linux-kernel - Re: [PATCH RFC net-next v1 5/5] net: devmem: Implement TX path

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Message-ID: <53192c45-df3c-4a65-9047-bbd59d4aee47@gmail.com>
Date: Wed, 5 Feb 2025 12:41:11 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 Mina Almasry <almasrymina@...gle.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, virtualization@...ts.linux.dev,
 kvm@...r.kernel.org, linux-kselftest@...r.kernel.org,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, Donald Hunter <donald.hunter@...il.com>,
 Jonathan Corbet <corbet@....net>, Andrew Lunn <andrew+netdev@...n.ch>,
 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>,
 Kaiyuan Zhang <kaiyuanz@...gle.com>, Willem de Bruijn <willemb@...gle.com>,
 Samiullah Khawaja <skhawaja@...gle.com>, Stanislav Fomichev
 <sdf@...ichev.me>, Joe Damato <jdamato@...tly.com>, dw@...idwei.uk
Subject: Re: [PATCH RFC net-next v1 5/5] net: devmem: Implement TX path

On 1/28/25 14:49, Willem de Bruijn wrote:
>>>> +struct net_devmem_dmabuf_binding *
>>>> +net_devmem_get_sockc_binding(struct sock *sk, struct sockcm_cookie *sockc)
>>>> +{
>>>> +     struct net_devmem_dmabuf_binding *binding;
>>>> +     int err = 0;
>>>> +
>>>> +     binding = net_devmem_lookup_dmabuf(sockc->dmabuf_id);
>>>
>>> This lookup is from global xarray net_devmem_dmabuf_bindings.
>>>
>>> Is there a check that the socket is sending out through the device
>>> to which this dmabuf was bound with netlink? Should there be?
>>> (e.g., SO_BINDTODEVICE).
>>>
>>
>> Yes, I think it may be an issue if the user triggers a send from a
>> different netdevice, because indeed when we bind a dmabuf we bind it
>> to a specific netdevice.
>>
>> One option is as you say to require TX sockets to be bound and to
>> check that we're bound to the correct netdev. I also wonder if I can
>> make this work without SO_BINDTODEVICE, by querying the netdev the
>> sock is currently trying to send out on and doing a check in the
>> tcp_sendmsg. I'm not sure if this is possible but I'll give it a look.
> 
> I was a bit quick on mentioning SO_BINDTODEVICE. Agreed that it is
> vastly preferable to not require that, but infer the device from
> the connected TCP sock.

I wonder why so? I'd imagine something like SO_BINDTODEVICE is a
better way to go. The user has to do it anyway, otherwise packets
might go to a different device and the user would suddenly start
getting errors with no good way to alleviate them (apart from
likes of SO_BINDTODEVICE). It's even worse if it works for a while
but starts to unpredictably fail as time passes. With binding at
least it'd fail fast if the setup is not done correctly.

-- 
Pavel Begunkov