Re: Sending CQE to a different ring

[Jens Axboe <axboe@kernel.dk>]

On 3/10/22 9:28 AM, Artyom Pavlov wrote:
>>> It looks like we have 3 options:
>>> 1) Copy sqe->len to cqe->result without any checks. Disadvantage:
>>> user-provided value may collide with EBADFD and EOVERFLOW.
>>> 2) Submit error CQE to the submitter ring.
>>> 3) Submit error CQE to the receiver ring (cqe->result will contain
>>> error code).
>>
>> #1 should not be an issue, as cqe->result for those values is the
>> original ring result code, not the target ring.
>>
>> I'd say the application should just case it to u32 and intepret it like
>> that, if it's worried about the signed nature of it?
> 
> Ah, indeed. I've missed that EBADFD and EOVERFLOW errors only can
> happen in the submitter ring, so the receiver ring can always
> interpret CQE with the IORING_CQE_F_MSG flag as a successfully
> received message from another ring.

Yes, so I don't think there's any confusion there. I did just make the
prep helper in liburing take an unsigned, and I did test that we don't
have any issues on that front.

Posted v2, passes testing too. I actually think this is quite a nifty
feature, and can be used quite flexibly to communicate between rings.

-- 
Jens Axboe