RE: CRC offload from application's POV

["Wu, Wenjun1" <wenjun1.wu@intel.com>]

> -----Original Message-----
> From: Ferruh Yigit <ferruh.yigit@amd.com>
> Sent: Tuesday, October 11, 2022 9:46 PM
> To: Viacheslav Galaktionov <Viacheslav.Galaktionov@arknetworks.am>; Su,
> Simei <simei.su@intel.com>; Wu, Wenjun1 <wenjun1.wu@intel.com>
> Cc: Denis Pryazhennikov <Denis.Pryazhennikov@arknetworks.am>;
> dev@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.