Re: [RFC 2/2] io_uring/net: allow to override notification tag

[Pavel Begunkov <asml.silence@gmail.com>]

On 8/17/22 13:04, Dylan Yudaken wrote:
> On Wed, 2022-08-17 at 11:48 +0100, Pavel Begunkov wrote:
>> On 8/16/22 09:37, Dylan Yudaken wrote:
>>> On Tue, 2022-08-16 at 08:42 +0100, Pavel Begunkov wrote:
>>>> Considering limited amount of slots some users struggle with
>>>> registration time notification tag assignment as it's hard to
>>>> manage
>>>> notifications using sequence numbers. Add a simple feature that
>>>> copies
>>>> sqe->user_data of a send(+flush) request into the notification
>>>> CQE it
>>>> flushes (and only when it's flushes).
>>>
>>> I think for this to be useful I think it would also be needed to
>>> have
>>> flags on the generated CQE.
>>>
>>> If there are more CQEs coming for the same request it should have
>>> IORING_CQE_F_MORE set. Otherwise user space would not be able to
>>> know
>>> if it is able to reuse local data.
>>
>> If you want to have:
>>
>> expect_more = cqe->flags & IORING_CQE_F_MORE;
>>
>> Then in the current form you can perfectly do that with
>>
>> // MSG_WAITALL
>> expect_more = (cqe->res == io_len);
>> // !MSG_WAITALL,
>> expect_more = (cqe->res >= 0);
>>
>> But might be more convenient to have IORING_CQE_F_MORE set,
>> one problem is a slight change of (implicit) semantics, i.e.
>> we don't execute linked requests when filling a IORING_CQE_F_MORE
>> CQE + CQE ordering implied from that.
>>
>> It's maybe worth to not rely on the link failing concept for
>> deciding whether to flush or not.
> 
> Is the ordering guaranteed then to be <send cqe>, <notif cqe>?

Not yet, need to send this patch

https://github.com/isilence/linux/commit/9a1464905be3fc0cee4f68b01e43c5ad14a05b06

> If so I would put the IORING_CQE_F_MORE more as a nice to have for
> consistency with other ops
> 
>>
>>
>>> Additionally it would need to provide a way of disambiguating the
>>> send
>>> CQE with the flush CQE.
>>
>> Do you mean like IORING_CQE_F_NOTIF from 1/2?
>>
> 
> Apologies - I missed that
> 

-- 
Pavel Begunkov