nfp: bpf: support optimizing dead branches

Verifier will now optimize out branches to dead code, implement
the replace_insn callback to take advantage of that optimization.
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

drivers/net/ethernet/netronome/nfp/bpf/main.h

@@ -412,6 +412,17 @@ static inline bool is_mbpf_div(const struct nfp_insn_meta *meta)
 	return is_mbpf_alu(meta) && mbpf_op(meta) == BPF_DIV;
 }
 
+static inline bool is_mbpf_cond_jump(const struct nfp_insn_meta *meta)
+{
+	u8 op;
+
+	if (BPF_CLASS(meta->insn.code) != BPF_JMP)
+		return false;
+
+	op = BPF_OP(meta->insn.code);
+	return op != BPF_JA && op != BPF_EXIT && op != BPF_CALL;
+}
+
 static inline bool is_mbpf_helper_call(const struct nfp_insn_meta *meta)
 {
 	struct bpf_insn insn = meta->insn;
@@ -520,6 +531,9 @@ int nfp_verify_insn(struct bpf_verifier_env *env, int insn_idx,
 		    int prev_insn_idx);
 int nfp_bpf_finalize(struct bpf_verifier_env *env);
 
+int nfp_bpf_opt_replace_insn(struct bpf_verifier_env *env, u32 off,
+			     struct bpf_insn *insn);
+
 extern const struct bpf_prog_offload_ops nfp_bpf_dev_ops;
 
 struct netdev_bpf;

drivers/net/ethernet/netronome/nfp/bpf/offload.c

@@ -592,6 +592,7 @@ int nfp_net_bpf_offload(struct nfp_net *nn, struct bpf_prog *prog,
 const struct bpf_prog_offload_ops nfp_bpf_dev_ops = {
 	.insn_hook	= nfp_verify_insn,
 	.finalize	= nfp_bpf_finalize,
+	.replace_insn	= nfp_bpf_opt_replace_insn,
 	.prepare	= nfp_bpf_verifier_prep,
 	.translate	= nfp_bpf_translate,
 	.destroy	= nfp_bpf_destroy,

drivers/net/ethernet/netronome/nfp/bpf/verifier.c

@@ -786,3 +786,37 @@ int nfp_bpf_finalize(struct bpf_verifier_env *env)
 
 	return 0;
 }
+
+int nfp_bpf_opt_replace_insn(struct bpf_verifier_env *env, u32 off,
+			     struct bpf_insn *insn)
+{
+	struct nfp_prog *nfp_prog = env->prog->aux->offload->dev_priv;
+	struct bpf_insn_aux_data *aux_data = env->insn_aux_data;
+	struct nfp_insn_meta *meta = nfp_prog->verifier_meta;
+
+	meta = nfp_bpf_goto_meta(nfp_prog, meta, aux_data[off].orig_idx);
+	nfp_prog->verifier_meta = meta;
+
+	/* conditional jump to jump conversion */
+	if (is_mbpf_cond_jump(meta) &&
+	    insn->code == (BPF_JMP | BPF_JA | BPF_K)) {
+		unsigned int tgt_off;
+
+		tgt_off = off + insn->off + 1;
+
+		if (!insn->off) {
+			meta->jmp_dst = list_next_entry(meta, l);
+			meta->jump_neg_op = false;
+		} else if (meta->jmp_dst->n != aux_data[tgt_off].orig_idx) {
+			pr_vlog(env, "branch hard wire at %d changes target %d -> %d\n",
+				off, meta->jmp_dst->n,
+				aux_data[tgt_off].orig_idx);
+			return -EINVAL;
+		}
+		return 0;
+	}
+
+	pr_vlog(env, "unsupported instruction replacement %hhx -> %hhx\n",
+		meta->insn.code, insn->code);
+	return -EINVAL;
+}