[dpdk-dev] [RFC] Add UDP GSO Support

[Jiayu Hu <jiayu.hu@intel.com>]

With the support of UDP Fragmentation Offload (UFO) and TCP Segmentation
Offload (TSO) in virtio, VMs can exchange large UDP and TCP packets
exceeding MTU between each other, which can greatly reduce per-packet
processing overheads.

When the destination of the large TCP and UDP packets is crossing
machines, the host application needs to call two different libraries,
GSO and IP fragmentation, to split the large packets respectively.
However,the GSO and IP fragmentation library have quite different APIs,
which greatly complicates the host application implementation.

To simplify application development, we propose to support UDP segmentation
offload in the GSO library. With supporting UDP GSO, host applicationss can
use the unified APIs to split large UDP and TCP packets.

Since we already have the IP fragmentation library, we can implement UDP
GSO in two ways:

1. Calling the IP fragmentation library inside the GSO library. This
   method can resue the existed code, but it also introduces some
   changes. Specifically, the GSO library ignores checksum, but the IP
   fragmentation library sets PKT_TX_IP_CKSUM for output packets by
   default. To avoid confusing applications, we need to change the GSO
   or IP fragmentation library design, but both of them would change the
   behaviors of the applications which already use the two libraries.

2. Implementing a new UDP fragmentation function in the GSO library. This
   method can keep current designs for the two libraries, but we cannot
   reuse the existed code.

Look forward to your opinions about this proposal.

Thanks,
Jiayu