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 <stefa...@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.) > .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. I was trying to support devices that can be resized below QEMU (e.g. vhost-user-blk, vhost-vdpa-blk, and virtio-blk-pci). That was unnecessary since QEMU doesn't support that model. If an LVM volume is resized, for example, you still need to execute a monitor command to let QEMU know. I'll drop .has_variable_length. Stefan
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