Re: [PATCH net-next] sfc: trace_xdp_exception on XDP failure

[Edward Cree <ecree@solarflare.com>]

On 11/11/2019 17:38, Arthur Fabre wrote:
> On Mon, Nov 11, 2019 at 5:27 PM Edward Cree <ecree@solarflare.com> wrote:
>>
>> On 11/11/2019 10:51, Arthur Fabre wrote:
>>> diff --git a/drivers/net/ethernet/sfc/rx.c b/drivers/net/ethernet/sfc/rx.c
>>> index a7d9841105d8..5bfe1f6112a1 100644
>>> --- a/drivers/net/ethernet/sfc/rx.c
>>> +++ b/drivers/net/ethernet/sfc/rx.c
>>> @@ -678,6 +678,7 @@ static bool efx_do_xdp(struct efx_nic *efx, struct efx_channel *channel,
>>>                                 "XDP is not possible with multiple receive fragments (%d)\n",
>>>                                 channel->rx_pkt_n_frags);
>>>               channel->n_rx_xdp_bad_drops++;
>>> +             trace_xdp_exception(efx->net_dev, xdp_prog, xdp_act);
>>>               return false;
>>>       }
>> AIUI trace_xdp_exception() is improper here as we have not run
>>  the XDP program (and xdp_act is thus uninitialised).
>>
>> The other three, below, appear to be correct.
>> -Ed
>>
> 
> Good point. Do you know under what conditions we'd end up with
> "fragmented" packets? As far as I can tell this isn't IP
> fragmentation?

Fragments in this case means that the packet data are spread across
 multiple RX buffers (~= memory pages).  This should only happen if
 the RX packet is too big to fit in a single buffer, and when
 enabling XDP we ensure that the MTU is small enough to prevent
 that.  So in theory this can't happen if the NIC is functioning
 correctly.

-Ed