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

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

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.

Also in future, if uring_cmd needs personality, it still may reuse top
16bit of uring_cmd_flags for that.



Thanks,
Ming