Re: [RFC v2 1/8] blkio: add io_uring block driver using libblkio

[Kevin Wolf <kwolf@redhat.com>]

Am 05.04.2022 um 17:33 hat Stefan Hajnoczi geschrieben:
> libblkio (https://gitlab.com/libblkio/libblkio/) is a library for
> high-performance disk I/O. It currently supports io_uring with
> additional drivers planned.
> 
> One of the reasons for developing libblkio is that other applications
> besides QEMU can use it. This will be particularly useful for
> vhost-user-blk which applications may wish to use for connecting to
> qemu-storage-daemon.
> 
> libblkio also gives us an opportunity to develop in Rust behind a C API
> that is easy to consume from QEMU.
> 
> This commit adds an io_uring BlockDriver to QEMU using libblkio. For now
> I/O buffers are copied through bounce buffers if the libblkio driver
> requires it. Later commits add an optimization for pre-registering guest
> RAM to avoid bounce buffers. It will be easy to add other libblkio
> drivers since they will share the majority of code.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

> +static BlockDriver bdrv_io_uring = {
> +    .format_name                = "io_uring",
> +    .protocol_name              = "io_uring",
> +    .instance_size              = sizeof(BDRVBlkioState),
> +    .bdrv_needs_filename        = true,
> +    .bdrv_parse_filename        = blkio_parse_filename_io_uring,
> +    .bdrv_file_open             = blkio_file_open,
> +    .bdrv_close                 = blkio_close,
> +    .bdrv_getlength             = blkio_getlength,
> +    .has_variable_length        = true,

This one is a bad idea. It means that every request will call
blkio_getlength() first, which looks up the "capacity" property in
libblkio and then calls lseek() for the io_uring backend.

For other backends like the vhost_user one (where I just copied your
definition and then noticed this behaviour), it involve a message over
the vhost socket, which is even worse.

.has_variable_length was only meant for the host_floppy/cdrom drivers
that have to deal with media change. Everything else just requires an
explicit block_resize monitor command to be resized.

Kevin