net: Document netmem driver support

   netdevices

   netfilter-sysctl

   netif-msg

   netmem

   nexthop-group-resilient

   nf_conntrack-sysctl

   nf_flowtable

.. SPDX-License-Identifier: GPL-2.0

==================================

Netmem Support for Network Drivers

==================================

This document outlines the requirements for network drivers to support netmem,

an abstract memory type that enables features like device memory TCP. By

supporting netmem, drivers can work with various underlying memory types

with little to no modification.

Benefits of Netmem :

* Flexibility: Netmem can be backed by different memory types (e.g., struct

  page, DMA-buf), allowing drivers to support various use cases such as device

  memory TCP.

* Future-proof: Drivers with netmem support are ready for upcoming

  features that rely on it.

* Simplified Development: Drivers interact with a consistent API,

  regardless of the underlying memory implementation.

Driver Requirements

===================

1. The driver must support page_pool.

2. The driver must support the tcp-data-split ethtool option.

3. The driver must use the page_pool netmem APIs for payload memory. The netmem

   APIs currently 1-to-1 correspond with page APIs. Conversion to netmem should

   be achievable by switching the page APIs to netmem APIs and tracking memory

   via netmem_refs in the driver rather than struct page * :

   - page_pool_alloc -> page_pool_alloc_netmem

   - page_pool_get_dma_addr -> page_pool_get_dma_addr_netmem

   - page_pool_put_page -> page_pool_put_netmem

   Not all page APIs have netmem equivalents at the moment. If your driver

   relies on a missing netmem API, feel free to add and propose to netdev@, or

   reach out to the maintainers and/or almasrymina@google.com for help adding

   the netmem API.

4. The driver must use the following PP_FLAGS:

   - PP_FLAG_DMA_MAP: netmem is not dma-mappable by the driver. The driver

     must delegate the dma mapping to the page_pool, which knows when

     dma-mapping is (or is not) appropriate.

   - PP_FLAG_DMA_SYNC_DEV: netmem dma addr is not necessarily dma-syncable

     by the driver. The driver must delegate the dma syncing to the page_pool,

     which knows when dma-syncing is (or is not) appropriate.

   - PP_FLAG_ALLOW_UNREADABLE_NETMEM. The driver must specify this flag iff

     tcp-data-split is enabled.

5. The driver must not assume the netmem is readable and/or backed by pages.

   The netmem returned by the page_pool may be unreadable, in which case

   netmem_address() will return NULL. The driver must correctly handle

   unreadable netmem, i.e. don't attempt to handle its contents when

   netmem_address() is NULL.

   Ideally, drivers should not have to check the underlying netmem type via

   helpers like netmem_is_net_iov() or convert the netmem to any of its

   underlying types via netmem_to_page() or netmem_to_net_iov(). In most cases,

   netmem or page_pool helpers that abstract this complexity are provided

   (and more can be added).

6. The driver must use page_pool_dma_sync_netmem_for_cpu() in lieu of

   dma_sync_single_range_for_cpu(). For some memory providers, dma_syncing for

   CPU will be done by the page_pool, for others (particularly dmabuf memory

   provider), dma syncing for CPU is the responsibility of the userspace using

   dmabuf APIs. The driver must delegate the entire dma-syncing operation to

   the page_pool which will do it correctly.

7. Avoid implementing driver-specific recycling on top of the page_pool. Drivers

   cannot hold onto a struct page to do their own recycling as the netmem may

   not be backed by a struct page. However, you may hold onto a page_pool

   reference with page_pool_fragment_netmem() or page_pool_ref_netmem() for

   that purpose, but be mindful that some netmem types might have longer

   circulation times, such as when userspace holds a reference in zerocopy

   scenarios.