netdev - Re: [PATCH net-next v3 5/6] net: devmem: Implement TX path

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Message-ID: <09a0caba-db29-4100-a46d-b104b282ea0a@redhat.com>
Date: Tue, 4 Feb 2025 13:15:38 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Mina Almasry <almasrymina@...gle.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 kvm@...r.kernel.org, virtualization@...ts.linux.dev,
 linux-kselftest@...r.kernel.org
Cc: Donald Hunter <donald.hunter@...il.com>, Jakub Kicinski
 <kuba@...nel.org>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Simon Horman <horms@...nel.org>,
 Jonathan Corbet <corbet@....net>, Andrew Lunn <andrew+netdev@...n.ch>,
 Neal Cardwell <ncardwell@...gle.com>, David Ahern <dsahern@...nel.org>,
 "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez
 <eperezma@...hat.com>, Stefan Hajnoczi <stefanha@...hat.com>,
 Stefano Garzarella <sgarzare@...hat.com>, Shuah Khan <shuah@...nel.org>,
 sdf@...ichev.me, asml.silence@...il.com, dw@...idwei.uk,
 Jamal Hadi Salim <jhs@...atatu.com>, Victor Nogueira <victor@...atatu.com>,
 Pedro Tammela <pctammela@...atatu.com>,
 Samiullah Khawaja <skhawaja@...gle.com>, Kaiyuan Zhang <kaiyuanz@...gle.com>
Subject: Re: [PATCH net-next v3 5/6] net: devmem: Implement TX path

On 2/3/25 11:39 PM, 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@...ichev.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@...ichev.me>
> Signed-off-by: Kaiyuan Zhang <kaiyuanz@...gle.com>
> Signed-off-by: Mina Almasry <almasrymina@...gle.com>

Very minor nit: you unexpectedly leaved a lot of empty lines after the SoB.

[...]
@@ -240,13 +249,23 @@ net_devmem_bind_dmabuf(struct net_device *dev,
unsigned int dmabuf_fd,
>  	 * binding can be much more flexible than that. We may be able to
>  	 * allocate MTU sized chunks here. Leave that for future work...
>  	 */
> -	binding->chunk_pool =
> -		gen_pool_create(PAGE_SHIFT, dev_to_node(&dev->dev));
> +	binding->chunk_pool = gen_pool_create(PAGE_SHIFT,
> +					      dev_to_node(&dev->dev));
>  	if (!binding->chunk_pool) {
>  		err = -ENOMEM;
>  		goto err_unmap;
>  	}
>  
> +	if (direction == DMA_TO_DEVICE) {
> +		binding->tx_vec = kvmalloc_array(dmabuf->size / PAGE_SIZE,
> +						 sizeof(struct net_iov *),
> +						 GFP_KERNEL);
> +		if (!binding->tx_vec) {
> +			err = -ENOMEM;
> +			goto err_free_chunks;

It looks like the later error paths (in the for_each_sgtable_dma_sg()
loop) could happen even for 'direction == DMA_TO_DEVICE', so I guess an
additional error label is needed to clean tx_vec on such paths.

/P