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

From: Hari Bathini <[email-masked]>
Date: Tue, 14 Jun 2022 00:44:19 +0530

On 14/06/22 12:41 am, Hari Bathini wrote:
> 
> 
> 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 <[email-masked]>
>>> ---
>>>
>>> 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..

Sorry, the above should read:
   "should be removed" instead of "should not be removed"..