netdev - Re: IORING_SEND_NOTIF_USER_DATA (was Re: IORING_CQE_F

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Date:   Fri, 21 Oct 2022 12:20:57 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Stefan Metzmacher <metze@...ba.org>,
        io-uring <io-uring@...r.kernel.org>, Jens Axboe <axboe@...nel.dk>
Cc:     Jakub Kicinski <kuba@...nel.org>, netdev <netdev@...r.kernel.org>,
        Dylan Yudaken <dylany@...com>
Subject: Re: IORING_SEND_NOTIF_USER_DATA (was Re: IORING_CQE_F_COPIED)

On 10/21/22 10:45, Stefan Metzmacher wrote:
> Am 21.10.22 um 11:27 schrieb Pavel Begunkov:
>> On 10/21/22 09:32, Stefan Metzmacher wrote:
>>> Hi Pavel,
>>>
>>>>>>> Experimenting with this stuff lets me wish to have a way to
>>>>>>> have a different 'user_data' field for the notif cqe,
>>>>>>> maybe based on a IORING_RECVSEND_ flag, it may make my life
>>>>>>> easier and would avoid some complexity in userspace...
>>>>>>> As I need to handle retry on short writes even with MSG_WAITALL
>>>>>>> as EINTR and other errors could cause them.
>>>>>>>
>>>>>>> What do you think?
>>>>>
>>>>> Any comment on this?
>>>>>
>>>>> IORING_SEND_NOTIF_USER_DATA could let us use
>>>>> notif->cqe.user_data = sqe->addr3;
>>>>
>>>> I'd rather not use the last available u64, tbh, that was the
>>>> reason for not adding a second user_data in the first place.
>>>
>>> As far as I can see io_send_zc_prep has this:
>>>
>>>          if (unlikely(READ_ONCE(sqe->__pad2[0]) || READ_ONCE(sqe->addr3)))
>>>                  return -EINVAL;
>>>
>>> both are u64...
>>
>> Hah, true, completely forgot about that one
> 
> So would a commit like below be fine for you?
> 
> Do you have anything in mind for SEND[MSG]_ZC that could possibly use
> another u64 in future?

It'll most likely be taken in the future, some features are planned
some I can imagine. The question is how necessary this one is and/or
how much simpler it would make it considering that CQEs are ordered
and apps still need to check for F_MORE. It shouldn't even require
refcounting. Can you elaborate on the simplifying userspace part?

-- 
Pavel Begunkov