mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2026-08-09 06:14:34 +02:00
Bobby EshlemanandJakub Kicinski
6ce2bb0480
Some virtual devices like netkit (or ifb) never DMA and never touch frag contents, they just forward the skb to another device. They are unable to forward unreadable skbs, however, because they fail to pass TX validation checks on dev->netmem_tx. The existing two-state NETMEM_TX_NONE / NETMEM_TX_DMA doesn't give the TX validator enough information to differentiate devices that will attempt DMA on the unreadable skb from those that will simply route it untouched. Add a third mode to the enum so drivers can indicate 1) if they have netmem TX support, and 2) if they do, whether they are DMA-capable: NETMEM_TX_NO_DMA - pass-through, device never DMAs Widen dev->netmem_tx from a 1-bit field to 2 bits to fit the new value, and declare netkit as NETMEM_TX_NO_DMA. Devmem TX support over these devices comes in a follow-up patch. Acked-by: Stanislav Fomichev <sdf@fomichev.me> Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com> Link: https://patch.msgid.link/20260514-tcp-dm-netkit-v5-2-408c59b91e66@meta.com Signed-off-by: Jakub Kicinski <kuba@kernel.org>
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105 lines
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.. SPDX-License-Identifier: GPL-2.0
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==================================
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Netmem Support for Network Drivers
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==================================
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This document outlines the requirements for network drivers to support netmem,
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an abstract memory type that enables features like device memory TCP. By
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supporting netmem, drivers can work with various underlying memory types
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with little to no modification.
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Benefits of Netmem :
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* Flexibility: Netmem can be backed by different memory types (e.g., struct
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page, DMA-buf), allowing drivers to support various use cases such as device
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memory TCP.
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* Future-proof: Drivers with netmem support are ready for upcoming
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features that rely on it.
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* Simplified Development: Drivers interact with a consistent API,
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regardless of the underlying memory implementation.
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Driver RX Requirements
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======================
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1. The driver must support page_pool.
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2. The driver must support the tcp-data-split ethtool option.
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3. The driver must use the page_pool netmem APIs for payload memory. The netmem
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APIs currently 1-to-1 correspond with page APIs. Conversion to netmem should
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be achievable by switching the page APIs to netmem APIs and tracking memory
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via netmem_refs in the driver rather than struct page * :
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- page_pool_alloc -> page_pool_alloc_netmem
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- page_pool_get_dma_addr -> page_pool_get_dma_addr_netmem
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- page_pool_put_page -> page_pool_put_netmem
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Not all page APIs have netmem equivalents at the moment. If your driver
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relies on a missing netmem API, feel free to add and propose to netdev@, or
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reach out to the maintainers and/or almasrymina@google.com for help adding
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the netmem API.
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4. The driver must use the following PP_FLAGS:
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- PP_FLAG_DMA_MAP: netmem is not dma-mappable by the driver. The driver
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must delegate the dma mapping to the page_pool, which knows when
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dma-mapping is (or is not) appropriate.
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- PP_FLAG_DMA_SYNC_DEV: netmem dma addr is not necessarily dma-syncable
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by the driver. The driver must delegate the dma syncing to the page_pool,
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which knows when dma-syncing is (or is not) appropriate.
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- PP_FLAG_ALLOW_UNREADABLE_NETMEM. The driver must specify this flag iff
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tcp-data-split is enabled.
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5. The driver must not assume the netmem is readable and/or backed by pages.
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The netmem returned by the page_pool may be unreadable, in which case
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netmem_address() will return NULL. The driver must correctly handle
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unreadable netmem, i.e. don't attempt to handle its contents when
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netmem_address() is NULL.
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Ideally, drivers should not have to check the underlying netmem type via
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helpers like netmem_is_net_iov() or convert the netmem to any of its
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underlying types via netmem_to_page() or netmem_to_net_iov(). In most cases,
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netmem or page_pool helpers that abstract this complexity are provided
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(and more can be added).
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6. The driver must use page_pool_dma_sync_netmem_for_cpu() in lieu of
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dma_sync_single_range_for_cpu(). For some memory providers, dma_syncing for
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CPU will be done by the page_pool, for others (particularly dmabuf memory
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provider), dma syncing for CPU is the responsibility of the userspace using
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dmabuf APIs. The driver must delegate the entire dma-syncing operation to
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the page_pool which will do it correctly.
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7. Avoid implementing driver-specific recycling on top of the page_pool. Drivers
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cannot hold onto a struct page to do their own recycling as the netmem may
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not be backed by a struct page. However, you may hold onto a page_pool
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reference with page_pool_fragment_netmem() or page_pool_ref_netmem() for
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that purpose, but be mindful that some netmem types might have longer
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circulation times, such as when userspace holds a reference in zerocopy
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scenarios.
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Driver TX Requirements
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======================
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1. The Driver must not pass the netmem dma_addr to any of the dma-mapping APIs
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directly. This is because netmem dma_addrs may come from a source like
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dma-buf that is not compatible with the dma-mapping APIs.
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Helpers like netmem_dma_unmap_page_attrs() & netmem_dma_unmap_addr_set()
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should be used in lieu of dma_unmap_page[_attrs](), dma_unmap_addr_set().
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The netmem variants will handle netmem dma_addrs correctly regardless of the
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source, delegating to the dma-mapping APIs when appropriate.
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Not all dma-mapping APIs have netmem equivalents at the moment. If your
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driver relies on a missing netmem API, feel free to add and propose to
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netdev@, or reach out to the maintainers and/or almasrymina@google.com for
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help adding the netmem API.
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2. Driver should declare support by setting `netdev->netmem_tx` to the
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appropriate mode:
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- `NETMEM_TX_DMA`: for physical devices that perform DMA.
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- `NETMEM_TX_NO_DMA`: for virtual or passthrough devices that do
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not DMA, but still support handling of netmem-backed skbs.
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