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.. 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.
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