Message-ID: <036c80e5-4844-5c84-304c-7e553fe17a9b@kernel.dk>
Date:   Tue, 11 Apr 2023 08:54:49 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        David Ahern <dsahern@...nel.org>,
        Breno Leitao <leitao@...ian.org>
Cc:     Willem de Bruijn <willemb@...gle.com>, io-uring@...r.kernel.org,
        netdev@...r.kernel.org, kuba@...nel.org, asml.silence@...il.com,
        leit@...com, edumazet@...gle.com, pabeni@...hat.com,
        davem@...emloft.net, dccp@...r.kernel.org, mptcp@...ts.linux.dev,
        linux-kernel@...r.kernel.org, matthieu.baerts@...sares.net,
        marcelo.leitner@...il.com
Subject: Re: [PATCH 0/5] add initial io_uring_cmd support for sockets

On 4/11/23 8:51?AM, Willem de Bruijn wrote:
> Jens Axboe wrote:
>> On 4/11/23 8:36?AM, David Ahern wrote:
>>> On 4/11/23 6:00 AM, Breno Leitao wrote:
>>>> I am not sure if avoiding io_uring details in network code is possible.
>>>>
>>>> The "struct proto"->uring_cmd callback implementation (tcp_uring_cmd()
>>>> in the TCP case) could be somewhere else, such as in the io_uring/
>>>> directory, but, I think it might be cleaner if these implementations are
>>>> closer to function assignment (in the network subsystem).
>>>>
>>>> And this function (tcp_uring_cmd() for instance) is the one that I am
>>>> planning to map io_uring CMDs to ioctls. Such as SOCKET_URING_OP_SIOCINQ
>>>> -> SIOCINQ.
>>>>
>>>> Please let me know if you have any other idea in mind.
>>>
>>> I am not convinced that this io_uring_cmd is needed. This is one
>>> in-kernel subsystem calling into another, and there are APIs for that.
>>> All of this set is ioctl based and as Willem noted a little refactoring
>>> separates the get_user/put_user out so that in-kernel can call can be
>>> made with existing ops.
>>
>> How do you want to wire it up then? We can't use fops->unlocked_ioctl()
>> obviously, and we already have ->uring_cmd() for this purpose.
> 
> Does this suggestion not work?

Not sure I follow, what suggestion?

>>> I was thinking just having sock_uring_cmd call sock->ops->ioctl, like
>>> sock_do_ioctl.
>  
>> I do think the right thing to do is have a common helper that returns
>> whatever value you want (or sets it), and split the ioctl parts into a
>> wrapper around that that simply copies in/out as needed. Then
>> ->uring_cmd() could call that, or you could some exported function that
>> does supports that.
>>
>> This works for the basic cases, though I do suspect we'll want to go
>> down the ->uring_cmd() at some point for more advanced cases or cases
>> that cannot sanely be done in an ioctl fashion.
> 
> Right now the two examples are ioctls that return an integer. Do you 
> already have other calls in mind? That would help estimate whether
> ->uring_cmd() indeed will be needed and we might as well do it now.

Right, it's a proof of concept. But we'd want to support anything that
setsockopt/getsockopt would do. This is necessary so that direct
descriptors (eg ones that describe a struct file that isn't in the
process file table or have a regular fd) can be used for anything that a
regular file can. Beyond that, perhaps various things necessary for
efficient zero copy rx.

I do think we can make the ->uring_cmd() hookup a bit more palatable in
terms of API. It really should be just a sub-opcode and then arguments
to support that. The grunt of the work is really refactoring the ioctl
and set/getsockopt bits so that they can be called in-kernel rather than
assuming copy in/out is needed. Once that is done, the actual uring_cmd
hookup should be simple and trivial.

-- 
Jens Axboe

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