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

Sagi Grimberg
2022-07-04T11:17:57Z

> 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?