Re: [PATCH RFC] io_uring: extend io_uring_sqe flags bits

[Jens Axboe <axboe@kernel.dk>]

On 11/1/24 9:01 AM, Ming Lei wrote:
> On Fri, Nov 01, 2024 at 08:42:42AM -0600, Jens Axboe wrote:
>> On 11/1/24 8:34 AM, Ming Lei wrote:
>>> On Fri, Nov 01, 2024 at 07:59:38AM -0600, Jens Axboe wrote:
>>>> On 10/31/24 8:42 PM, Ming Lei wrote:
>>>>> On Fri, Nov 01, 2024 at 10:12:25AM +0800, Ming Lei wrote:
>>>>>> On Thu, Oct 31, 2024 at 03:22:18PM -0600, Jens Axboe wrote:
>>>>>>> In hindsight everything is clearer, but it probably should've been known
>>>>>>> that 8 bits of ->flags would run out sooner than later. Rather than
>>>>>>> gobble up the last bit for a random use case, add a bit that controls
>>>>>>> whether or not ->personality is used as a flags2 argument. If that is
>>>>>>> the case, then there's a new IOSQE2_PERSONALITY flag that tells io_uring
>>>>>>> which personality field to read.
>>>>>>>
>>>>>>> While this isn't the prettiest, it does allow extending with 15 extra
>>>>>>> flags, and retains being able to use personality with any kind of
>>>>>>> command. The exception is uring cmd, where personality2 will overlap
>>>>>>> with the space set aside for SQE128. If they really need that, then that
>>>>>>
>>>>>> The space is the 1st `short` for uring_cmd, instead of SQE128 only.
>>>>>>
>>>>>> Also it is overlapped with ->optval and ->addr3, so just wondering why not
>>>>>> use ->__pad2?
>>>>>>
>>>>>> Another ways is to use __pad2 for sqe2_flags for non-uring_cmd, and for
>>>>>> uring_cmd, use its top 16 as sqe2_flags, this way does work, but it is
>>>>>> just a bit ugly to use.
>>>>>
>>>>> Also IOSQE2_PERSONALITY doesn't have to be per-SQE, and it can be one
>>>>> feature of IORING_FEAT_IOSQE2_PERSONALITY, that is why I thought it is
>>>>> fine to take the 7th bit as SQE_GROUP now.
>>>>
>>>> Not sure I follow your thinking there, can you expand?
>>>
>>> It could be one io_uring setup flag, such as
>>> IORING_SETUP_IOSQE2_PERSONALITY.
>>>
>>> If this flag is set, take __pad2 as sqe2_flags, otherwise use current
>>> way, so it doesn't have to take bit7 of sqe_flags for this purpose.
>>
>> Would probably have to be a IORING_SETUP_IOSQE2_FLAGS or something in
>> general. And while that could work, not a huge fan of that. I think we
>> should retain that for when a v2 of the sqe is done, to coordinate which
>> version to use.
> 
> Fair enough.
> 
> Now there are 16bits for new features, which may put v2 off long enough.

Exactly, hopefully that'll push the need out quite a bit, so we have
time to do something nice for v2.

>>> Also in future, if uring_cmd needs personality, it still may reuse top
>>> 16bit of uring_cmd_flags for that.
>>
>> Right, that's what I referred to in terms of uring_cmd just having its
>> own way to set personality.
> 
> Then this approach is safe to go, imo.

Thanks I think so too, and it'll unblock the sqe grouping. So at least
that paves the way for the first part of your patchset. I'll post a v2
of it shortly.

-- 
Jens Axboe