Re: [PATCH V2 1/1] Docs: ublk: add ublk document

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

On Thu, Sep 01, 2022 at 11:04:24AM +0800, Ziyang Zhang wrote:
> On 2022/9/1 10:30, Ming Lei wrote:
> > +
> > +- ``UBLK_IO_NEED_GET_DATA``
> > +
> > +  ublk server pre-allocates IO buffer for each IO by default. Any new projects
> > +  should use this buffer to communicate with ublk driver. However, existing
> > +  projects may break or not able to consume the new buffer interface; that's
> > +  why this command is added for backwards compatibility so that existing
> > +  projects can still consume existing buffers.
> 
> Hi, Ming.
> 
> Could you please add more information on UBLK_IO_NEED_GET_DATA. stefanha
> found it hard to understand.
> 
> Myabe we should write like this:
> 
> With UBLK_F_NEED_GET_DATA enabled, the WRITE request will be firstly issued to
> ublksrv without data copy. Then, IO backend receives the request and it can allocate
> data buffer and embed its addr inside a new ioucmd. After the kernel driver gets the
> ioucmd, the data copy happens(from biovecs to backend's buffer). Finally,
> the backend receives the request again with data to be written and it can truly
> handle the request.
> 
> UBLK_IO_NEED_GET_DATA add one additional round-trip in ublk_drv and one
> io_uring_enter() syscall. Any user thinks that it may lower performance
> should not enable UBLK_F_NEED_GET_DATA. ublk server pre-allocates IO buffer
> for each IO by default. Any new projects should use this buffer to communicate
> with ublk driver. However, existing projects may break or not able to consume
> the new buffer interface; that's why this command is added for backwards
> compatibility so that existing projects can still consume existing buffers.

I am fine to add it if V3 is needed. If not, please send a new patch.

BTW, I guess Jens may consider it for v6.0.

Thanks,
Ming