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

[Kevin Wolf]

Am 07.04.2022 um 09:22 hat Stefan Hajnoczi geschrieben:
> On Wed, Apr 06, 2022 at 07:32:04PM +0200, Kevin Wolf wrote:
> > 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.
> 
> Thanks for pointing this out. I didn't think this through. More below on
> what I was trying to do.
> 
> > 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.
> 
> (A vhost-user-blk driver could cache the capacity field and update it
> when a Configuration Change Notification is received. There is no need
> to send a vhost-user protocol message every time.)

In theory we could cache in libblkio, but then we would need a mechanism
to invalidate the cache so we can support resizing an image (similar to
what block_resize does in QEMU, except that it wouldn't set the new
size from a parameter, but just get the new value from the backend).

I think it's simpler to leave caching to the application - and QEMU
already does this automatically if we don't set .has_variable_length =
true.

Kevin

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