[PATCH v4 0/4] LoongArch bpf kptr xchg inline support

From: Chenguang Zhao <[email protected]>

This series enables the BPF verifier to inline bpf_kptr_xchg()
into BPF_XCHG on LoongArch64, and fixes the underlying JIT
atomic ordering that makes such inlining safe.

The BPF verifier can lower bpf_kptr_xchg() to a single BPF_XCHG
atomic when the JIT advertises ptr xchg support via
bpf_jit_supports_ptr_xchg(). This removes helper-call overhead
from the kptr exchange fast path. Inlining is only correct when
the JITed exchange provides the same sequentially consistent
ordering as the bpf_kptr_xchg() helper.

Patch 1 fixes memory ordering for all value-returning BPF atomic
RMW operations emitted by the LoongArch bpf JIT. Per LKMM,
value-returning atomic RMW must provide sequentially consistent
ordering. Plain AMO instructions and bare ll/sc loops on LoongArch
do not satisfy this by themselves:
 - BPF_FETCH (ADD/AND/OR/XOR): switch to am*_db.{b,h,w,d}
 - BPF_XCHG: switch to amswap_db.{b,h,w,d}
 - BPF_CMPXCHG: emit dbar 0x700 after the ll/sc loop, matching
   __WEAK_LLSC_MB in cmpxchg.h
Non-value-returning RMW ops (plain BPF_ADD, BPF_AND, etc.)
remain weakly ordered, consistent with LKMM. This fix is independent
of kptr inlining and benefits all BPF programs using value-returning
atomics on LoongArch.

Patch 2 implements bpf_jit_supports_ptr_xchg() so the verifier
may inline bpf_kptr_xchg() on LoongArch64.

Patches 3 and 4 extend bpf selftests: functional coverage via
kptr_xchg_inline, and an optional kptr-xchg benchmark to compare
helper vs inlined paths.

Chenguang Zhao (4):
  LoongArch: bpf: Fix memory ordering for value-returning atomics
  LoongArch: bpf: Advertise JIT support for kptr xchg inline
  selftests/bpf: Enable kptr_xchg_inline test on LoongArch
  selftests/bpf: Add kptr-xchg benchmark

 arch/loongarch/include/asm/inst.h             | 18 ++++
 arch/loongarch/net/bpf_jit.c                  | 37 ++++---
 tools/testing/selftests/bpf/Makefile          |  2 +
 tools/testing/selftests/bpf/bench.c           |  2 +
 .../selftests/bpf/benchs/bench_kptr_xchg.c    | 96 +++++++++++++++++++
 .../bpf/prog_tests/kptr_xchg_inline.c         |  3 +-
 .../selftests/bpf/progs/kptr_xchg_bench.c     | 49 ++++++++++
 7 files changed, 192 insertions(+), 15 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/benchs/bench_kptr_xchg.c
 create mode 100644 tools/testing/selftests/bpf/progs/kptr_xchg_bench.c

---
v4:
 - Resend patches against new kernel.

v3:
 - https://lore.kernel.org/all/[email protected]/

v2:
 - https://lore.kernel.org/all/[email protected]/

v1:
 - https://lore.kernel.org/all/[email protected]/

-- 
2.25.1