Re: where to find ethernet CRC when stripping is off

[Ivan Boule <ivan.boule@6wind.com>]

Hi Francesco,

On 01/20/2016 06:15 PM, Montorsi, Francesco wrote:
> Hi Ivan,
>
>
>> -----Original Message-----
>> You would be right... if the PMDs did not transparently strip the CRC in
>> software when hardware CRC stripping is disabled at port configuration (as
>> described above).
>> See for instance how the function ixgbe_recv_pkts_lro() in file
>> drivers/net/ixgbe/ixgbe_rxtx.c deals with crc_len.
>
> Yeah, I see. However, I wonder what's the utility of the hw_strip_crc feature if finally it is completely masked to the mbuf user.
> However, to my understanding, looking at that ixgbe code, I think that what I wrote before:
>
>     uint32_t crc = *(rte_pktmbuf_mtod_offset (mymbuf, uint32_t*, mymbuf->pkt_len)) ;
>
> should work, since the pkt_len and data_len has the "crc_len" removed, but the CRC itself should be there.

In most cases, yes. But not in the [rare] case where the CRC would have 
been the only data stored into the last segment of a scattered input 
packet. See how the function ixgbe_recv_pkts_lro() checks this 
particular case, and free the associated mbuf.

> I know it is kind of an hack, but at least for ixgbe that sounds like a possible (temporary) solution for me....
>
>
>> Considering your need, I think now that PMDs should keep the CRC that are
>> stored in received packets when hardware CRC stripping is disabled by the
>> application, so that the application can access it as needed.
>>
> Yes, that would be very useful.
>
>> Note that this would impose that the input packet processing of such DPDK
>> applications be aware of the CRC presence (+4 in the packet length , for
>> instance).
> Or perhaps, to maintain backward compatibility, just a flag inside the mbuf could be set that informs the user that at the end of the mbuf packet, you can find 4 bytes with the CRC.
>
>>
>> Let's see what others, if any, that might care think about such a change into
>> the CRC stripping semantics.
>
> Thanks!
> Francesco
>


-- 
Ivan Boule
6WIND Development Engineer