Re: [PATCH net-next v9 04/14] netdev: support binding dma-buf to netdevice

[Nikolay Aleksandrov <razor@blackwall.org>]

On 11/05/2024 02:21, 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 <willemb@google.com>
> Signed-off-by: Kaiyuan Zhang <kaiyuanz@google.com>
> Signed-off-by: Mina Almasry <almasrymina@google.com>
> 
> ---
> 
> v9: https://lore.kernel.org/all/20240403002053.2376017-5-almasrymina@google.com/
> - Removed net_devmem_restart_rx_queues and put it in its own patch
>   (David).
> 
> v8:
> - move dmabuf_devmem_ops usage to later patch to avoid patch-by-patch
>   build error.
> 
> v7:
> - Use IS_ERR() instead of IS_ERR_OR_NULL() for the dma_buf_get() return
>   value.
> - Changes netdev_* naming in devmem.c to net_devmem_* (Yunsheng).
> - DMA_BIDIRECTIONAL -> DMA_FROM_DEVICE (Yunsheng).
> - Added a comment around recovering of the old rx queue in
>   net_devmem_restart_rx_queue(), and added freeing of old_mem if the
>   restart of the old queue fails. (Yunsheng).
> - Use kernel-family sock-priv (Jakub).
> - Put pp_memory_provider_params in netdev_rx_queue instead of the
>   dma-buf specific binding (Pavel & David).
> - Move queue management ops to queue_mgmt_ops instead of netdev_ops
>   (Jakub).
> - Remove excess whitespaces (Jakub).
> - Use genlmsg_iput (Jakub).
> 
> v6:
> - Validate rx queue index
> - Refactor new functions into devmem.c (Pavel)
> 
> v5:
> - Renamed page_pool_iov to net_iov, and moved that support to devmem.h
>   or netmem.h.
> 
> v1:
> - Introduce devmem.h instead of bloating netdevice.h (Jakub)
> - ENOTSUPP -> EOPNOTSUPP (checkpatch.pl I think)
> - Remove unneeded rcu protection for binding->list (rtnl protected)
> - Removed extraneous err_binding_put: label.
> - Removed dma_addr += len (Paolo).
> - Don't override err on netdev_bind_dmabuf_to_queue failure.
> - Rename devmem -> dmabuf (David).
> - Add id to dmabuf binding (David/Stan).
> - Fix missing xa_destroy bound_rq_list.
> - Use queue api to reset bound RX queues (Jakub).
> - Update netlink API for rx-queue type (tx/re) (Jakub).
> 
> RFC v3:
> - Support multi rx-queue binding
> 
> ---
>  Documentation/netlink/specs/netdev.yaml |   4 +
>  include/net/devmem.h                    | 111 +++++++++++
>  include/net/netdev_rx_queue.h           |   2 +
>  include/net/netmem.h                    |  10 +
>  include/net/page_pool/types.h           |   5 +
>  net/core/Makefile                       |   2 +-
>  net/core/dev.c                          |   3 +
>  net/core/devmem.c                       | 254 ++++++++++++++++++++++++
>  net/core/netdev-genl-gen.c              |   4 +
>  net/core/netdev-genl-gen.h              |   4 +
>  net/core/netdev-genl.c                  | 105 +++++++++-
>  11 files changed, 501 insertions(+), 3 deletions(-)
>  create mode 100644 include/net/devmem.h
>  create mode 100644 net/core/devmem.c
> 
[snip]
> +/* Protected by rtnl_lock() */
> +static DEFINE_XARRAY_FLAGS(net_devmem_dmabuf_bindings, XA_FLAGS_ALLOC1);
> +
> +void net_devmem_unbind_dmabuf(struct net_devmem_dmabuf_binding *binding)
> +{
> +	struct netdev_rx_queue *rxq;
> +	unsigned long xa_idx;
> +	unsigned int rxq_idx;
> +
> +	if (!binding)
> +		return;
> +
> +	if (binding->list.next)
> +		list_del(&binding->list);
> +

minor nit:
In theory list.next can still be != null if it's poisoned (e.g. after del). You can
use the list api here (!list_empty(&binding->list) -> list_del_init(&binding->list))
if you initialize it in net_devmem_bind_dmabuf(), then you'll also get nice list
debugging.

> +	xa_for_each(&binding->bound_rxq_list, xa_idx, rxq) {
> +		if (rxq->mp_params.mp_priv == binding) {
> +			/* 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 page_pool
> +			 * may read this config while it's creating its
> +			 * rx-queues. WRITE_ONCE() here to match the
> +			 * READ_ONCE() in the page_pool.
> +			 */
> +			WRITE_ONCE(rxq->mp_params.mp_ops, NULL);
> +			WRITE_ONCE(rxq->mp_params.mp_priv, NULL);
> +
> +			rxq_idx = get_netdev_rx_queue_index(rxq);
> +
> +			netdev_rx_queue_restart(binding->dev, rxq_idx);
> +		}
> +	}
> +
> +	xa_erase(&net_devmem_dmabuf_bindings, binding->id);
> +
> +	net_devmem_dmabuf_binding_put(binding);
> +}
[snip]

Cheers,
 Nik