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

[Jens Axboe <axboe@kernel.dk>]

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.

> 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.

-- 
Jens Axboe