[Libguestfs] Re: Passing block size preferences from NBD -> qemu -> Linux

This is expanding on the commit message I wrote here: https://gitlab.com/nbdkit/nbdkit/-/commit/780599d2e77c7cc4c1a7e99d0a93328... A simple "one-liner" to test if NBD block size preferences are passed correctly through qemu and into a Linux guest is this: $ nbdkit memory 1G --filter=blocksize-policy \ blocksize-minimum=4096 \ blocksize-preferred=65536 \ blocksize-maximum=8M \ --run ' LIBGUESTFS_HV=/path/to/qemu-system-x86_64 \ LIBGUESTFS_BACKEND=direct \ guestfish --format=raw -a "$uri" \ run : \ debug sh "head -1 /sys/block/*/queue/*_io_size" : \ debug sh "for d in /dev/sd? ; do sg_inq -p 0xb0 \$d ; done" \ ' Current qemu (9.0.0) does not pass the block size preferences correctly. It's a problem in qemu, not in Linux. qemu's NBD client requests the block size preferences from nbdkit and reads them correctly. I verified this by adding some print statements into nbd/client.c. The sizes are stored in BDRVNBDState 'info' field. qemu's virtio-scsi driver *can* present a block limits VPD page (0xb0) containing these limits (see hw/scsi/scsi-disk.c), and Linux is able to see the contents of this page using tools like 'sg_inq'. Linux appears to translate the information faithfully into /sys/block/sdX/queue/{minimum,optimal}_io_size files. However the virtio-scsi driver in qemu populates this information from the qemu command line (-device [...]min_io_size=512,opt_io_size=4096). It doesn't pass the information through from the NBD source backing the drive.

Fixing this seems like a non-trivial amount of work.