[Catpowder] Windows: Allocate DemiBuffers directly from XDP user memory region

[kyleholohan]

Context

Currently on send and receive in Catpowder for Windows, DemiBuffer content is copied to/from the allocated XDP umem region. This creates an additional unnecessary copy on the data path.

Proposed Solution

Leverage the XDP umem "headroom" configuration to allocate a pool of DemiBuffers on initialization of the LibOS. On sgaalooc/sgafree, allocate and free buffers directly from the umem pool. On send/receive path, avoid the additional copy by putting/pulling the DemiBuffer packet content directly into/from the Tx/Rx queues.

Alternative Solutions

The solution could be left as is to reduce coupling between components at the cost of the additional copy. Alternatively, DemiBuffer metadata could point to the umem pool, and the metadata/data association could be reformed through some pointer-based lookup on receive.