ublk: document zero copy feature

  ``UBLK_IO_COMMIT_AND_FETCH_REQ`` to the server, ublkdrv needs to copy

  the server buffer (pages) read to the IO request pages.

Future development

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Zero copy

---------

Zero copy is a generic requirement for nbd, fuse or similar drivers. A

problem [#xiaoguang]_ Xiaoguang mentioned is that pages mapped to userspace

can't be remapped any more in kernel with existing mm interfaces. This can

occurs when destining direct IO to ``/dev/ublkb*``. Also, he reported that

big requests (IO size >= 256 KB) may benefit a lot from zero copy.

ublk zero copy relies on io_uring's fixed kernel buffer, which provides

two APIs: `io_buffer_register_bvec()` and `io_buffer_unregister_bvec`.

ublk adds IO command of `UBLK_IO_REGISTER_IO_BUF` to call

`io_buffer_register_bvec()` for ublk server to register client request

buffer into io_uring buffer table, then ublk server can submit io_uring

IOs with the registered buffer index. IO command of `UBLK_IO_UNREGISTER_IO_BUF`

calls `io_buffer_unregister_bvec()` to unregister the buffer, which is

guaranteed to be live between calling `io_buffer_register_bvec()` and

`io_buffer_unregister_bvec()`. Any io_uring operation which supports this

kind of kernel buffer will grab one reference of the buffer until the

operation is completed.

ublk server implementing zero copy or user copy has to be CAP_SYS_ADMIN and

be trusted, because it is ublk server's responsibility to make sure IO buffer

filled with data for handling read command, and ublk server has to return

correct result to ublk driver when handling READ command, and the result

has to match with how many bytes filled to the IO buffer. Otherwise,

uninitialized kernel IO buffer will be exposed to client application.

ublk server needs to align the parameter of `struct ublk_param_dma_align`

with backend for zero copy to work correctly.

For reaching best IO performance, ublk server should align its segment

parameter of `struct ublk_param_segment` with backend for avoiding

unnecessary IO split, which usually hurts io_uring performance.

References

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