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

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Message-ID: <a9858183-8f69-7aff-51ac-122f627ba66f@kernel.org>
Date:   Tue, 11 Apr 2023 09:27:41 -0600
From:   David Ahern <dsahern@...nel.org>
To:     Jens Axboe <axboe@...nel.dk>, 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 9:17 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?