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

From: Breno Leitao <[email-masked]>
Date: Thu, 13 Apr 2023 09:47:56 -0700

Hello Ming,

On Thu, Apr 13, 2023 at 10:56:49AM +0800, Ming Lei wrote:
> 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.
> > 
> > Suggested-by: Pavel Begunkov <[email-masked]>
> > Signed-off-by: Breno Leitao <[email-masked]>
> > ---
> 
> ...
> 
> > diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
> > index 2e4c483075d3..9648134ccae1 100644
> > --- a/io_uring/uring_cmd.c
> > +++ b/io_uring/uring_cmd.c
> > @@ -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);
> >  
> > -	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);
> 
> The copy will make some fields of sqe become READ TWICE, and driver may see
> different sqe field value compared with the one observed in io_init_req().

This copy only happens if the operation goes to the async path
(calling io_uring_cmd_prep_async()).  This only happens if
f_op->uring_cmd() returns -EAGAIN.

          ret = file->f_op->uring_cmd(ioucmd, issue_flags);
          if (ret == -EAGAIN) {
                  if (!req_has_async_data(req)) {
                          if (io_alloc_async_data(req))
                                  return -ENOMEM;
                          io_uring_cmd_prep_async(req);
                  }
                  return -EAGAIN;
          }

Are you saying that after this copy, the operation is still reading from
sqe instead of req->async_data?

If you have an example of the two copes flow, that would be great.

Thanks for the review,
Breno