Subject: Re: [RFC PATCH net-next v7 04/14] netdev: support binding dma-buf to netdevice

From: Simon Horman <[email-masked]>
Date: Thu, 28 Mar 2024 18:28:12 +0000

On Tue, Mar 26, 2024 at 03:50:35PM -0700, Mina Almasry wrote:
> Add a netdev_dmabuf_binding struct which represents the
> dma-buf-to-netdevice binding. The netlink API will bind the dma-buf to
> rx queues on the netdevice. On the binding, the dma_buf_attach
> & dma_buf_map_attachment will occur. The entries in the sg_table from
> mapping will be inserted into a genpool to make it ready
> for allocation.
> 
> The chunks in the genpool are owned by a dmabuf_chunk_owner struct which
> holds the dma-buf offset of the base of the chunk and the dma_addr of
> the chunk. Both are needed to use allocations that come from this chunk.
> 
> We create a new type that represents an allocation from the genpool:
> net_iov. We setup the net_iov allocation size in the
> genpool to PAGE_SIZE for simplicity: to match the PAGE_SIZE normally
> allocated by the page pool and given to the drivers.
> 
> The user can unbind the dmabuf from the netdevice by closing the netlink
> socket that established the binding. We do this so that the binding is
> automatically unbound even if the userspace process crashes.
> 
> The binding and unbinding leaves an indicator in struct netdev_rx_queue
> that the given queue is bound, but the binding doesn't take effect until
> the driver actually reconfigures its queues, and re-initializes its page
> pool.
> 
> The netdev_dmabuf_binding struct is refcounted, and releases its
> resources only when all the refs are released.
> 
> Signed-off-by: Willem de Bruijn <[email-masked]>
> Signed-off-by: Kaiyuan Zhang <[email-masked]>
> Signed-off-by: Mina Almasry <[email-masked]>

...

> +int net_devmem_bind_dmabuf_to_queue(struct net_device *dev, u32 rxq_idx,
> +				    struct net_devmem_dmabuf_binding *binding)
> +{
> +	struct netdev_rx_queue *rxq;
> +	u32 xa_idx;
> +	int err;
> +
> +	if (rxq_idx >= dev->num_rx_queues)
> +		return -ERANGE;
> +
> +	rxq = __netif_get_rx_queue(dev, rxq_idx);
> +	if (rxq->mp_params.mp_priv)
> +		return -EEXIST;
> +
> +	err = xa_alloc(&binding->bound_rxq_list, &xa_idx, rxq, xa_limit_32b,
> +		       GFP_KERNEL);
> +	if (err)
> +		return err;
> +
> +	/* We hold the rtnl_lock while binding/unbinding dma-buf, so we can't
> +	 * race with another thread that is also modifying this value. However,
> +	 * the driver may read this config while it's creating its * rx-queues.
> +	 * WRITE_ONCE() here to match the READ_ONCE() in the driver.
> +	 */
> +	WRITE_ONCE(rxq->mp_params.mp_ops, &dmabuf_devmem_ops);

Hi Mina,

This causes a build failure because mabuf_devmem_ops is not added until a
subsequent patch in this series.

> +	WRITE_ONCE(rxq->mp_params.mp_priv, binding);
> +
> +	err = net_devmem_restart_rx_queue(dev, rxq_idx);
> +	if (err)
> +		goto err_xa_erase;
> +
> +	return 0;
> +
> +err_xa_erase:
> +	WRITE_ONCE(rxq->mp_params.mp_ops, NULL);
> +	WRITE_ONCE(rxq->mp_params.mp_priv, NULL);
> +	xa_erase(&binding->bound_rxq_list, xa_idx);
> +
> +	return err;
> +}

...