On Thu, Mar 21, 2024 at 2:08 PM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
>
> 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
>
here I could control that it won't be taken in common case, so if we
are to do that in BPF helper itself, we should use unlikely() and
check that compiler actually honored it. I can add that in the next
revision.
> 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
> >
> >
