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bpf: inline bpf_get_branch_snapshot() helper

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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.

Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240404002640.1774210-3-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Andrii Nakryiko 2024-04-03 17:26:40 -07:00 committed by Alexei Starovoitov
parent 5e6a3c1ee6
commit 314a53623c
1 changed files with 55 additions and 0 deletions
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55
kernel/bpf/verifier.c
View file

@ -20188,6 +20188,61 @@ 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.
*/
insn_buf[1] = BPF_MOV32_IMM(BPF_REG_0, 0xaaaaaaab);
insn_buf[2] = BPF_ALU64_REG(BPF_MUL, BPF_REG_2, BPF_REG_0);
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 &&