Document expectations from drivers looking to add support for device memory tcp or other netmem based features. Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx> --- 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@xxxxxxxxxx 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