[PATCH bpf-next v2 2/2] bpf, arm64: inline bpf_get_smp_processor_id() helper

[Puranjay Mohan <puranjay@kernel.org>]

As ARM64 JIT now implements BPF_MOV64_PERCPU_REG instruction, inline
bpf_get_smp_processor_id().

ARM64 uses the per-cpu variable cpu_number to store the cpu id.

Here is how the BPF and ARM64 JITed assembly changes after this commit:

                                         BPF
         		                =====
              BEFORE                                       AFTER
             --------                                     -------

int cpu = bpf_get_smp_processor_id();           int cpu = bpf_get_smp_processor_id();
(85) call bpf_get_smp_processor_id#229032       (18) r0 = 0xffff800082072008
                                                (bf) r0 = r0
                                                (61) r0 = *(u32 *)(r0 +0)

				      ARM64 JIT
				     ===========

              BEFORE                                       AFTER
             --------                                     -------

int cpu = bpf_get_smp_processor_id();      int cpu = bpf_get_smp_processor_id();
mov     x10, #0xfffffffffffff4d0           mov     x7, #0xffff8000ffffffff
movk    x10, #0x802b, lsl #16              movk    x7, #0x8207, lsl #16
movk    x10, #0x8000, lsl #32              movk    x7, #0x2008
blr     x10                                mrs     x10, tpidr_el1
add     x7, x0, #0x0                       add     x7, x7, x10
                                           ldr     w7, [x7]

Performance improvement using benchmark[1]

             BEFORE                                       AFTER
            --------                                     -------

glob-arr-inc   :   23.817 ± 0.019M/s      glob-arr-inc   :   24.631 ± 0.027M/s
arr-inc        :   23.253 ± 0.019M/s      arr-inc        :   23.742 ± 0.023M/s
hash-inc       :   12.258 ± 0.010M/s      hash-inc       :   12.625 ± 0.004M/s

[1] https://github.com/anakryiko/linux/commit/8dec900975ef

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 kernel/bpf/verifier.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 9715c88cc025..3373be261889 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -20205,7 +20205,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
 			goto next_insn;
 		}
 
-#ifdef CONFIG_X86_64
+#if defined(CONFIG_X86_64) || defined(CONFIG_ARM64)
 		/* Implement bpf_get_smp_processor_id() inline. */
 		if (insn->imm == BPF_FUNC_get_smp_processor_id &&
 		    prog->jit_requested && bpf_jit_supports_percpu_insn()) {
@@ -20214,11 +20214,20 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
 			 * changed in some incompatible and hard to support
 			 * way, it's fine to back out this inlining logic
 			 */
+#if defined(CONFIG_X86_64)
 			insn_buf[0] = BPF_MOV32_IMM(BPF_REG_0, (u32)(unsigned long)&pcpu_hot.cpu_number);
 			insn_buf[1] = BPF_MOV64_PERCPU_REG(BPF_REG_0, BPF_REG_0);
 			insn_buf[2] = BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, 0);
 			cnt = 3;
+#elif defined(CONFIG_ARM64)
+			struct bpf_insn cpu_number_addr[2] = { BPF_LD_IMM64(BPF_REG_0, (u64)&cpu_number) };
 
+			insn_buf[0] = cpu_number_addr[0];
+			insn_buf[1] = cpu_number_addr[1];
+			insn_buf[2] = BPF_MOV64_PERCPU_REG(BPF_REG_0, BPF_REG_0);
+			insn_buf[3] = BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, 0);
+			cnt = 4;
+#endif
 			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
 			if (!new_prog)
 				return -ENOMEM;
-- 
2.40.1