[Libguestfs] [PATCH libnbd] ublk: Add new nbdublk program

This patch adds simple support for a ublk-based NBD client. It is also available here: https://gitlab.com/rwmjones/libnbd/-/tree/nbdublk/ublk ublk is a way to write Linux block device drivers in userspace: https://lwn.net/Articles/903855/ For simplicity of implementation and because I don't currently understand the thread model of ublksrv, this only implements synchronous calls for now. It should be possible to extend this to a fully asynchronous client without too much difficulty. It does appear to work, at least for simple cases. I have created filesystems, files, etc on a ublk device backed by an nbdkit RAM disk, eg: On one machine do: $ nbdkit memory 1G On the client machine with the right kernel etc [see below] do: # modprobe ublk_drv # nbdublk /dev/ublkb0 nbd://remote # ublk list # blockdev --getsize64 /dev/ublkb0 # mke2fs /dev/ublkb0 # ... # ublk del -n 0 Testing this is not for the fainthearted. I would start with a throwaway Fedora Rawhide virtual machine, fully upgraded. You will need to recompile the kernel with CONFIG_BLK_DEV_UBLK=m You will need to upgrade to liburing 2.2 (I pushed this to Rawhide a few days ago). You will need to download & compile: https://github.com/ming1/ubdsrv Apply this patch to libnbd and compile it with: export PKG_CONFIG_PATH=$HOME/ubdsrv export CFLAGS="$CFLAGS -I$HOME/ubdsrv/include" export CXXFLAGS="$CXXFLAGS -I$HOME/ubdsrv/include" export LDFLAGS="$LDFLAGS -L$HOME/ubdsrv/lib" ./configure make (Check that ublk dependencies are found and nbdublk is compiled) You will then be able to run nbdublk from the compile directory using: sudo ./run nbdublk --help Rich.