Re: [RFC PATCHv2 1/1] io_uring: add support for IORING_SETUP_SQE_MIXED

[Jens Axboe <axboe@kernel.dk>]

On 9/10/25 11:44 AM, Caleb Sander Mateos wrote:
>> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
>> index 04ebff33d0e62..9cef9085f52ee 100644
>> --- a/include/uapi/linux/io_uring.h
>> +++ b/include/uapi/linux/io_uring.h
>> @@ -146,6 +146,7 @@ enum io_uring_sqe_flags_bit {
>>         IOSQE_ASYNC_BIT,
>>         IOSQE_BUFFER_SELECT_BIT,
>>         IOSQE_CQE_SKIP_SUCCESS_BIT,
>> +       IOSQE_SQE_128B_BIT,
> 
> Have you given any thought to how we would handle the likely scenario
> that we want to define more SQE flags in the future? Are there
> existing unused bytes of the SQE where the new flags could go? If not,
> we may need to repurpose some existing but rarely used field. And then
> we'd likely want to reserve this last flag bit to specify whether the
> SQE is using this "extended flags" field.

Yep this is my main problem with this change. If you search the io_uring
list on lore you can find discussions about this in relation to when
Ming had his SQE grouping patches that also used this last bit. My
suggestion then was indeed to have this last flag be "look at XX for
IOSQE2_* flags". But it never quite got finalized. IIRC, my suggestion
back then was to use the personality field, since it's a pretty
specialized use case. Only issue with that is that you could then not
use IOSQE2_* flags with personality.

IOW, I think the IOSQE_SQE_128B flag is fine for prototyping and testing
these patches, but we unfortunately do need to iron out how on earth
we'll expose some more flags before this can go in.

Suggestions welcome...

-- 
Jens Axboe