[PATCH bpf] powerpc/bpf: enforce full ordering for ATOMIC operations with BPF_FETCH

[Christophe Leroy]

Le 08/05/2024 à 07:05, Michael Ellerman a écrit :
> Puranjay Mohan <puranjay at kernel.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?

Yes indeed, thanks for spotting it, the sync is only required on SMP and 
is worth avoiding on non SMP.

Thanks
Christophe