Re: [PATCH RFC net-next v2 5/6] net: devmem: Implement TX path

On Thu, Jan 30, 2025 at 09:15:38PM +0000, Mina Almasry wrote:

Augment dmabuf binding to be able to handle TX. Additional to all the RX binding, we also create tx_vec needed for the TX path.

Provide API for sendmsg to be able to send dmabufs bound to this device:

• Provide a new dmabuf_tx_cmsg which includes the dmabuf to send from.
• MSG_ZEROCOPY with SCM_DEVMEM_DMABUF cmsg indicates send from dma-buf.

Devmem is uncopyable, so piggyback off the existing MSG_ZEROCOPY implementation, while disabling instances where MSG_ZEROCOPY falls back to copying.

We additionally pipe the binding down to the new zerocopy_fill_skb_from_devmem which fills a TX skb with net_iov netmems instead of the traditional page netmems.

We also special case skb_frag_dma_map to return the dma-address of these dmabuf net_iovs instead of attempting to map pages.

Based on work by Stanislav Fomichev sdf@fomichev.me. A lot of the meat of the implementation came from devmem TCP RFC v1[1], which included the TX path, but Stan did all the rebasing on top of netmem/net_iov.

Cc: Stanislav Fomichev sdf@fomichev.me Signed-off-by: Kaiyuan Zhang kaiyuanz@google.com Signed-off-by: Mina Almasry almasrymina@google.com

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diff --git a/net/core/netdev-genl.c b/net/core/netdev-genl.c

index 0e41699df419..9ba6994e2a05 100644 --- a/net/core/netdev-genl.c +++ b/net/core/netdev-genl.c

...

@@ -911,10 +912,68 @@ int netdev_nl_bind_rx_doit(struct sk_buff *skb, struct genl_info *info) return err; } -/* stub */ int netdev_nl_bind_tx_doit(struct sk_buff *skb, struct genl_info *info) {

• return 0;

• struct net_devmem_dmabuf_binding *binding;
• struct list_head *sock_binding_list;
• struct net_device *netdev;
• u32 ifindex, dmabuf_fd;
• struct sk_buff *rsp;
• int err = 0;
• void *hdr;
• if (GENL_REQ_ATTR_CHECK(info, NETDEV_A_DEV_IFINDEX) ||

•    GENL_REQ_ATTR_CHECK(info, NETDEV_A_DMABUF_FD))
  

• return -EINVAL;
  

• ifindex = nla_get_u32(info->attrs[NETDEV_A_DEV_IFINDEX]);
• dmabuf_fd = nla_get_u32(info->attrs[NETDEV_A_DMABUF_FD]);
• sock_binding_list =

• genl_sk_priv_get(&netdev_nl_family, NETLINK_CB(skb).sk);
  

• if (IS_ERR(sock_binding_list))

• return PTR_ERR(sock_binding_list);
  

• rsp = genlmsg_new(GENLMSG_DEFAULT_SIZE, GFP_KERNEL);
• if (!rsp)

• return -ENOMEM;
  

• hdr = genlmsg_iput(rsp, info);
• if (!hdr) {

• err = -EMSGSIZE;
  

• goto err_genlmsg_free;
  

• }
• rtnl_lock();
• netdev = __dev_get_by_index(genl_info_net(info), ifindex);
• if (!netdev || !netif_device_present(netdev)) {

• err = -ENODEV;
  

• goto err_unlock;
  

• }
• binding = net_devmem_bind_dmabuf(netdev, DMA_TO_DEVICE, dmabuf_fd,

• 			 info->extack);
  

• if (IS_ERR(binding)) {

• err = PTR_ERR(binding);
  

• goto err_unlock;
  

• }
• list_add(&binding->list, sock_binding_list);
• nla_put_u32(rsp, NETDEV_A_DMABUF_ID, binding->id);
• genlmsg_end(rsp, hdr);
• rtnl_unlock();
• return genlmsg_reply(rsp, info);
• net_devmem_unbind_dmabuf(binding);

Hi Mina,

It appears that the line above is unreachable. I guess it was part of an unwind that is no-longer needed and thus can now be removed.

Flagged by Smatch.

+err_unlock:

• rtnl_unlock();

+err_genlmsg_free:

• nlmsg_free(rsp);
• return err;

}

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