Re: [RFC dwarves 3/4] pahole: add --btf_features=feature1[,feature2...] support

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Thu, Oct 12, 2023 at 01:35:41PM +0100, Alan Maguire escreveu:
> On 11/10/2023 23:14, Eduard Zingerman wrote:
> > On Wed, 2023-10-11 at 23:05 +0100, Alan Maguire wrote:
> > [...]
> >>>>> I'm not sure I understand the logic behind "skip" features.
> >>>>> Take `decl_tag` for example:
> >>>>> - by default conf_load->skip_encoding_btf_decl_tag is 0;
> >>>>> - if `--btf_features=decl_tag` is passed it is still 0 because of the
> >>>>>   `skip ? false : true` logic.
> >>>>>
> >>>>> If there is no way to change "skip" features why listing these at all?
> >>>>>
> >>>> You're right; in the case of a skip feature, I think we need the
> >>>> following behaviour
> >>>>
> >>>> 1. we skip the encoding by default (so the equivalent of
> >>>> --skip_encoding_btf_decl_tag, setting skip_encoding_btf_decl_tag
> >>>> to true
> >>>>
> >>>> 2. if the user however specifies the logical inversion of the skip
> >>>> feature in --btf_features (in this case "decl_tag" - or "all")
> >>>> skip_encoding_btf_decl_tag is set to false.
> >>>>
> >>>> So in my code we had 2 above but not 1. If both were in place I think
> >>>> we'd have the right set of behaviours. Does that sound right?
> >>>
> >>> You mean when --features=? is specified we default to
> >>> conf_load->skip_encoding_btf_decl_tag = true, and set it to false only
> >>> if "all" or "decl_tag" is listed in features, right?
> >>>
> >>
> >> Yep. Here's the comment I was thinking of adding for the next version,
> >> hopefully it clarifies this all a bit more than the original.
> >>
> >> +/* --btf_features=feature1[,feature2,..] allows us to specify
> >> + * a list of requested BTF features or "all" to enable all features.
> >> + * These are translated into the appropriate conf_load values via
> >> + * struct btf_feature which specifies the associated conf_load
> >> + * boolean field and whether its default (representing the feature being
> >> + * off) is false or true.
> >> + *
> >> + * btf_features is for opting _into_ features so for a case like
> >> + * conf_load->btf_gen_floats, the translation is simple; the presence
> >> + * of the "float" feature in --btf_features sets conf_load->btf_gen_floats
> >> + * to true.
> >> + *
> >> + * The more confusing case is for features that are enabled unless
> >> + * skipping them is specified; for example
> >> + * conf_load->skip_encoding_btf_type_tag.  By default - to support
> >> + * the opt-in model of only enabling features the user asks for -
> >> + * conf_load->skip_encoding_btf_type_tag is set to true (meaning no
> >> + * type_tags) and it is only set to false if --btf_features contains
> >> + * the "type_tag" feature.
> >> + *
> >> + * So from the user perspective, all features specified via
> >> + * --btf_features are enabled, and if a feature is not specified,
> >> + * it is disabled.
> >>   */
> > 
> > Sounds reasonable. Maybe also add a line saying that
> > skip_encoding_btf_decl_tag defaults to false if --btf_features is not
> > specified to remain backwards compatible?
> >
> 
> good idea, will do! Thanks!

I have to catch up with all the comments on this thread, but does this
mean you're respinning the series now?

- Arnaldo