QEMU SCSI Tape passthrough
Uh oh, what am I doing?
Just a note to myself since I searched the internet for a long time to make this work…
I needed to test a really trivial change to the scsi tape driver code, but didn’t want to reboot an actual machine that has a tape drive. So I thought, maybe QEMU can pass-through an arbitrary device?
Indeed it can, but all the examples one finds are either
- referring to libvirt or openstack or another random virtualisation tool, and their xml config files;
- and most likely referring to disks/block devices, for which there are simpler ways to do things.
After many fruitless searches, finally found a reddit comment that shows how it is done for, well, (cdrom) disks again, but in a generic way. A bit of massaging results in:
$ qemu … \ -drive id=footape,if=none,format=raw,readonly=off,file=/dev/sgX \ -device virtio-scsi-pci,id=scsi0 \ -device scsi-generic,bus=scsi0.0,drive=footape
Note 1: The special note is that tapes shouldn’t be addressed by their regular devices (
nstX), since that would actually open the tape and hangs without a tape. You need to find the scsi generic device for it (via
sg_map), and use that as the
file argument to the
Note 2: Of course, if running qemu unprivileged, don’t forget permissions on the ‘sg’ device.
After that, all good. Almost, that is: while
mt status and
mt eject work, at least for my tape drive,
mt tell said (in the virtual machine):
# mt tell /dev/nst0: Input/output error # dmesg … [ 128.799466] st 1:0:0:0: [st0] Sense Key : Illegal Request [current] [ 128.800172] st 1:0:0:0: [st0] Add. Sense: Invalid field in cdb
Well, even with that, it was good enough for me, so thought to write this down. And yes, I ejected the tape from the VM.