Re: [PATCH V6 7/8] io_uring/uring_cmd: support provide group kernel buffer

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

On Fri, Oct 04, 2024 at 04:44:54PM +0100, Pavel Begunkov wrote:
> On 9/12/24 11:49, Ming Lei wrote:
> > Allow uring command to be group leader for providing kernel buffer,
> > and this way can support generic device zero copy over device buffer.
> > 
> > The following patch will use the way to support zero copy for ublk.
> > 
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >   include/linux/io_uring/cmd.h  |  7 +++++++
> >   include/uapi/linux/io_uring.h |  7 ++++++-
> >   io_uring/uring_cmd.c          | 28 ++++++++++++++++++++++++++++
> >   3 files changed, 41 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/io_uring/cmd.h b/include/linux/io_uring/cmd.h
> > index 447fbfd32215..fde3a2ec7d9a 100644
> > --- a/include/linux/io_uring/cmd.h
> > +++ b/include/linux/io_uring/cmd.h
> > @@ -48,6 +48,8 @@ void __io_uring_cmd_do_in_task(struct io_uring_cmd *ioucmd,
> >   void io_uring_cmd_mark_cancelable(struct io_uring_cmd *cmd,
> >   		unsigned int issue_flags);
> > +int io_uring_cmd_provide_kbuf(struct io_uring_cmd *ioucmd,
> > +		const struct io_uring_kernel_buf *grp_kbuf);
> >   #else
> >   static inline int io_uring_cmd_import_fixed(u64 ubuf, unsigned long len, int rw,
> >   			      struct iov_iter *iter, void *ioucmd)
> > @@ -67,6 +69,11 @@ static inline void io_uring_cmd_mark_cancelable(struct io_uring_cmd *cmd,
> >   		unsigned int issue_flags)
> >   {
> >   }
> > +static inline int io_uring_cmd_provide_kbuf(struct io_uring_cmd *ioucmd,
> > +		const struct io_uring_kernel_buf *grp_kbuf)
> > +{
> > +	return -EOPNOTSUPP;
> > +}
> >   #endif
> >   /*
> > diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
> > index 2af32745ebd3..11985eeac10e 100644
> > --- a/include/uapi/linux/io_uring.h
> > +++ b/include/uapi/linux/io_uring.h
> > @@ -271,9 +271,14 @@ 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_PROVIDE_GROUP_KBUF	this command provides group kernel buffer
> > + *				for member requests which can retrieve
> > + *				any sub-buffer with offset(sqe->addr) and
> > + *				len(sqe->len)
> 
> Is there a good reason it needs to be a cmd generic flag instead of
> ublk specific?

io_uring request isn't visible for drivers, so driver can't know if the
uring command is one group leader.

Another way is to add new API of io_uring_cmd_may_provide_buffer(ioucmd)
so driver can check if device buffer can be provided with this uring_cmd,
but I prefer to the new uring_cmd flag:

- IORING_PROVIDE_GROUP_KBUF can provide device buffer in generic way.
- ->prep() can fail fast in case that it isn't one group request

> 
> 1. Extra overhead for files / cmds that don't even care about the
> feature.

It is just checking ioucmd->flags in ->prep(), and basically zero cost.

> 
> 2. As it stands with this patch, the flag is ignored by all other
> cmd implementations, which might be quite confusing as an api,
> especially so since if we don't set that REQ_F_GROUP_KBUF memeber
> requests will silently try to import a buffer the "normal way",

The usage is same with buffer select or fixed buffer, and consumer
has to check the flag.

And same with IORING_URING_CMD_FIXED which is ignored by other
implementations except for nvme, :-)

I can understand the concern, but it exits since uring cmd is born.

> i.e. interpret sqe->addr or such as the target buffer.

> 3. We can't even put some nice semantics on top since it's
> still cmd specific and not generic to all other io_uring
> requests.
> 
> I'd even think that it'd make sense to implement it as a
> new cmd opcode, but that's the business of the file implementing
> it, i.e. ublk.
> 
> >    */
> >   #define IORING_URING_CMD_FIXED	(1U << 0)
> > -#define IORING_URING_CMD_MASK	IORING_URING_CMD_FIXED
> > +#define IORING_PROVIDE_GROUP_KBUF	(1U << 1)
> > +#define IORING_URING_CMD_MASK	(IORING_URING_CMD_FIXED | IORING_PROVIDE_GROUP_KBUF)

It needs one new file operation, and we shouldn't work toward
this way.



Thanks,
Ming