Re: [PATCH V5 2/2] io_uring: uring_cmd: add multishot support

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

On Thu, Aug 21, 2025 at 10:38:57AM -0600, Jens Axboe wrote:
> On 8/21/25 10:29 AM, Caleb Sander Mateos wrote:
> > On Wed, Aug 20, 2025 at 9:02?PM Ming Lei <ming.lei@redhat.com> wrote:
> >>
> >> Add UAPI flag IORING_URING_CMD_MULTISHOT for supporting multishot
> >> uring_cmd operations with provided buffer.
> >>
> >> This enables drivers to post multiple completion events from a single
> >> uring_cmd submission, which is useful for:
> >>
> >> - Notifying userspace of device events (e.g., interrupt handling)
> >> - Supporting devices with multiple event sources (e.g., multi-queue devices)
> >> - Avoiding the need for device poll() support when events originate
> >>   from multiple sources device-wide
> >>
> >> The implementation adds two new APIs:
> >> - io_uring_cmd_select_buffer(): selects a buffer from the provided
> >>   buffer group for multishot uring_cmd
> >> - io_uring_mshot_cmd_post_cqe(): posts a CQE after event data is
> >>   pushed to the provided buffer
> >>
> >> Multishot uring_cmd must be used with buffer select (IOSQE_BUFFER_SELECT)
> >> and is mutually exclusive with IORING_URING_CMD_FIXED for now.
> >>
> >> The ublk driver will be the first user of this functionality:
> >>
> >>         https://github.com/ming1/linux/commits/ublk-devel/
> >>
> >> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> >> ---
> >>  include/linux/io_uring/cmd.h  | 27 +++++++++++++
> >>  include/uapi/linux/io_uring.h |  6 ++-
> >>  io_uring/opdef.c              |  1 +
> >>  io_uring/uring_cmd.c          | 71 ++++++++++++++++++++++++++++++++++-
> >>  4 files changed, 103 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/include/linux/io_uring/cmd.h b/include/linux/io_uring/cmd.h
> >> index cfa6d0c0c322..856d343b8e2a 100644
> >> --- a/include/linux/io_uring/cmd.h
> >> +++ b/include/linux/io_uring/cmd.h
> >> @@ -70,6 +70,21 @@ void io_uring_cmd_mark_cancelable(struct io_uring_cmd *cmd,
> >>  /* Execute the request from a blocking context */
> >>  void io_uring_cmd_issue_blocking(struct io_uring_cmd *ioucmd);
> >>
> >> +/*
> >> + * Select a buffer from the provided buffer group for multishot uring_cmd.
> >> + * Returns the selected buffer address and size.
> >> + */
> >> +struct io_br_sel io_uring_cmd_buffer_select(struct io_uring_cmd *ioucmd,
> >> +                                           unsigned buf_group, size_t *len,
> >> +                                           unsigned int issue_flags);
> >> +
> >> +/*
> >> + * Complete a multishot uring_cmd event. This will post a CQE to the completion
> >> + * queue and update the provided buffer.
> >> + */
> >> +bool io_uring_mshot_cmd_post_cqe(struct io_uring_cmd *ioucmd,
> >> +                                struct io_br_sel *sel, unsigned int issue_flags);
> >> +
> >>  #else
> >>  static inline int
> >>  io_uring_cmd_import_fixed(u64 ubuf, unsigned long len, int rw,
> >> @@ -102,6 +117,18 @@ static inline void io_uring_cmd_mark_cancelable(struct io_uring_cmd *cmd,
> >>  static inline void io_uring_cmd_issue_blocking(struct io_uring_cmd *ioucmd)
> >>  {
> >>  }
> >> +static inline int io_uring_cmd_select_buffer(struct io_uring_cmd *ioucmd,
> >> +                               unsigned buf_group,
> >> +                               void **buf, size_t *len,
> >> +                               unsigned int issue_flags)
> >> +{
> >> +       return -EOPNOTSUPP;
> >> +}
> >> +static inline bool io_uring_mshot_cmd_post_cqe(struct io_uring_cmd *ioucmd,
> >> +                               ssize_t ret, unsigned int issue_flags)
> >> +{
> >> +       return true;
> >> +}
> >>  #endif
> >>
> >>  /*
> >> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
> >> index 6957dc539d83..1e935f8901c5 100644
> >> --- a/include/uapi/linux/io_uring.h
> >> +++ b/include/uapi/linux/io_uring.h
> >> @@ -298,9 +298,13 @@ enum io_uring_op {
> >>   * sqe->uring_cmd_flags                top 8bits aren't available for userspace
> >>   * IORING_URING_CMD_FIXED      use registered buffer; pass this flag
> >>   *                             along with setting sqe->buf_index.
> >> + * IORING_URING_CMD_MULTISHOT  must be used with buffer select, like other
> >> + *                             multishot commands. Not compatible with
> >> + *                             IORING_URING_CMD_FIXED, for now.
> >>   */
> >>  #define IORING_URING_CMD_FIXED (1U << 0)
> >> -#define IORING_URING_CMD_MASK  IORING_URING_CMD_FIXED
> >> +#define IORING_URING_CMD_MULTISHOT     (1U << 1)
> >> +#define IORING_URING_CMD_MASK  (IORING_URING_CMD_FIXED | IORING_URING_CMD_MULTISHOT)
> > 
> > One other question: what is the purpose of this additional flag?
> > io_uring_cmd_prep() checks that it matches IOSQE_BUFFER_SELECT, so
> > could we just check that flag instead and drop the check that
> > IORING_URING_CMD_MULTISHOT matches REQ_F_BUFFER_SELECT?
> 
> This is a good question - if we don't strictly needs to exist, eg it
> overlaps 100% with IOSQE_BUFFER_SELECT, we should just drop it.

Without this flag, who knows it is one mshot command?

Other mshot OPs use new OP for showing the purpose, here I just want to
avoid to add another uring_cmd variant.


Thanks,
Ming