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

From: Simon Horman <[email-masked]>
Date: Fri, 31 Jan 2025 09:09:27 +0000

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 <[email-masked]>. 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 <[email-masked]>
> Signed-off-by: Kaiyuan Zhang <[email-masked]>
> Signed-off-by: Mina Almasry <[email-masked]>

...

> 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;
>  }

...