CRC offload from application's POV

[Ferruh Yigit]

On 10/12/2022 9:18 AM, Wu, Wenjun1 wrote:
> 
> 
>> -----Original Message-----
>> From: Ferruh Yigit <ferruh.yigit at amd.com>
>> Sent: Wednesday, October 12, 2022 4:07 PM
>> To: Wu, Wenjun1 <wenjun1.wu at intel.com>; Viacheslav Galaktionov
>> <Viacheslav.Galaktionov at arknetworks.am>; Su, Simei <simei.su at intel.com>
>> Cc: Denis Pryazhennikov <Denis.Pryazhennikov at arknetworks.am>;
>> dev at dpdk.org
>> Subject: Re: CRC offload from application's POV
>>
>> On 10/12/2022 3:29 AM, Wu, Wenjun1 wrote:
>>>
>>>
>>>> -----Original Message-----
>>>> From: Ferruh Yigit <ferruh.yigit at amd.com>
>>>> Sent: Tuesday, October 11, 2022 9:46 PM
>>>> To: Viacheslav Galaktionov <Viacheslav.Galaktionov at arknetworks.am>;
>>>> Su, Simei <simei.su at intel.com>; Wu, Wenjun1 <wenjun1.wu at intel.com>
>>>> Cc: Denis Pryazhennikov <Denis.Pryazhennikov at arknetworks.am>;
>>>> dev at dpdk.org
>>>> Subject: Re: CRC offload from application's POV
>>>>
>>>> On 10/11/2022 12:54 PM, Viacheslav Galaktionov wrote:
>>>>> On 10/11/22 15:36, Ferruh Yigit wrote:
>>>>>> On 10/11/2022 11:48 AM, Viacheslav Galaktionov wrote:
>>>>>>> Hi!
>>>>>>>
>>>>>>> We're looking to implement CRC offload in our driver and we're
>>>>>>> having difficulties understanding what the feature changes from
>>>>>>> the application's point of view. If we enable the KEEP_CRC
>>>>>>> offload, then the NIC is supposed to preserve the CRC in the
>>>>>>> packet, that much is clear. But we checked other drivers and it
>>>>>>> seems common for PMDs to remove the CRC from the final mbufs.
>>>>>>> Why is that?
>>>>>>>
>>>>>>> We couldn't find any place where the CRC would be stored after
>>>>>>> removal, so it looks like the application doesn't have access to
>>>>>>> this piece of data. And if so, what's the point of having this
>>>>>>> feature if the CRC is discarded either way?
>>>>>>>
>>>>>>> We're probably missing something and would really appreciate any
>>>>>>> help with this.
>>>>>>>
>>>>>>
>>>>>> Hi Viacheslav,
>>>>>>
>>>>>> As you said default behavior is to strip the CRC from packet, even
>>>>>> some devices doesn't support having CRC in the packet it is removed
>>>>>> by HW automatically. In this case application can't access to the CRC.
>>>>>>
>>>>>> For the devices that has capability to keep CRC, KEEP_CRC offload
>>>>>> should enable having CRC as part of the packet. There is no special
>>>>>> field to store the CRC.
>>>>>>
>>>>> I'm asking because I'm seeing a common pattern in the code base: if
>>>>> the hardware didn't remove the CRC, the driver does this itself.
>>>>> Grepping the code for "crc_len" will show you what I mean. One of
>>>>> the most apparent examples of this happening can be seen in
>>>>> drivers/net/e1000/em_rxtx.c:
>>>>>
>>>>> /*
>>>>>     * This is the last buffer of the received packet.
>>>>>     * If the CRC is not stripped by the hardware:
>>>>>     *   - Subtract the CRC    length from the total packet length.
>>>>>     *   - If the last buffer only contains the whole CRC or a part
>>>>>     *     of it, free the mbuf associated to the last buffer.
>>>>>     *     If part of the CRC is also contained in the previous
>>>>>     *     mbuf, subtract the length of that CRC part from the
>>>>>     *     data length of the previous mbuf.
>>>>>     */
>>>>>
>>>>> I don't understand why this is necessary, and whether this is just a
>>>>> particularity of this driver or how the feature is supposed to be
>>>>> implemented everywhere. I haven't checked every driver, but it seems
>>>>> like a lot of them do something similar to this.
>>>>
>>>> That looks wrong to me too, cc'ed maintainers for comment.
>>>>
>>>> That piece of code seems remaining from first upstream of the driver
>>>> (2012), it is before KEEP_CRC change, looks like it is missed.
>>>>
>>>> CRC should be kept in the packet if driver supports it and user
>>>> requested KEEP_CRC offload.
>>>>
>>>> But Rx stats should not include CRC, as it is common to use 'm->pkt_len'
>>>> for received packet stat, when CRC is in packet that should taken
>>>> into account for stats.
>>>
>>> I agree with Ferruh. PMD will advertise KEEP_CRC offload in
>>> rte_eth_dev_info->rx_offload_capa. If it is supported, CRC will always
>>> keep in the packets. If user request KEEP_CRC offload in
>>> rte_eth_rxmode, the driver will subtract CRC length from data length
>>> to remove CRC from packet data, and user can get the CRC after the end of
>> the packet.
>>>
>>
>> Hi Wenjun,
>>
>> What we said is slightly different, it is OK to subtract the length from *stats*,
>> but I think driver shouldn't remove the CRC from packet data.
> You are right, the driver will never remove the CRC from packet data.

Just to be sure we are on same page, driver won't remove the CRC only 
when KEEP_CRC is requested by user. Otherwise it will remove the CRC by 
default.