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@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
Thanks for the updates, looks good.
Reviewed-by: Shannon Nelson <shannon.nelson@xxxxxxx>
