bpf: Inline calls to bpf_loop when callback is known

Calls to `bpf_loop` are replaced with direct loops to avoid
indirection. E.g. the following:

  bpf_loop(10, foo, NULL, 0);

Is replaced by equivalent of the following:

  for (int i = 0; i < 10; ++i)
    foo(i, NULL);

This transformation could be applied when:
- callback is known and does not change during program execution;
- flags passed to `bpf_loop` are always zero.

Inlining logic works as follows:

- During execution simulation function `update_loop_inline_state`
  tracks the following information for each `bpf_loop` call
  instruction:
  - is callback known and constant?
  - are flags constant and zero?
- Function `optimize_bpf_loop` increases stack depth for functions
  where `bpf_loop` calls can be inlined and invokes `inline_bpf_loop`
  to apply the inlining. The additional stack space is used to spill
  registers R6, R7 and R8. These registers are used as loop counter,
  loop maximal bound and callback context parameter;

Measurements using `benchs/run_bench_bpf_loop.sh` inside QEMU / KVM on
i7-4710HQ CPU show a drop in latency from 14 ns/op to 2 ns/op.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/r/20220620235344.569325-4-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -1286,6 +1286,9 @@ struct bpf_array {
 #define BPF_COMPLEXITY_LIMIT_INSNS      1000000 /* yes. 1M insns */
 #define MAX_TAIL_CALL_CNT 33
 
+/* Maximum number of loops for bpf_loop */
+#define BPF_MAX_LOOPS	BIT(23)
+
 #define BPF_F_ACCESS_MASK	(BPF_F_RDONLY |		\
 				 BPF_F_RDONLY_PROG |	\
 				 BPF_F_WRONLY |		\

include/linux/bpf_verifier.h

@@ -344,6 +344,14 @@ struct bpf_verifier_state_list {
 	int miss_cnt, hit_cnt;
 };
 
+struct bpf_loop_inline_state {
+	int initialized:1; /* set to true upon first entry */
+	int fit_for_inline:1; /* true if callback function is the same
+			       * at each call and flags are always zero
+			       */
+	u32 callback_subprogno; /* valid when fit_for_inline is true */
+};
+
 /* Possible states for alu_state member. */
 #define BPF_ALU_SANITIZE_SRC		(1U << 0)
 #define BPF_ALU_SANITIZE_DST		(1U << 1)
@@ -373,6 +381,10 @@ struct bpf_insn_aux_data {
 				u32 mem_size;	/* mem_size for non-struct typed var */
 			};
 		} btf_var;
+		/* if instruction is a call to bpf_loop this field tracks
+		 * the state of the relevant registers to make decision about inlining
+		 */
+		struct bpf_loop_inline_state loop_inline_state;
 	};
 	u64 map_key_state; /* constant (32 bit) key tracking for maps */
 	int ctx_field_size; /* the ctx field size for load insn, maybe 0 */