Re: [PATCH RFC] io_uring: Pass whole sqe to commands

Keith Busch
2023-04-07T19:03:09Z

On Thu, Apr 06, 2023 at 09:57:05AM -0700, Breno Leitao wrote:
> Currently uring CMD operation relies on having large SQEs, but future
> operations might want to use normal SQE.
>
> The io_uring_cmd currently only saves the payload (cmd) part of the SQE,
> but, for commands that use normal SQE size, it might be necessary to
> access the initial SQE fields outside of the payload/cmd block. So,
> saves the whole SQE other than just the pdu.
>
> This changes slighlty how the io_uring_cmd works, since the cmd
> structures and callbacks are not opaque to io_uring anymore. I.e, the
> callbacks can look at the SQE entries, not only, in the cmd structure.
>
> The main advantage is that we don't need to create custom structures for
> simple commands.

This looks good to me. The only disadvantage I can see is that the async
fallback allocates just a tiny bit more data than before, but no biggie.

Reviewed-by: Keith Busch <kbusch@kernel.org>

> @@ -63,14 +63,15 @@ EXPORT_SYMBOL_GPL(io_uring_cmd_done);
> int io_uring_cmd_prep_async(struct io_kiocb *req)
> {
> struct io_uring_cmd *ioucmd = io_kiocb_to_cmd(req, struct io_uring_cmd);
> - size_t cmd_size;
> + size_t size = sizeof(struct io_uring_sqe);
>
> BUILD_BUG_ON(uring_cmd_pdu_size(0) != 16);
> BUILD_BUG_ON(uring_cmd_pdu_size(1) != 80);

One minor suggestion. The above is the only user of uring_cmd_pdu_size() now,
which is kind of a convoluted way to enfoce the offset of the 'cmd' field. It
may be more clear to replace these with:

BUILD_BUG_ON(offsetof(struct io_uring_sqe, cmd) == 48);

> - cmd_size = uring_cmd_pdu_size(req->ctx->flags & IORING_SETUP_SQE128);
> + if (req->ctx->flags & IORING_SETUP_SQE128)
> + size <<= 1;
>
> - memcpy(req->async_data, ioucmd->cmd, cmd_size);
> + memcpy(req->async_data, ioucmd->sqe, size);
> return 0;
> }