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

[Eduard Zingerman <eddyz87@gmail.com>]

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?

Thanks,
Eduard

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