Re: [PATCH V3 1/1] ublk: add io_uring based userspace block driver

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

On Mon, Jul 04, 2022 at 02:17:44PM +0300, Sagi Grimberg wrote:
> 
> > This is the driver part of userspace block driver(ublk driver), the other
> > part is userspace daemon part(ublksrv)[1].
> > 
> > The two parts communicate by io_uring's IORING_OP_URING_CMD with one
> > shared cmd buffer for storing io command, and the buffer is read only for
> > ublksrv, each io command is indexed by io request tag directly, and
> > is written by ublk driver.
> > 
> > For example, when one READ io request is submitted to ublk block driver, ublk
> > driver stores the io command into cmd buffer first, then completes one
> > IORING_OP_URING_CMD for notifying ublksrv, and the URING_CMD is issued to
> > ublk driver beforehand by ublksrv for getting notification of any new io request,
> > and each URING_CMD is associated with one io request by tag.
> > 
> > After ublksrv gets the io command, it translates and handles the ublk io
> > request, such as, for the ublk-loop target, ublksrv translates the request
> > into same request on another file or disk, like the kernel loop block
> > driver. In ublksrv's implementation, the io is still handled by io_uring,
> > and share same ring with IORING_OP_URING_CMD command. When the target io
> > request is done, the same IORING_OP_URING_CMD is issued to ublk driver for
> > both committing io request result and getting future notification of new
> > io request.
> > 
> > Another thing done by ublk driver is to copy data between kernel io
> > request and ublksrv's io buffer:
> > 
> > 1) before ubsrv handles WRITE request, copy the request's data into
> > ublksrv's userspace io buffer, so that ublksrv can handle the write
> > request
> > 
> > 2) after ubsrv handles READ request, copy ublksrv's userspace io buffer
> > into this READ request, then ublk driver can complete the READ request
> > 
> > Zero copy may be switched if mm is ready to support it.
> > 
> > ublk driver doesn't handle any logic of the specific user space driver,
> > so it should be small/simple enough.
> > 
> > [1] ublksrv
> > 
> > https://github.com/ming1/ubdsrv
> > 
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >   drivers/block/Kconfig         |    6 +
> >   drivers/block/Makefile        |    2 +
> >   drivers/block/ublk_drv.c      | 1603 +++++++++++++++++++++++++++++++++
> >   include/uapi/linux/ublk_cmd.h |  158 ++++
> >   4 files changed, 1769 insertions(+)
> >   create mode 100644 drivers/block/ublk_drv.c
> >   create mode 100644 include/uapi/linux/ublk_cmd.h
> > 
> > diff --git a/drivers/block/Kconfig b/drivers/block/Kconfig
> > index fdb81f2794cd..d218089cdbec 100644
> > --- a/drivers/block/Kconfig
> > +++ b/drivers/block/Kconfig
> > @@ -408,6 +408,12 @@ config BLK_DEV_RBD
> >   	  If unsure, say N.
> > +config BLK_DEV_UBLK
> > +	bool "Userspace block driver"
> 
> Really? why compile this to the kernel and not tristate as loadable
> module?

So far, this is only one reason: task_work_add() is required, which
isn't exported for modules.


Thanks,
Ming