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

[Ming Lei <ming.lei@redhat.com>]

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.

> 
> > 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,
Ming