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Wed, 11 Nov 2026 14:47:47 -0800 (PST) Received: from localhost ([2a03:2880:9ff:71::]) by smtp.gmail.com with ESMTPSA id a92af1059eb24-1b3b5b4efc2sm16602523c88.15.2026.11.11.14.47.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 11 Nov 2026 14:47:46 -0800 (PST) From: Stanislav Fomichev X-Google-Original-From: Stanislav Fomichev To: netdev@vger.kernel.org Cc: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, michael.chan@broadcom.com, pavan.chebbi@broadcom.com, andrew+netdev@lunn.ch, edumazet@kernel.org, horms@kernel.org, shuah@kernel.org, almasrymina@google.com, sdf@fomichev.me, leitao@debian.org, dtatulea@nvidia.com, asml.silence@gmail.com, bobbyeshleman@meta.com, daniel@iogearbox.net, razor@blackwall.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: [PATCH net-next v4 4/5] net: devmem: size TX net_iovs from the dmabuf sg table Date: Wed, 11 Nov 2026 14:47:39 -0800 Message-ID: <20261111224740.2744522-5-sdf@fomichev.me> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20261111224740.2744522-1-sdf@fomichev.me> References: <20261111224740.2744522-1-sdf@fomichev.me> X-Mailing-List: netdev@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit PAGE_SIZE TX net_iovs cap devmem skbs at MAX_SKB_FRAGS pages (~68KB), which makes BIG TCP ineffective. TX net_iovs only need to be DMA contiguous. Use the largest power of 2, from PAGE_SIZE up to 2MB, which aligns every DMA segment of the dmabuf: - 2MB hugepage-backed udmabufs get 2MB net_iovs - no UAPI change, TX addresses are dmabuf offsets - skbs keep their size, only the frags get larger The net_iov size doesn't depend on the device. A frag is never larger than its skb, and the skb is never larger than the device's gso_max_size. TCP sizes skbs to fit, and validate_xmit_skb() segments bigger ones. gso_max_size is 64KB unless BIG TCP is enabled. With BIG TCP, page frags can already be 64KB or larger, for example with MSG_ZEROCOPY from huge pages or with 64KB pages. So drivers that enable BIG TCP already have to handle them. bnxt doesn't handle them yet. It has a 16-bit BD length and needs a separate fix. Signed-off-by: Stanislav Fomichev Reviewed-by: Mina Almasry Reviewed-by: Dragos Tatulea --- net/core/devmem.c | 28 ++++++++++++++++++++++++++-- net/core/netdev-genl.c | 2 +- 2 files changed, 27 insertions(+), 3 deletions(-) diff --git a/net/core/devmem.c b/net/core/devmem.c index c6868f39261a..713528048897 100644 --- a/net/core/devmem.c +++ b/net/core/devmem.c @@ -303,6 +303,20 @@ int net_devmem_bind_dmabuf_to_queue(struct net_device *dev, u32 rxq_idx, return err; } +static bool net_devmem_sgt_aligned(struct sg_table *sgt, size_t size) +{ + struct scatterlist *sg; + unsigned int i; + + for_each_sgtable_dma_sg(sgt, sg, i) { + if (!IS_ALIGNED(sg_dma_address(sg), size) || + !IS_ALIGNED(sg_dma_len(sg), size)) + return false; + } + return true; +} + +/* niov_shift == 0 picks the net_iov size from the dmabuf, TX only */ struct net_devmem_dmabuf_binding * net_devmem_bind_dmabuf(struct net_device *dev, void *vdev, struct device *dma_dev, @@ -313,11 +327,11 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev, struct netlink_ext_ack *extack) { struct net_devmem_dmabuf_binding *binding; - size_t niov_size = 1UL << niov_shift; static u32 id_alloc_next; struct scatterlist *sg; struct dma_buf *dmabuf; unsigned int sg_idx; + size_t niov_size; size_t niov_idx; size_t i; long err; @@ -342,7 +356,6 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev, binding->vdev = vdev; write_pnet(&binding->net, sock_net(nl_sk)); binding->uid = sk_uid(nl_sk); - binding->niov_shift = niov_shift; xa_init_flags(&binding->bound_rxqs, XA_FLAGS_ALLOC); err = percpu_ref_init(&binding->ref, @@ -387,6 +400,17 @@ net_devmem_bind_dmabuf(struct net_device *dev, void *vdev, goto err_unmap; } + if (!niov_shift) { + niov_size = SZ_2M; + while (niov_size > PAGE_SIZE && + (!IS_ALIGNED(dmabuf->size, niov_size) || + !net_devmem_sgt_aligned(binding->sgt, niov_size))) + niov_size /= 2; + niov_shift = ilog2(niov_size); + } + binding->niov_shift = niov_shift; + niov_size = 1UL << niov_shift; + if (!IS_ALIGNED(dmabuf->size, niov_size)) { err = -EINVAL; NL_SET_ERR_MSG_FMT(extack, diff --git a/net/core/netdev-genl.c b/net/core/netdev-genl.c index 9c5bf17b04e5..d2ccb1090496 100644 --- a/net/core/netdev-genl.c +++ b/net/core/netdev-genl.c @@ -1261,7 +1261,7 @@ int netdev_nl_bind_tx_doit(struct sk_buff *skb, struct genl_info *info) binding = net_devmem_bind_dmabuf(bind_dev, bind_dev != netdev ? netdev : NULL, dma_dev, DMA_TO_DEVICE, dmabuf_fd, - PAGE_SHIFT, priv, NETLINK_CB(skb).sk, + 0, priv, NETLINK_CB(skb).sk, info->extack); if (IS_ERR(binding)) { err = PTR_ERR(binding); -- 2.53.0-Meta