Re: [RFC v3 1/8] blkio: add io_uring block driver using libblkio

[Kevin Wolf]

Am 08.07.2022 um 06:17 hat Stefan Hajnoczi geschrieben:
> libblkio (https://gitlab.com/libblkio/libblkio/) is a library for
> high-performance disk I/O. It currently supports io_uring and
> virtio-blk-vhost-vdpa with additional drivers under development.
> 
> 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 io_uring and virtio-blk-vhost-vdpa BlockDrivers to QEMU
> using libblkio. It will be easy to add other libblkio drivers since they
> will share the majority of code.
> 
> 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.
> 
> The syntax is:
> 
>   --blockdev 
> io_uring,node-name=drive0,filename=test.img,readonly=on|off,cache.direct=on|off
> 
> and:
> 
>   --blockdev 
> virtio-blk-vhost-vdpa,node-name=drive0,path=/dev/vdpa...,readonly=on|off
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

The subject line implies only io_uring, but you actually add vhost-vdpa
support, too. I think the subject line should be changed.

I think it would also make sense to already implement support for
vhost-user-blk on the QEMU side even if support isn't compiled in
libblkio by default and opening vhost-user-blk images would therefore
always fail with a default build.

But then you could run QEMU with a custom build of libblkio to make use
of it without patching QEMU. This is probably useful for getting libvirt
support for using a storage daemon implemented without having to wait
for another QEMU release. (Peter, do you have any opinion on this?)

Kevin