[dpdk-dev] [PATCH v2] net/e1000: correct VLAN tag byte order for i35x LB packets

Roger B. Melton rmelton at cisco.com
Wed Oct 25 22:45:13 CEST 2017

On 10/25/17 4:22 PM, Ferruh Yigit wrote:
> On 10/25/2017 1:16 PM, Bruce Richardson wrote:
>> On Wed, Oct 25, 2017 at 11:11:08AM -0700, Ferruh Yigit wrote:
>>> On 10/23/2017 10:42 AM, Roger B. Melton wrote:
>>>> On 10/20/17 3:04 PM, Ferruh Yigit wrote:
>>>>> On 10/12/2017 10:24 AM, Roger B Melton wrote:
>>>>>> When copying VLAN tags from the RX descriptor to the vlan_tci field
>>>>>> in the mbuf header,  igb_rxtx.c:eth_igb_recv_pkts() and
>>>>>> eth_igb_recv_scattered_pkts() both assume that the VLAN tag is always
>>>>>> little endian.  While i350, i354 and /i350vf VLAN non-loopback
>>>>>> packets are stored little endian, VLAN tags in loopback packets for
>>>>>> those devices are big endian.
>>>>>> For i350, i354 and i350vf VLAN loopback packets, swap the tag when
>>>>>> copying from the RX descriptor to the mbuf header.  This will ensure
>>>>>> that the mbuf vlan_tci is always little endian.
>>>>>> Signed-off-by: Roger B Melton <rmelton at cisco.com>
>>>>> <...>
>>>>>> @@ -946,9 +954,16 @@ eth_igb_recv_pkts(void *rx_queue, struct rte_mbuf **rx_pkts,
>>>>>>    		rxm->hash.rss = rxd.wb.lower.hi_dword.rss;
>>>>>>    		hlen_type_rss = rte_le_to_cpu_32(rxd.wb.lower.lo_dword.data);
>>>>>> -		/* Only valid if PKT_RX_VLAN_PKT set in pkt_flags */
>>>>>> -		rxm->vlan_tci = rte_le_to_cpu_16(rxd.wb.upper.vlan);
>>>>>> -
>>>>>> +		/*
>>>>>> +		 * The vlan_tci field is only valid when PKT_RX_VLAN_PKT is
>>>>>> +		 * set in the pkt_flags field and must be in CPU byte order.
>>>>>> +		 */
>>>>>> +		if ((staterr & rte_cpu_to_le_32(E1000_RXDEXT_STATERR_LB)) &&
>>>>>> +			(rxq->flags & IGB_RXQ_FLAG_LB_BSWAP_VLAN)) {
>>>>> This is adding more condition checks into Rx path.
>>>>> What is the performance cost of this addition?
>>>> I have not measured the performance cost, but I can collect data. What
>>>> specifically are you looking for?
>>>> To be clear the current implementation incorrect as it does not
>>>> normalize the vlan tag to CPU byte order before copying it into mbuf and
>>>> applications have no visibility to determine if the tag in the mbuf is
>>>> big or little endian.
>>>> Do you have any suggestions for an alternative approach to avoid rx
>>>> patch checks?
>>> No suggestion indeed. And correctness matters.
>>> But this add a cost and I wonder how much it is, based on that result it may be
>>> possible to do more investigation for alternate solutions or trade-offs.
>>> Konstantin, Bruce, Wenzhuo,
>>> What do you think, do you have any comment?
>> For a 1G driver, is performance really that big an issue?
> I don't know. So is this an Ack from you for the patch?

I can tell you that from the perspective of my application the 
performance impact for 1G is not a concern.

FWIW, I did go through a few iterations with Wenzhou to minimize the 
performance impact before we settled on this implementation, and Wenzhou 
did Ack it btw.

I'm hoping we can get this into 17.11.


>> Unless you
>> have a *lot* of 1G ports, I would expect most platforms not to notice an
>> extra couple of cycles when dealing with 1G line rates.
>> /Bruce
> .

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