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

[Alan Maguire <alan.maguire@oracle.com>]

On 13/10/2023 15:17, Arnaldo Carvalho de Melo wrote:
> 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?
>

Yep, I'm respinning now. There were a few small things in Andrii's
suggestions that we might need to figure out - mostly around names - but
there's enough change since the RFC that it would probably be best to
discuss those with the v2 series. Thanks!

Alan
> - Arnaldo
>