On Thu, Mar 21, 2024 at 11:05:00AM -0700, Andrii Nakryiko wrote:
> Inline bpf_get_branch_snapshot() helper using architecture-agnostic
> inline BPF code which calls directly into underlying callback of
> perf_snapshot_branch_stack static call. This callback is set early
> during kernel initialization and is never updated or reset, so it's ok
> to fetch actual implementation using static_call_query() and call
> directly into it.
>
> This change eliminates a full function call and saves one LBR entry
> in PERF_SAMPLE_BRANCH_ANY LBR mode.
>
> Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> ---
> kernel/bpf/verifier.c | 37 +++++++++++++++++++++++++++++++++++++
> 1 file changed, 37 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index de7813947981..4fb6c468e199 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -20130,6 +20130,43 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
> goto next_insn;
> }
>
> + /* Implement bpf_get_branch_snapshot inline. */
> + if (prog->jit_requested && BITS_PER_LONG == 64 &&
> + insn->imm == BPF_FUNC_get_branch_snapshot) {
> + /* We are dealing with the following func protos:
> + * u64 bpf_get_branch_snapshot(void *buf, u32 size, u64 flags);
> + * int perf_snapshot_branch_stack(struct perf_branch_entry *entries, u32 cnt);
> + */
> + const u32 br_entry_size = sizeof(struct perf_branch_entry);
> +
> + /* if (unlikely(flags)) return -EINVAL */
> + insn_buf[0] = BPF_JMP_IMM(BPF_JNE, BPF_REG_3, 0, 5);
nit, you moved the flags check on top, which I think makes sense and
we should do it in bpf_get_branch_snapshot as well to keep it same
jirka
> + /* transform size (bytes) into entry_cnt */
> + insn_buf[1] = BPF_ALU32_IMM(BPF_DIV, BPF_REG_2, br_entry_size);
> + /* call perf_snapshot_branch_stack implementation */
> + insn_buf[2] = BPF_EMIT_CALL(static_call_query(perf_snapshot_branch_stack));
> + /* if (entry_cnt == 0) return -ENOENT */
> + insn_buf[3] = BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 4);
> + /* return entry_cnt * sizeof(struct perf_branch_entry) */
> + insn_buf[4] = BPF_ALU32_IMM(BPF_MUL, BPF_REG_0, br_entry_size);
> + insn_buf[5] = BPF_JMP_A(3);
> + /* return -EINVAL; */
> + insn_buf[6] = BPF_MOV64_IMM(BPF_REG_0, -EINVAL);
> + insn_buf[7] = BPF_JMP_A(1);
> + /* return -ENOENT; */
> + insn_buf[8] = BPF_MOV64_IMM(BPF_REG_0, -ENOENT);
> + cnt = 9;
> +
> + new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
> + if (!new_prog)
> + return -ENOMEM;
> +
> + delta += cnt - 1;
> + env->prog = prog = new_prog;
> + insn = new_prog->insnsi + i + delta;
> + continue;
> + }
> +
> /* Implement bpf_kptr_xchg inline */
> if (prog->jit_requested && BITS_PER_LONG == 64 &&
> insn->imm == BPF_FUNC_kptr_xchg &&
> --
> 2.43.0
>
>
