Re: [PATCH v2 bpf-next 2/2] bpf: inline bpf_get_branch_snapshot() helper

[John Fastabend <john.fastabend@gmail.com>]

Andrii Nakryiko wrote:
> On Tue, Apr 2, 2024 at 12:05 PM Andrii Nakryiko <andrii@kernel.org> 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@kernel.org>
> > ---
> >  kernel/bpf/verifier.c | 55 +++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 55 insertions(+)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index fcb62300f407..49789da56f4b 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -20157,6 +20157,61 @@ 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);
> > +
> > +                       /* struct perf_branch_entry is part of UAPI and is
> > +                        * used as an array element, so extremely unlikely to
> > +                        * ever grow or shrink
> > +                        */
> > +                       BUILD_BUG_ON(br_entry_size != 24);
> > +
> > +                       /* if (unlikely(flags)) return -EINVAL */
> > +                       insn_buf[0] = BPF_JMP_IMM(BPF_JNE, BPF_REG_3, 0, 7);
> > +
> > +                       /* Transform size (bytes) into number of entries (cnt = size / 24).
> > +                        * But to avoid expensive division instruction, we implement
> > +                        * divide-by-3 through multiplication, followed by further
> > +                        * division by 8 through 3-bit right shift.
> > +                        * Refer to book "Hacker's Delight, 2nd ed." by Henry S. Warren, Jr.,
> > +                        * p. 227, chapter "Unsigned Divison by 3" for details and proofs.
> > +                        *
> > +                        * N / 3 <=> M * N / 2^33, where M = (2^33 + 1) / 3 = 0xaaaaaaab.
> > +                        */

Nice bit of magic. Thanks for the reference.

> > +                       insn_buf[1] = BPF_MOV32_IMM(BPF_REG_0, 0xaaaaaaab);
> > +                       insn_buf[2] = BPF_ALU64_REG(BPF_REG_2, BPF_REG_0, 0);
> 
> Doh, this should be:
> 
> insn_buf[2] = BPF_ALU64_REG(BPF_MUL, BPF_REG_2, BPF_REG_0);
> 
> I'll wait a bit for any other feedback, will retest everything on real
> hardware again, and will submit v2 tomorrow.
> 
> pw-bot: cr
> 

LGTM. With above fix,

Acked-by: John Fastabend <john.fastabend@gmail.com>

> 
> > +                       insn_buf[3] = BPF_ALU64_IMM(BPF_RSH, BPF_REG_2, 36);
> > +
> > +                       /* call perf_snapshot_branch_stack implementation */
> > +                       insn_buf[4] = BPF_EMIT_CALL(static_call_query(perf_snapshot_branch_stack));
> > +                       /* if (entry_cnt == 0) return -ENOENT */
> > +                       insn_buf[5] = BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 4);
> > +                       /* return entry_cnt * sizeof(struct perf_branch_entry) */
> > +                       insn_buf[6] = BPF_ALU32_IMM(BPF_MUL, BPF_REG_0, br_entry_size);
> > +                       insn_buf[7] = BPF_JMP_A(3);
> > +                       /* return -EINVAL; */
> > +                       insn_buf[8] = BPF_MOV64_IMM(BPF_REG_0, -EINVAL);
> > +                       insn_buf[9] = BPF_JMP_A(1);
> > +                       /* return -ENOENT; */
> > +                       insn_buf[10] = BPF_MOV64_IMM(BPF_REG_0, -ENOENT);
> > +                       cnt = 11;
> > +
> > +                       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
> >
>