netdev - Re: [PATCH net-next] erspan: set bso bit based on mirrored packet's len

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Message-ID: <CALDO+SaDVJtagChiQdvue6tY-N8G=jAT40GcD7U9kpa9qcVvQA@mail.gmail.com>
Date:   Wed, 16 May 2018 07:05:34 -0700
From:   William Tu <u9012063@...il.com>
To:     "Tobin C. Harding" <tobin@...orbit.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] erspan: set bso bit based on mirrored packet's len

On Mon, May 14, 2018 at 10:33 PM, Tobin C. Harding <tobin@...orbit.com> wrote:
> On Mon, May 14, 2018 at 04:54:36PM -0700, William Tu wrote:
>> Before the patch, the erspan BSO bit (Bad/Short/Oversized) is not
>> handled.  BSO has 4 possible values:
>>   00 --> Good frame with no error, or unknown integrity
>>   11 --> Payload is a Bad Frame with CRC or Alignment Error
>>   01 --> Payload is a Short Frame
>>   10 --> Payload is an Oversized Frame
>>
>> Based the short/oversized definitions in RFC1757, the patch sets
>> the bso bit based on the mirrored packet's size.
>>
>> Reported-by: Xiaoyan Jin <xiaoyanj@...are.com>
>> Signed-off-by: William Tu <u9012063@...il.com>
>> ---
>>  include/net/erspan.h | 25 +++++++++++++++++++++++++
>>  1 file changed, 25 insertions(+)
>>
>> diff --git a/include/net/erspan.h b/include/net/erspan.h
>> index d044aa60cc76..5eb95f78ad45 100644
>> --- a/include/net/erspan.h
>> +++ b/include/net/erspan.h
>> @@ -219,6 +219,30 @@ static inline __be32 erspan_get_timestamp(void)
>>       return htonl((u32)h_usecs);
>>  }
>>
>> +/* ERSPAN BSO (Bad/Short/Oversized)
>> + *   00b --> Good frame with no error, or unknown integrity
>> + *   01b --> Payload is a Short Frame
>> + *   10b --> Payload is an Oversized Frame
>> + *   11b --> Payload is a Bad Frame with CRC or Alignment Error
>> + */
>> +enum erspan_bso {
>> +     BSO_NOERROR,
>> +     BSO_SHORT,
>> +     BSO_OVERSIZED,
>> +     BSO_BAD,
>> +};
>
> If we are relying on the values perhaps this would be clearer
>
>         BSO_NOERROR     = 0x00,
>         BSO_SHORT       = 0x01,
>         BSO_OVERSIZED   = 0x02,
>         BSO_BAD         = 0x03,
>

Yes, thanks. I will change in v2.

>> +
>> +static inline u8 erspan_detect_bso(struct sk_buff *skb)
>> +{
>> +     if (skb->len < ETH_ZLEN)
>> +             return BSO_SHORT;
>> +
>> +     if (skb->len > ETH_FRAME_LEN)
>> +             return BSO_OVERSIZED;
>> +
>> +     return BSO_NOERROR;
>> +}
>
> Without having much contextual knowledge around this patch; should we be
> doing some check on CRC or alignment (at some stage)?  Having BSO_BAD
> seems to imply so?
>

The definition of BSO_BAD:
etherStatsCRCAlignErrors OBJECT-TYPE
              SYNTAX Counter
              ACCESS read-only
              STATUS mandatory
              DESCRIPTION
                  "The total number of packets received that
                  had a length (excluding framing bits, but
                  including FCS octets) of between 64 and 1518
                  octets, inclusive, but but had either a bad
                  Frame Check Sequence (FCS) with an integral
                  number of octets (FCS Error) or a bad FCS with
                  a non-integral number of octets (Alignment Error)."

But I don't know how to check CRC error at this code point.
Isn't it done by the NIC hardware?

Thanks for your review!
William