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

Queued the first 3 patches, since the last one is not LoongArch-specific.

Huacai

On Wed, Jul 29, 2026 at 10:29 AM Chenguang Zhao
<[email protected]> wrote:
>
> 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
>
>