Re: [PATCH] net/hns3: fix Rx packet truncation when KEEP CRC enabled

[Ferruh Yigit <ferruh.yigit@amd.com>]

On 2/26/2024 3:16 AM, Jie Hai wrote:
> On 2024/2/23 21:53, Ferruh Yigit wrote:
>> On 2/20/2024 3:58 AM, Jie Hai wrote:
>>> Hi, Ferruh,
>>>
>>> Thanks for your review.
>>>
>>> On 2024/2/7 22:15, Ferruh Yigit wrote:
>>>> On 2/6/2024 1:10 AM, Jie Hai wrote:
>>>>> From: Dengdui Huang <huangdengdui@huawei.com>
>>>>>
>>>>> When KEEP_CRC offload is enabled, some packets will be truncated and
>>>>> the CRC is still be stripped in following cases:
>>>>> 1. For HIP08 hardware, the packet type is TCP and the length
>>>>>      is less than or equal to 60B.
>>>>> 2. For other hardwares, the packet type is IP and the length
>>>>>      is less than or equal to 60B.
>>>>>
>>>>
>>>> If a device doesn't support the offload by some packets, it can be
>>>> option to disable offload for that device, instead of calculating it in
>>>> software and append it.
>>>
>>> The KEEP CRC feature of hns3 is faulty only in the specific packet
>>> type and small packet(<60B) case.
>>> What's more, the small ethernet packet is not common.
>>>
>>>> Unless you have a specific usecase, or requirement to support the
>>>> offload.
>>>
>>> Yes, some users of hns3 are already using this feature.
>>> So we cannot drop this offload
>>>
>>>> <...>
>>>>
>>>>> @@ -2492,10 +2544,16 @@ hns3_recv_pkts_simple(void *rx_queue,
>>>>>                goto pkt_err;
>>>>>              rxm->packet_type = hns3_rx_calc_ptype(rxq, l234_info,
>>>>> ol_info);
>>>>> -
>>>>>            if (rxm->packet_type == RTE_PTYPE_L2_ETHER_TIMESYNC)
>>>>>                rxm->ol_flags |= RTE_MBUF_F_RX_IEEE1588_PTP;
>>>>>    +        if (unlikely(rxq->crc_len > 0)) {
>>>>> +            if (hns3_need_recalculate_crc(rxq, rxm))
>>>>> +                hns3_recalculate_crc(rxq, rxm);
>>>>> +            rxm->pkt_len -= rxq->crc_len;
>>>>> +            rxm->data_len -= rxq->crc_len;
>>>>>
>>>>
>>>> Removing 'crc_len' from 'mbuf->pkt_len' & 'mbuf->data_len' is
>>>> practically same as stripping CRC.
>>>>
>>>> We don't count CRC length in the statistics, but it should be
>>>> accessible
>>>> in the payload by the user.
>>> Our drivers are behaving exactly as you say.
>>>
>>
>> If so I missed why mbuf 'pkt_len' and 'data_len' reduced by
>> 'rxq->crc_len', can you please explain what above lines does?
>>
>>
> @@ -2470,8 +2523,7 @@ hns3_recv_pkts_simple(void *rx_queue,
>          rxdp->rx.bd_base_info = 0;
> 
>          rxm->data_off = RTE_PKTMBUF_HEADROOM;
> -        rxm->pkt_len = (uint16_t)(rte_le_to_cpu_16(rxd.rx.pkt_len)) -
> -                rxq->crc_len;
> +        rxm->pkt_len = rte_le_to_cpu_16(rxd.rx.pkt_len);
> 
> In the previous code above, the 'pkt_len' is set to the length obtained
> from the BD. the length obtained from the BD already contains CRC length.
> But as you said above, the DPDK requires that the length of the mbuf
> does not contain CRC length . So we subtract 'rxq->crc_len' from
> mbuf'pkt_len' and 'data_len'. This patch doesn't change the logic, it
> just moves the code around.
>

Nope, I am not saying mbuf length shouldn't contain CRC length, indeed
it is other way around and this is our confusion.

CRC length shouldn't be in the statistics, I mean in received bytes stats.
Assume that received packet is 128 bytes and we know it has the CRC,
Rx received bytes stat should be 124 (rx_bytes = 128 - CRC = 124)

But mbuf->data_len & mbuf->pkt_len should have full frame length,
including CRC.

As application explicitly requested to KEEP CRC, it will know last 4
bytes are CRC.
Anything after 'mbuf->data_len' in the mbuf buffer is not valid, so if
you reduce 'mbuf->data_len' by CRC size, application can't know if 4
bytes after 'mbuf->data_len' is valid CRC or not.