Visitor *v, void **obj)
> >  
> >  bool visit_optional(Visitor *v, const char *name, bool *present)
> >  {
> > +    trace_visit_optional(v, name, present);
> >      if (v->optional) {
> >          v->optional(v, name, present);
> >      }
> > @@ -127,6 +139,7 @@ bool visit_is_input(Visitor *v)
> >  void visit_type_int(Visitor *v, const char *name, int64_t *obj, Error 
> > **errp)
> >  {
> >      assert(obj);
> > +    trace_visit_type_int(v, name, obj);
> >      v->type_int64(v, name, obj, errp);
> >  }
> >  
> > @@ -151,6 +164,7 @@ void visit_type_uint8(Visitor *v, const char *name, 
> > uint8_t *obj,
> >                        Error **errp)
> >  {
> >      uint64_t value = *obj;
> > +    trace_visit_type_uint8(v, name, obj);
> >      visit_type_uintN(v, &value, name, UINT8_MAX, "uint8_t", errp);
> >      *obj = value;
> >  }
> 
> Putting the trace in the middle of the "value = *obj;
> visit_type_uintN(v, &value, ...); *obj = value" pattern makes it less
> visible.
> 
> Preserving the pattern requires replacing the initializer by an
> assignment:
> 
>        uint64_t value;
> 
>        trace_visit_type_uint8(v, name, obj);
>        value = *obj;
>        visit_type_uintN(v, &value, name, UINT8_MAX, "uint8_t", errp);
>        *obj = value;
> 
> Looks slightly more legible to me.  What do you think?

I'm fine with whatever you prefer.

Regards,
Daniel
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