Re: [PATCH v2 4/5] bpf ppc32: add support for BPF_ATOMIC bitwise operations

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 10/06/2022 à 17:55, Hari Bathini a écrit :
> Adding instructions for ppc32 for
> 
> atomic_and
> atomic_or
> atomic_xor
> atomic_fetch_add
> atomic_fetch_and
> atomic_fetch_or
> atomic_fetch_xor
> 
> Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
> ---
> 
> Changes in v2:
> * Used an additional register (BPF_REG_AX)
>      - to avoid clobbering src_reg.
>      - to keep the lwarx reservation as intended.
>      - to avoid the odd switch/goto construct.

Might be a stupid question as I don't know the internals of BPF: Are we 
sure BPF_REG_AX cannot be the src reg or the dst reg ?


> * Zero'ed out the higher 32-bit explicitly when required.
> 
>   arch/powerpc/net/bpf_jit_comp32.c | 53 ++++++++++++++++++++++++-------
>   1 file changed, 41 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c
> index e46ed1e8c6ca..28dc6a1a8f2f 100644
> --- a/arch/powerpc/net/bpf_jit_comp32.c
> +++ b/arch/powerpc/net/bpf_jit_comp32.c
> @@ -294,6 +294,7 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
>   		u32 dst_reg_h = dst_reg - 1;
>   		u32 src_reg = bpf_to_ppc(insn[i].src_reg);
>   		u32 src_reg_h = src_reg - 1;
> +		u32 ax_reg = bpf_to_ppc(BPF_REG_AX);
>   		u32 tmp_reg = bpf_to_ppc(TMP_REG);
>   		u32 size = BPF_SIZE(code);
>   		s16 off = insn[i].off;
> @@ -798,25 +799,53 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
>   		 * BPF_STX ATOMIC (atomic ops)
>   		 */
>   		case BPF_STX | BPF_ATOMIC | BPF_W:
> -			if (imm != BPF_ADD) {
> -				pr_err_ratelimited("eBPF filter atomic op code %02x (@%d) unsupported\n",
> -						   code, i);
> -				return -ENOTSUPP;
> -			}
> -
> -			/* *(u32 *)(dst + off) += src */
> -
>   			bpf_set_seen_register(ctx, tmp_reg);
> +			bpf_set_seen_register(ctx, ax_reg);
> +
>   			/* Get offset into TMP_REG */
>   			EMIT(PPC_RAW_LI(tmp_reg, off));
> +			tmp_idx = ctx->idx * 4;
>   			/* load value from memory into r0 */
>   			EMIT(PPC_RAW_LWARX(_R0, tmp_reg, dst_reg, 0));
> -			/* add value from src_reg into this */
> -			EMIT(PPC_RAW_ADD(_R0, _R0, src_reg));
> -			/* store result back */
> +
> +			/* Save old value in BPF_REG_AX */
> +			if (imm & BPF_FETCH)
> +				EMIT(PPC_RAW_MR(ax_reg, _R0));
> +
> +			switch (imm) {
> +			case BPF_ADD:
> +			case BPF_ADD | BPF_FETCH:
> +				EMIT(PPC_RAW_ADD(_R0, _R0, src_reg));
> +				break;
> +			case BPF_AND:
> +			case BPF_AND | BPF_FETCH:
> +				EMIT(PPC_RAW_AND(_R0, _R0, src_reg));
> +				break;
> +			case BPF_OR:
> +			case BPF_OR | BPF_FETCH:
> +				EMIT(PPC_RAW_OR(_R0, _R0, src_reg));
> +				break;
> +			case BPF_XOR:
> +			case BPF_XOR | BPF_FETCH:
> +				EMIT(PPC_RAW_XOR(_R0, _R0, src_reg));
> +				break;
> +			default:
> +				pr_err_ratelimited("eBPF filter atomic op code %02x (@%d) unsupported\n",
> +						   code, i);
> +				return -EOPNOTSUPP;
> +			}
> +
> +			/* store new value */
>   			EMIT(PPC_RAW_STWCX(_R0, tmp_reg, dst_reg));
>   			/* we're done if this succeeded */
> -			PPC_BCC_SHORT(COND_NE, (ctx->idx - 3) * 4);
> +			PPC_BCC_SHORT(COND_NE, tmp_idx);
> +
> +			/* For the BPF_FETCH variant, get old data into src_reg */
> +			if (imm & BPF_FETCH) {
> +				EMIT(PPC_RAW_MR(src_reg, ax_reg));
> +				if (!fp->aux->verifier_zext)
> +					EMIT(PPC_RAW_LI(src_reg_h, 0));
> +			}
>   			break;
>   
>   		case BPF_STX | BPF_ATOMIC | BPF_DW: /* *(u64 *)(dst + off) += src */