powerpc64/bpf: Implement PROBE_ATOMIC instructions

powerpc supports BPF atomic operations using a loop around
Load-And-Reserve(LDARX/LWARX) and Store-Conditional(STDCX/STWCX)
instructions gated by sync instructions to enforce full ordering.

To implement arena_atomics, arena vm start address is added to the
dst_reg to be used for both the LDARX/LWARX and STDCX/STWCX instructions.
Further, an exception table entry is added for LDARX/LWARX
instruction to land after the loop on fault. At the end of sequence,
dst_reg is restored by subtracting arena vm start address.

bpf_jit_supports_insn() is introduced to selectively enable instruction
support as in other architectures like x86 and arm64.

Reviewed-by: Hari Bathini <[email protected]>
Tested-by: Venkat Rao Bagalkote <[email protected]>
Signed-off-by: Saket Kumar Bhaskar <[email protected]>

Author: Saket Kumar Bhaskar

arch/powerpc/net/bpf_jit_comp.c

@@ -450,6 +450,22 @@ bool bpf_jit_supports_far_kfunc_call(void)
 	return IS_ENABLED(CONFIG_PPC64);
 }
 
+bool bpf_jit_supports_insn(struct bpf_insn *insn, bool in_arena)
+{
+	if (!in_arena)
+		return true;
+	switch (insn->code) {
+	case BPF_STX | BPF_ATOMIC | BPF_H:
+	case BPF_STX | BPF_ATOMIC | BPF_B:
+	case BPF_STX | BPF_ATOMIC | BPF_W:
+	case BPF_STX | BPF_ATOMIC | BPF_DW:
+		if (bpf_atomic_is_load_store(insn))
+			return false;
+		return IS_ENABLED(CONFIG_PPC64);
+	}
+	return true;
+}
+
 void *arch_alloc_bpf_trampoline(unsigned int size)
 {
 	return bpf_prog_pack_alloc(size, bpf_jit_fill_ill_insns);

arch/powerpc/net/bpf_jit_comp64.c

@@ -1163,6 +1163,32 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, u32 *fimage, struct code
 
 			break;
 
+		/*
+		 * BPF_STX PROBE_ATOMIC (arena atomic ops)
+		 */
+		case BPF_STX | BPF_PROBE_ATOMIC | BPF_W:
+		case BPF_STX | BPF_PROBE_ATOMIC | BPF_DW:
+			EMIT(PPC_RAW_ADD(dst_reg, dst_reg, bpf_to_ppc(ARENA_VM_START)));
+			ret = bpf_jit_emit_atomic_ops(image, ctx, &insn[i],
+						      &jmp_off, &tmp_idx, &addrs[i + 1]);
+			if (ret) {
+				if (ret == -EOPNOTSUPP) {
+					pr_err_ratelimited(
+						"eBPF filter atomic op code %02x (@%d) unsupported\n",
+						code, i);
+				}
+				return ret;
+			}
+			/* LDARX/LWARX should land here on exception. */
+			ret = bpf_add_extable_entry(fp, image, fimage, pass, ctx,
+						    tmp_idx, jmp_off, dst_reg, code);
+			if (ret)
+				return ret;
+
+			/* Retrieve the dst_reg */
+			EMIT(PPC_RAW_SUB(dst_reg, dst_reg, bpf_to_ppc(ARENA_VM_START)));
+			break;
+
 		/*
 		 * BPF_STX ATOMIC (atomic ops)
 		 */