[dpdk-dev] where to find ethernet CRC when stripping is off

Montorsi, Francesco fmontorsi at empirix.com
Wed Jan 20 18:15:16 CET 2016

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.
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.


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