Re: [PATCH v2 5/5] bpf ppc32: Add instructions for atomic_[cmp]xchg

[Hari Bathini <hbathini@linux.ibm.com>]

On 11/06/22 11:04 pm, Christophe Leroy wrote:
> 
> 
> Le 10/06/2022 à 17:55, Hari Bathini a écrit :
>> This adds two atomic opcodes BPF_XCHG and BPF_CMPXCHG on ppc32, both
>> of which include the BPF_FETCH flag.  The kernel's atomic_cmpxchg
>> operation fundamentally has 3 operands, but we only have two register
>> fields. Therefore the operand we compare against (the kernel's API
>> calls it 'old') is hard-coded to be BPF_REG_R0. Also, kernel's
>> atomic_cmpxchg returns the previous value at dst_reg + off. JIT the
>> same for BPF too with return value put in BPF_REG_0.
>>
>>     BPF_REG_R0 = atomic_cmpxchg(dst_reg + off, BPF_REG_R0, src_reg);
>>
>> Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
>> ---
>>
>> Changes in v2:
>> * Moved variable declaration to avoid late declaration error on
>>     some compilers.
>> * Tried to make code readable and compact.
>>
>>
>>    arch/powerpc/net/bpf_jit_comp32.c | 25 ++++++++++++++++++++++---
>>    1 file changed, 22 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c
>> index 28dc6a1a8f2f..43f1c76d48ce 100644
>> --- a/arch/powerpc/net/bpf_jit_comp32.c
>> +++ b/arch/powerpc/net/bpf_jit_comp32.c
>> @@ -297,6 +297,7 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
>>    		u32 ax_reg = bpf_to_ppc(BPF_REG_AX);
>>    		u32 tmp_reg = bpf_to_ppc(TMP_REG);
>>    		u32 size = BPF_SIZE(code);
>> +		u32 save_reg, ret_reg;
>>    		s16 off = insn[i].off;
>>    		s32 imm = insn[i].imm;
>>    		bool func_addr_fixed;
>> @@ -799,6 +800,9 @@ 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:
>> +			save_reg = _R0;
>> +			ret_reg = src_reg;
>> +
>>    			bpf_set_seen_register(ctx, tmp_reg);
>>    			bpf_set_seen_register(ctx, ax_reg);
>>    
>> @@ -829,6 +833,21 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
>>    			case BPF_XOR | BPF_FETCH:
>>    				EMIT(PPC_RAW_XOR(_R0, _R0, src_reg));
>>    				break;
>> +			case BPF_CMPXCHG:
>> +				/*
>> +				 * Return old value in BPF_REG_0 for BPF_CMPXCHG &
>> +				 * in src_reg for other cases.
>> +				 */
>> +				ret_reg = bpf_to_ppc(BPF_REG_0);
>> +
>> +				/* Compare with old value in BPF_REG_0 */
>> +				EMIT(PPC_RAW_CMPW(bpf_to_ppc(BPF_REG_0), _R0));
>> +				/* Don't set if different from old value */
>> +				PPC_BCC_SHORT(COND_NE, (ctx->idx + 3) * 4);
>> +				fallthrough;
>> +			case BPF_XCHG:
>> +				save_reg = src_reg;
> 
> I'm a bit lost, when save_reg is src_reg, don't we expect the upper part
> (ie src_reg - 1) to be explicitely zeroised ?
> 

For BPF_FETCH variants, old value is returned in src_reg (ret_reg).
In all such cases, higher 32-bit is zero'ed. But in case of BPF_CMPXCHG,
src_reg is untouched as BPF_REG_0 is used instead. So, higher 32-bit
remains untouched for that case alone..


>> +				break;
>>    			default:
>>    				pr_err_ratelimited("eBPF filter atomic op code %02x (@%d) unsupported\n",
>>    						   code, i);
>> @@ -836,15 +855,15 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
>>    			}
>>    
>>    			/* store new value */
>> -			EMIT(PPC_RAW_STWCX(_R0, tmp_reg, dst_reg));
>> +			EMIT(PPC_RAW_STWCX(save_reg, tmp_reg, dst_reg));
>>    			/* we're done if this succeeded */
>>    			PPC_BCC_SHORT(COND_NE, tmp_idx);
>>    

>>    			/* For the BPF_FETCH variant, get old data into src_reg */

With this commit, this comment is not true for BPF_CMPXCHG. So, this
comment should not be removed..

Thanks
Hari