1. 14 Jul, 2022 1 commit
    • Ming Lei's avatar
      ublk_drv: add io_uring based userspace block driver · 71f28f31
      Ming Lei authored
      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 is small/simple enough.
      
      [1] ublksrv
      
      https://github.com/ming1/ubdsrvSigned-off-by: default avatarMing Lei <ming.lei@redhat.com>
      Link: https://lore.kernel.org/r/20220713140711.97356-2-ming.lei@redhat.comSigned-off-by: default avatarJens Axboe <axboe@kernel.dk>
      71f28f31
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