On Wed, Oct 11, 2023 at 2:17 AM Alan Maguire <alan.maguire@xxxxxxxxxx> wrote:
>
> Currently, the kernel uses pahole version checking as the way to
> determine which BTF encoding features to request from pahole. This
> means that such features have to be tied to a specific version and
> as new features are added, additional clauses in scripts/pahole-flags.sh
> have to be added; for example
>
> if [ "${pahole_ver}" -ge "125" ]; then
> extra_paholeopt="${extra_paholeopt} --skip_encoding_btf_inconsistent_proto --btf_gen_optimized"
> fi
>
> To better future-proof this process, we can have a catch-all
> single "btf_features" parameter that uses a comma-separated list
> of encoding options. What makes this better is that any version
> of pahole that supports btf_features can accept the option list;
> unknown options are silently ignored. However there would be no need
> to add additional version clauses beyond
>
> if [ "${pahole_ver}" -ge "126" ]; then
> extra_pahole_opt="-j --lang_exclude=rust
> --btf_features=encode_force,var,float,decl_tag,type_tag,enum64,optimized,consistent"
I've seen a nice convention used in a lot of CLIs that allow multiple
values for a given parameter:
--btf_feature=encode_force --btf=var --btf=float and so on
It simplifies parsing code, and is also a bit more "modular" in
scripts. But it's just a minor nit, feel free to ignore.
> fi
>
> Newly-supported features would simply be appended to the btf_features
> list, and these would have impact on BTF encoding only if the features
> were supported by pahole. This means pahole will not require a version
> bump when new BTF features are added, and should ease the burden of
> coordinating such changes between bpf-next and dwarves.
Yep, this is great.
>
> Patches 1 and 2 are preparatory work, while patch 3 adds the
> --btf_features support. Patch 4 provides a means of querying
> the supported feature set since --btf_features will not error
> out when it encounters an unrecognized features (this ensures
> an older pahole without a requested feature will not dump warnings
> in the build log for kernel/module BTF generation).
List of features is great. I think we can also have something like
--btf_features_strict to make pahole error out if an unsupported
feature is specified.
>
> See [1] for more background on this topic.
>
> [1] https://lore.kernel.org/bpf/CAEf4Bzaz1UqqxuZ7Q+KQee-HLyY1nwhAurBE2n9YTWchqoYLbg@xxxxxxxxxxxxxx/
>
> Alan Maguire (4):
> btf_encoder, pahole: move btf encoding options into conf_load
> dwarves: move ARRAY_SIZE() to dwarves.h
> pahole: add --btf_features=feature1[,feature2...] support
> pahole: add --supported_btf_features to display feature support
>
> btf_encoder.c | 8 +--
> btf_encoder.h | 2 +-
> dwarves.c | 16 ------
> dwarves.h | 19 +++++++
> man-pages/pahole.1 | 24 +++++++++
> pahole.c | 127 ++++++++++++++++++++++++++++++++++++++++-----
> 6 files changed, 162 insertions(+), 34 deletions(-)
>
> --
> 2.31.1
>
