netdev - Re: [RFC PATCH 00/10] Device Memory TCP

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Date: Tue, 18 Jul 2023 16:35:17 -0600
From: David Ahern <dsahern@...nel.org>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Jason Gunthorpe <jgg@...pe.ca>, Mina Almasry <almasrymina@...gle.com>,
 Andy Lutomirski <luto@...nel.org>, linux-kernel@...r.kernel.org,
 linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linaro-mm-sig@...ts.linaro.org, netdev@...r.kernel.org,
 linux-arch@...r.kernel.org, linux-kselftest@...r.kernel.org,
 Sumit Semwal <sumit.semwal@...aro.org>,
 Christian König <christian.koenig@....com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Paolo Abeni <pabeni@...hat.com>, Jesper Dangaard Brouer <hawk@...nel.org>,
 Ilias Apalodimas <ilias.apalodimas@...aro.org>, Arnd Bergmann
 <arnd@...db.de>, Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 Shuah Khan <shuah@...nel.org>
Subject: Re: [RFC PATCH 00/10] Device Memory TCP

On 7/18/23 12:29 PM, Jakub Kicinski wrote:
> On Tue, 18 Jul 2023 12:20:59 -0600 David Ahern wrote:
>> On 7/18/23 12:15 PM, Jakub Kicinski wrote:
>>> On Tue, 18 Jul 2023 15:06:29 -0300 Jason Gunthorpe wrote:  
>>>> netlink feels like a weird API choice for that, in particular it would
>>>> be really wrong to somehow bind the lifecycle of a netlink object to a
>>>> process.  
>>>
>>> Netlink is the right API, life cycle of objects can be easily tied to
>>> a netlink socket.  
>>
>> That is an untuitive connection -- memory references, h/w queues, flow
>> steering should be tied to the datapath socket, not a control plane socket.
> 
> There's one RSS context for may datapath sockets. Plus a lot of the
> APIs already exist, and it's more of a question of packaging them up 
> at the user space level. For things which do not have an API, however,
> netlink, please.

I do not see how 1 RSS context (or more specifically a h/w Rx queue) can
be used properly with memory from different processes (or dma-buf
references). When the process dies, that memory needs to be flushed from
the H/W queues. Queues with interlaced submissions make that more
complicated.

I guess the devil is in the details; I look forward to the evolution of
the patches.