From 5febf432df1cfa5b25d99b54c32103fafbdd0eb9 Mon Sep 17 00:00:00 2001 From: Miguel Ojeda Date: Sun, 16 Aug 2026 15:32:33 +0200 Subject: kbuild: rust: keep Rust objects out of Clang LTO with inline helpers Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit `objtool` errors such as: vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a: can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd The reason is that in such builds, the Clang invocation that compiles the combined Rust plus helpers bitcode emits LLVM bitcode (again) -- the final code generation happens in the linker's LTO step, which the `-mllvm` trap options passed to Clang do not reach. This, in turn, means that unreachable traps are missing, and the impossible paths do not merely fallthrough to the next symbol, but past the end of their own section, since LTO builds place each function in its own section. Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always emits machine code directly, with the traps in place. Assisted-by: LLM Cc: Gary Guo Cc: Boqun Feng Cc: Alice Ryhl Cc: Matthew Maurer Cc: Josh Poimboeuf Cc: Peter Zijlstra Cc: stable@vger.kernel.org Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust") Acked-by: Gary Guo Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org Signed-off-by: Miguel Ojeda --- Documentation/networking/netmem.rst | 104 ++++++++++++++++++++++++++++++++++++ 1 file changed, 104 insertions(+) create mode 100644 Documentation/networking/netmem.rst (limited to 'Documentation/networking/netmem.rst') diff --git a/Documentation/networking/netmem.rst b/Documentation/networking/netmem.rst new file mode 100644 index 000000000..217869d11 --- /dev/null +++ b/Documentation/networking/netmem.rst @@ -0,0 +1,104 @@ +.. 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 RX 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. + +Driver TX Requirements +====================== + +1. The Driver must not pass the netmem dma_addr to any of the dma-mapping APIs + directly. This is because netmem dma_addrs may come from a source like + dma-buf that is not compatible with the dma-mapping APIs. + + Helpers like netmem_dma_unmap_page_attrs() & netmem_dma_unmap_addr_set() + should be used in lieu of dma_unmap_page[_attrs](), dma_unmap_addr_set(). + The netmem variants will handle netmem dma_addrs correctly regardless of the + source, delegating to the dma-mapping APIs when appropriate. + + Not all dma-mapping 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. + +2. Driver should declare support by setting `netdev->netmem_tx` to the + appropriate mode: + + - `NETMEM_TX_DMA`: for physical devices that perform DMA. + + - `NETMEM_TX_NO_DMA`: for virtual or passthrough devices that do + not DMA, but still support handling of netmem-backed skbs. -- cgit v1.3.1