[RFC PATCH bpf-next 0/2] bpf: Introduce global percpu data

[Leon Hwang <leon.hwang@linux.dev>]

This patch set introduces global per-CPU data, similar to commit
6316f78306c1 ("Merge branch 'support-global-data'"), to reduce restrictions
in C for BPF programs.

With this enhancement, it becomes possible to define and use global per-CPU
variables, much like the DEFINE_PER_CPU() macro in the kernel[0].

The idea stems from the bpflbr project[1], which itself was inspired by
retsnoop[2]. During testing of bpflbr on the v6.6 kernel, two LBR
(Last Branch Record) entries were observed related to the
bpf_get_smp_processor_id() helper.

Since commit 1ae6921009e5 ("bpf: inline bpf_get_smp_processor_id() helper"),
the bpf_get_smp_processor_id() helper has been inlined on x86_64, reducing
the overhead and consequently minimizing these two LBR records.

However, the introduction of global per-CPU data offers a more robust
solution. By leveraging the percpu_array map and percpu instructions,
global per-CPU data can be implemented intrinsically.

This feature also facilitates sharing per-CPU information between tail
callers and callees or between freplace callers and callees through a
shared global per-CPU variable. Previously, this was achieved using a
1-entry percpu map, which this patch set aims to improve upon.

Links:
[0] https://github.com/torvalds/linux/blob/fbfd64d25c7af3b8695201ebc85efe90be28c5a3/include/linux/percpu-defs.h#L114
[1] https://github.com/Asphaltt/bpflbr
[2] https://github.com/anakryiko/retsnoop

Leon Hwang (2):
  bpf: Introduce global percpu data
  selftests/bpf: Add a case to test global percpu data

 kernel/bpf/arraymap.c                         |  39 +++++-
 kernel/bpf/verifier.c                         |  45 +++++++
 tools/lib/bpf/libbpf.c                        | 112 ++++++++++++++----
 .../bpf/prog_tests/global_data_init.c         |  83 ++++++++++++-
 .../bpf/progs/test_global_percpu_data.c       |  21 ++++
 5 files changed, 274 insertions(+), 26 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/test_global_percpu_data.c

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2.47.1