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 <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.)
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
signature.asc
Description: PGP signature