linux-kernel - Re: [PATCH bpf] powerpc/bpf: enforce full ordering for ATOMIC operations with BPF

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Date: Wed, 08 May 2024 15:05:40 +1000
From: Michael Ellerman <mpe@...erman.id.au>
To: Puranjay Mohan <puranjay@...nel.org>, Alexei Starovoitov
 <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko
 <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, Eduard
 Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, Yonghong Song
 <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, KP
 Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>, Hao Luo
 <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, "Naveen N. Rao"
 <naveen.n.rao@...ux.ibm.com>, Nicholas Piggin <npiggin@...il.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>, "Aneesh Kumar K.V"
 <aneesh.kumar@...nel.org>, Hari Bathini <hbathini@...ux.ibm.com>,
 bpf@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
 linux-kernel@...r.kernel.org
Cc: puranjay12@...il.com
Subject: Re: [PATCH bpf] powerpc/bpf: enforce full ordering for ATOMIC
 operations with BPF_FETCH

Puranjay Mohan <puranjay@...nel.org> writes:
> The Linux Kernel Memory Model [1][2] requires RMW operations that have a
> return value to be fully ordered.
>
> BPF atomic operations with BPF_FETCH (including BPF_XCHG and
> BPF_CMPXCHG) return a value back so they need to be JITed to fully
> ordered operations. POWERPC currently emits relaxed operations for
> these.

Thanks for catching this.

> diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c
> index 2f39c50ca729..b635e5344e8a 100644
> --- a/arch/powerpc/net/bpf_jit_comp32.c
> +++ b/arch/powerpc/net/bpf_jit_comp32.c
> @@ -853,6 +853,15 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, u32 *fimage, struct code
>  			/* Get offset into TMP_REG */
>  			EMIT(PPC_RAW_LI(tmp_reg, off));
>  			tmp_idx = ctx->idx * 4;
> +			/*
> +			 * Enforce full ordering for operations with BPF_FETCH by emitting a 'sync'
> +			 * before and after the operation.
> +			 *
> +			 * This is a requirement in the Linux Kernel Memory Model.
> +			 * See __cmpxchg_u64() in asm/cmpxchg.h as an example.
> +			 */
> +			if (imm & BPF_FETCH)
> +				EMIT(PPC_RAW_SYNC());
>  			/* load value from memory into r0 */
>  			EMIT(PPC_RAW_LWARX(_R0, tmp_reg, dst_reg, 0));
>  
> @@ -905,6 +914,8 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, u32 *fimage, struct code
>  
>  			/* For the BPF_FETCH variant, get old data into src_reg */
>  			if (imm & BPF_FETCH) {
> +				/* Emit 'sync' to enforce full ordering */
> +				EMIT(PPC_RAW_SYNC());
>  				EMIT(PPC_RAW_MR(ret_reg, ax_reg));
>  				if (!fp->aux->verifier_zext)
>  					EMIT(PPC_RAW_LI(ret_reg - 1, 0)); /* higher 32-bit */

On 32-bit there are non-SMP systems where those syncs will probably be expensive.

I think just adding an IS_ENABLED(CONFIG_SMP) around the syncs is
probably sufficient. Christophe?

cheers