Re: [PATCH net-next v5] net: Document netmem driver support

["Nelson, Shannon" <shannon.nelson@amd.com>]

On 12/17/2024 12:12 PM, Mina Almasry wrote:
> 
> Document expectations from drivers looking to add support for device
> memory tcp or other netmem based features.
> 
> Signed-off-by: Mina Almasry <almasrymina@google.com>
> 
> ---
> 
> v5 (forked from the merged series):
> - Describe benefits of netmem (Shannon).
> - Specify that netmem is for payload pages (Jakub).
> - Clarify what  recycling the driver can do (Jakub).
> - Clarify why the driver needs to use DMA_SYNC and DMA_MAP pp flags
>    (Shannon).
> 
> v4:
> - Address comments from Randy.
> - Change docs to netmem focus (Jakub).
> - Address comments from Jakub.
> 
> ---
>   Documentation/networking/index.rst  |  1 +
>   Documentation/networking/netmem.rst | 79 +++++++++++++++++++++++++++++
>   2 files changed, 80 insertions(+)
>   create mode 100644 Documentation/networking/netmem.rst
> 
> diff --git a/Documentation/networking/index.rst b/Documentation/networking/index.rst
> index 46c178e564b3..058193ed2eeb 100644
> --- a/Documentation/networking/index.rst
> +++ b/Documentation/networking/index.rst
> @@ -86,6 +86,7 @@ Contents:
>      netdevices
>      netfilter-sysctl
>      netif-msg
> +   netmem
>      nexthop-group-resilient
>      nf_conntrack-sysctl
>      nf_flowtable
> diff --git a/Documentation/networking/netmem.rst b/Documentation/networking/netmem.rst
> new file mode 100644
> index 000000000000..7de21ddb5412
> --- /dev/null
> +++ b/Documentation/networking/netmem.rst
> @@ -0,0 +1,79 @@
> +.. 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.
> --
> 2.47.1.613.gc27f4b7a9f-goog
> 
> 

Thanks for the updates, looks good.

Reviewed-by: Shannon Nelson <shannon.nelson@amd.com>