Date: Wed, 15 May 2024 13:19:47 +0300
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: Mina Almasry <almasrymina@...gle.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
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Cc: Donald Hunter <donald.hunter@...il.com>, Jakub Kicinski
 <kuba@...nel.org>, "David S. Miller" <davem@...emloft.net>,
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Subject: Re: [PATCH net-next v9 04/14] netdev: support binding dma-buf to
 netdevice

On 15/05/2024 13:01, Nikolay Aleksandrov wrote:
> 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@...gle.com>
>> Signed-off-by: Kaiyuan Zhang <kaiyuanz@...gle.com>
>> Signed-off-by: Mina Almasry <almasrymina@...gle.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.
> 

On second thought nevermind this, sorry for the noise.

>> +	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
> 

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