netdev - Re: [PATCH 0/5] add initial io_uring

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Message-ID: <78abce6b-709b-7309-2ee1-5370d05748b6@kernel.dk>
Date:   Tue, 11 Apr 2023 09:29:15 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     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, willemdebruijn.kernel@...il.com,
        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 9:27?AM, David Ahern wrote:
> On 4/11/23 9:17 AM, Jens Axboe wrote:
>> On 4/11/23 9:10?AM, David Ahern wrote:
>>> On 4/11/23 8:41 AM, 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.
>>>>
>>>> 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.
>>>>
>>>
>>> My meta point is that there are uapis today to return this information
>>> to applications (and I suspect this is just the start of more networking
>>> changes - both data retrieval and adjusting settings). io_uring is
>>> wanting to do this on behalf of the application without a syscall. That
>>> makes io_uring yet another subsystem / component managing a socket. Any
>>> change to the networking stack required by io_uring should be usable by
>>> all other in-kernel socket owners or managers. ie., there is no reason
>>> for io_uring specific code here.
>>
>> I think we are in violent agreement here, what I'm describing is exactly
>> that - it'd make ioctl/{set,get}sockopt call into the same helpers that
>> ->uring_cmd() would, with the only difference being that the former
>> would need copy in/out and the latter would not.
>>
>> But let me just stress that for direct descriptors, we cannot currently
>> call ioctl or set/getsockopt. This means we have to instantiate a
>> regular descriptor first, do those things, then register it to never use
>> the regular file descriptor again. That's wasteful, and this is what we
>> want to enable (direct use of ioctl set/getsockopt WITHOUT a normal file
>> descriptor). It's not just for "oh it'd be handy to also do this from
>> io_uring" even if that would be a worthwhile goal in itself.
>>
> 
> Christoph's patch set a few years back that removed set_fs broke the
> ability to do in-kernel ioctl and {s,g}setsockopt calls. I did not
> follow that change; was it a deliberate intent to not allow these
> in-kernel calls vs wanting to remove the set_fs? e.g., can we add a
> kioctl variant for in-kernel use of the APIs?

I think it'd be much better to cleanly split it out rather than try and
hack around it.

-- 
Jens Axboe