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

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

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.


> 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?

-- 
Pavel Begunkov