RE: CRC offload from application's POV

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

> -----Original Message-----
> From: Ferruh Yigit <ferruh.yigit@amd.com>
> Sent: Wednesday, October 12, 2022 4:07 PM
> To: Wu, Wenjun1 <wenjun1.wu@intel.com>; Viacheslav Galaktionov
> <Viacheslav.Galaktionov@arknetworks.am>; Su, Simei <simei.su@intel.com>
> Cc: Denis Pryazhennikov <Denis.Pryazhennikov@arknetworks.am>;
> dev@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@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.
> >
> 
> 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.