Re: [PATCH net-next v13 4/9] net: devmem: Implement TX path

Paolo Abeni
2025-05-02T11:51:11Z

On 5/2/25 1:47 PM, Paolo Abeni wrote:
> On 4/29/25 5:26 AM, 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.
>>
>> The TX path may release the dmabuf in a context where we cannot wait.
>> This happens when the user unbinds a TX dmabuf while there are still
>> references to its netmems in the TX path. In that case, the netmems will
>> be put_netmem'd from a context where we can't unmap the dmabuf, Resolve
>> this by making __net_devmem_dmabuf_binding_free schedule_work'd.
>>
>> Based on work by Stanislav Fomichev <sdf@fomichev.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@fomichev.me>
>> Signed-off-by: Kaiyuan Zhang <kaiyuanz@google.com>
>> Signed-off-by: Mina Almasry <almasrymina@google.com>
>> Acked-by: Stanislav Fomichev <sdf@fomichev.me>
>
> I'm sorry for the late feedback. A bunch of things I did not notice
> before...

The rest LGTM, and my feedback here ranges from nit to corner-cases, so
we are probably better off with a follow-up than with a repost, other
opinions welcome!

/P