virtio_net: add page_pool support

Introduce page_pool support in virtio_net driver to enable page recycling
in RX buffer allocation and avoid repeated page allocator calls. This
applies to mergeable and small buffer modes.

Beyond performance improvements, this patch is a prerequisite for enabling
memory provider-based zero-copy features in virtio_net, specifically devmem
TCP and io_uring ZCRX, which require drivers to use page_pool for buffer
management.

The implementation preserves the DMA premapping optimization introduced in
commit 31f3cd4e5756 ("virtio-net: rq submits premapped per-buffer") by
conditionally using PP_FLAG_DMA_MAP when the virtio backend supports
standard DMA API (vhost, virtio-pci), and falling back to allocation-only
mode for backends with custom DMA mechanisms (VDUSE).

Changes in v2
=============

Addressing reviewer feedback from v1:

- Add "select PAGE_POOL" to Kconfig (Jason Wang)
- Move page pool creation from ndo_open to probe for device lifetime
  management (Xuan Zhuo, Jason Wang)
- Implement conditional DMA strategy using virtqueue_dma_dev():
  - When non-NULL: use PP_FLAG_DMA_MAP for page_pool-managed DMA premapping
  - When NULL (VDUSE): page_pool handles allocation only
- Use page_pool_get_dma_addr() + virtqueue_add_inbuf_premapped() to
  preserve DMA premapping optimization from commit 31f3cd4e5756
  ("virtio-net: rq submits premapped per-buffer") (Jason Wang)
- Remove dual allocation code paths - page_pool now always used for
  small/mergeable modes (Jason Wang)
- Remove unused virtnet_rq_alloc/virtnet_rq_init_one_sg functions
- Add comprehensive performance data (Michael S. Tsirkin)
- v1 link: https://lore.kernel.org/virtualization/20260106221924.123...

Performance Results
===================

Tested using iperf3 TCP_STREAM with virtio-net on vhost backend.
300-second runs, results show throughput and TCP retransmissions.
The base kernel is synced to net tree and commit: 709bbb015538.

Mergeable Buffer Mode (mrg_rxbuf=on, GSO enabled, MTU 1500):
+--------+---------+---------+------------+------------+--------+--------+
| Queues | Streams |  Patch  | Throughput |   Retries  | Delta  | Retry% |
+--------+---------+---------+------------+------------+--------+--------+
|   1    |    1    |  base   |  25.7 Gbps |      0     |   -    |   -    |
|   1    |    1    |   pp    |  26.2 Gbps |      0     | +1.9%  |   0%   |
+--------+---------+---------+------------+------------+--------+--------+
|   8    |    8    |  base   |  95.6 Gbps |  236,432   |   -    |   -    |
|   8    |    8    |   pp    |  97.9 Gbps |  188,249   | +2.4%  | -20.4% |
+--------+---------+---------+------------+------------+--------+--------+

Small Buffer Mode (mrg_rxbuf=off, GSO disabled, MTU 1500):
+--------+---------+---------+------------+------------+--------+--------+
| Queues | Streams |  Patch  | Throughput |   Retries  | Delta  | Retry% |
+--------+---------+---------+------------+------------+--------+--------+
|   1    |    1    |  base   |  9.17 Gbps |    15,152  |   -    |   -    |
|   1    |    1    |   pp    |  9.19 Gbps |    12,203  | +0.2%  | -19.5% |
+--------+---------+---------+------------+------------+--------+--------+
|   8    |    8    |  base   | 43.0 Gbps  |   974,500  |   -    |   -    |
|   8    |    8    |   pp    | 44.7 Gbps  |   717,411  | +4.0%  | -26.4% |
+--------+---------+---------+------------+------------+--------+--------+

Testing
=======

The patches have been tested with:
- iperf3 bulk transfer workloads (multiple queue/stream configurations)
- Included selftests for buffer circulation verification
- Edge case testing: device unbind/bind cycles, rapid interface open/close,
  traffic during close, ethtool feature toggling, close with pending refill
  work, and data integrity verification

Vishwanath Seshagiri (2):
  virtio_net: add page_pool support for buffer allocation
  selftests: virtio_net: add buffer circulation test

 drivers/net/Kconfig                           |   1 +
 drivers/net/virtio_net.c                      | 353 ++++++++++--------
 .../drivers/net/virtio_net/basic_features.sh  |  70 ++++
 3 files changed, 273 insertions(+), 151 deletions(-)

--
2.47.3