2345

[chali]

The purpose of this feature is to split the header and the payload of
the packet.

| receive buffer |
| 0th descriptor | 1th descriptor |
| virtnet hdr | mac | ip hdr | tcp hdr|<-- hold -->| payload |

We can use a buffer plus a separate page when allocating the receive
buffer. In this way, we can ensure that all payloads can be
independently in a page, which is very beneficial for the zerocopy
implemented by the upper layer.