diskless virtio-scsi-ccw controller triggers an error

Bug #1806823 reported by Christian Ehrhardt  on 2018-12-05
12
This bug affects 1 person
Affects Status Importance Assigned to Milestone
Ubuntu on IBM z Systems
Medium
bugproxy
qemu (Ubuntu)
Undecided
Unassigned

Bug Description

The example starts with a basic call to run an image (it doesn't matter which image)

$ sudo qemu-system-s390x -enable-kvm -cpu host -nographic -m 512 -hda ubuntu-18.04-server-cloudimg-s390x.img
LOADPARM=[........]
Using virtio-blk.
Using SCSI scheme.
....
[ 0.451611] Linux version 4.15.0-42-generic (buildd@bos02-s390x-018) (gcc version 7.3.0 (Ubuntu 7.3.0-16ubuntu3)) #45-Ubuntu SMP Thu Nov 15 19:29:11 UTC 2018 (Ubuntu 4.15.0-42.45-generic 4.15.18)
[ 0.451617] setup.289988: Linux is running under KVM in 64-bit mode

Adding an empty virtio-scsi-ccw breaks it (different than x86)

$ sudo qemu-system-s390x -enable-kvm -cpu host -nographic -m 512 -hda ubuntu-18.04-server-cloudimg-s390x.img -device virtio-scsi-ccw,id=virtio-scsi-test
LOADPARM=[........]
Using virtio-scsi.
! Cannot locate virtio-scsi device !

It seems to be the bootloader code insisting on a device to scan, since skipping bootloader makes it work

$ sudo qemu-system-s390x -enable-kvm -cpu host -nographic -m 512 -hda ubuntu-18.04-server-cloudimg-s390x.img -device virtio-scsi-ccw,id=virtio-scsi-test -kernel /boot/vmlinuz-4.15.0-42-generic -initrd /boot/initrd.img-4.15.0-42-generic
[ 0.475643] Linux version 4.15.0-42-generic (buildd@bos02-s390x-018) (gcc version 7.3.0 (Ubuntu 7.3.0-16ubuntu3)) #45-Ubuntu SMP Thu Nov 15 19:29:11 UTC 2018 (Ubuntu 4.15.0-42.45-generic 4.15.18)
[ 0.475649] setup.289988: Linux is running under KVM in 64-bit mode

Those are just examples, but it seems you can add a disk-less virtio-scsi-ccw to any to make it break.
We wondered about two things:
- the behavior is different than with virtio-scsi-pci
- one use case could want add a virtio-scsi-ccw with some explicit config to later hot-add devices right?

All of this could as well be intentional, but I wanted to report it in case it is a bug for you as well.

Thanks @Rharper for finding and mentioning it

Changed in ubuntu-z-systems:
assignee: nobody → bugproxy (bugproxy)
tags: added: reverse-proxy-bugzilla s390x
Changed in ubuntu-z-systems:
status: New → Triaged
bugproxy (bugproxy) on 2018-12-05
tags: added: architecture-s39064 bugnameltc-173834 severity-high targetmilestone-inin1804

------- Comment From <email address hidden> 2019-01-09 04:03 EDT-------
This is certainly something where we can improve the heuristics to guess the right boot device, but it is low priority and severity. It will not happen with libivrt and it will not happen when the user specifies a boot priority.
Reducing severity.

tags: added: severity-medium
removed: severity-high
Changed in ubuntu-z-systems:
importance: Undecided → Medium
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