virtio-net driver rewrite

[IanSeyler]

This pull request significantly refactors the VirtIO network driver to support multiple network interfaces and improve descriptor management. The changes replace the use of global/static variables with per-interface data structures, update how descriptor tables and rings are managed, and streamline packet transmit/receive logic for better correctness and maintainability.

Key improvements and refactoring:

Per-Interface Data Management:

• Replaced global variables (such as os_net_mem, netrxdescindex, nettxdescindex, etc.) with per-interface fields accessed via offsets from an interface structure (using r8/rdx and offsets like nt_rx_desc, nt_tx_desc, etc.), enabling support for multiple network interfaces and eliminating shared state. [1] [2] [3] [4] [5] [6] [7] [8]

Descriptor Table and Ring Handling:

• Updated initialization routines to use per-interface descriptor table addresses, and improved how RX/TX descriptor rings are populated and managed. This includes new logic for resetting, populating, and wrapping descriptor indices and handling the available and used rings per interface. [1] [2] [3] [4] [5] [6] [7]

Transmit and Receive Logic:

• Refactored net_virtio_transmit and net_virtio_poll to use per-interface descriptor tables and indices, improved ring index management, and ensured correct address calculations for packet data. Also, improved handling of the used/available rings for packet transmission and reception, including proper wrap-around of indices. [1] [2] [3]

Cleanup and Removal of Global State:

• Removed global/static variables related to descriptor and ring indices (netrxdescindex, netrxavailindex, nettxdescindex, nettxavailindex), as these are now managed per interface.

General Code Organization and Comments:

• Improved comments and code organization, clarified TODOs, and commented out obsolete or now-redundant code sections related to old assumptions (e.g., fixed 256 entries).

These changes make the VirtIO network driver more robust, modular, and ready for environments with multiple network devices.