Re: [dpdk-dev] [PATCH v2 08/13] testpmd: rework csum forward engine

[Olivier MATZ <olivier.matz@6wind.com>]

Hi Jijiang,

On 11/17/2014 09:11 AM, Liu, Jijiang wrote:
>> +/* Calculate the checksum of outer header (only vxlan is supported,
>> + * meaning IP + UDP). The caller already checked that it's a vxlan
>> + * packet */
>> +static uint64_t
>> +process_outer_cksums(void *outer_l3_hdr, uint16_t outer_ethertype,
>> +	uint16_t outer_l3_len, uint16_t testpmd_ol_flags) {
>> +	struct ipv4_hdr *ipv4_hdr = outer_l3_hdr;
>> +	struct ipv6_hdr *ipv6_hdr = outer_l3_hdr;
>> +	struct udp_hdr *udp_hdr;
>> +	uint64_t ol_flags = 0;
>> +
>> +	if (testpmd_ol_flags & TESTPMD_TX_OFFLOAD_VXLAN_CKSUM)
>> +		ol_flags |= PKT_TX_VXLAN_CKSUM;
>> +
>> +	if (outer_ethertype == _htons(ETHER_TYPE_IPv4)) {
>> +		ipv4_hdr->hdr_checksum = 0;
>> +
>> +		if ((testpmd_ol_flags & TESTPMD_TX_OFFLOAD_VXLAN_CKSUM)
>> == 0)
>> +			ipv4_hdr->hdr_checksum = get_ipv4_cksum(ipv4_hdr);
>> +	}
> 
> As I mentioned, we should use TESTPMD_TX_OFFLOAD_IP_CKSUM instead of using TESTPMD_TX_OFFLOAD_VXLAN_CKSUM flag to check if we need to set outer IP checksum offload.
> In other words, even if VXLAN packet, outer IP TX checksum offload is also needed if  TESTPMD_TX_OFFLOAD_IP_CKSUM is set.

The csum forward engine works as follow after the rework. I can add
some more comments in the patch or testpmd help to describe it more
clearly.

Receive a burst of packets, and for each packet:
 - parse packet, and try to recognize a supported packet type (1)
 - if it's not a supported packet type, don't touch the packet, else:
 - modify the IPs in inner headers and in outer headers if any
 - reprocess the checksum of all supported layers. This is done in SW
   or HW, depending on testpmd command line configuration
 - if TSO is enabled in testpmd command line, also flag the mbuf for TCP
   segmentation offload (this implies HW TCP checksum)
Then transmit packets on the output port.

(1) Supported packets are:
  Ether / (vlan) / IP|IP6 / UDP|TCP|SCTP .
  Ether / (vlan) / outer IP|IP6 / outer UDP / VxLAN / Ether / IP|IP6 /
UDP|TCP|SCTP

The testpmd command line for csum takes the following arguments:
  tx_cksum set (ip|udp|tcp|sctp|vxlan) (hw|sw) (port_id)

- "ip|udp|tcp|sctp" always concern the inner layer
- "vxlan" concerns the outer IP and UDP layer (if packet is recognized
  as a vxlan packet)

Hope it's enough precisely described to be able to predict the output
of testpmd without reading the code or the i40e datasheets. This was
not so clear before.

So, following this description, there is not reason to check the
TESTPMD_TX_OFFLOAD_IP_CKSUM when scheduling the hardware VxLAN checksum.
One thing may be wrong however, it's the mbuf flags set in the packet.
But we cannot say it's wrong today because the API is not documented.
But the VXLAN feature is not enough documented to be sure it's wrong.


Regards,
Olivier