Ubuntu 20.04.3 - ilzlnx3g1 - virtio-scsi devs on KVM guest having miscompares on disktests when there is a failed path.

== Comment: #63 - Halil Pasic <email address hidden> - 2022-03-28 17:33:34 ==
I'm pretty confident I've figured out what is going on. 

From the guest side, the decision whether the SCSI command was completed successfully or not comes down to looking at the sense data. Prior to commit
a108557bbf ("scsi: inline sg_io_sense_from_errno() into the callers."), we don't
build sense data as a response to seeing a host status presented by the host SCSI stack (e.g. kernel).

Thus when the kernel tells us that  a given SCSI command did not get completed via
SCSI_HOST_TRANSPORT_DISRUPTED or SCSI_HOST_NO_LUN, we end up  fooling the guest into believing that the command completed successfully.

The guest kernel, and especially virtio and multipath are at no fault (AFAIU). Given these facts, it isn't all that surprising, that we end up with corruptions.

All we have to do is do backports for QEMU (when necessary). I didn't investigate vhost-scsi -- my guess is, that it ain't affected.

How do we want to handle the back-ports?

== Comment: #66 - Halil Pasic <email address hidden> - 2022-04-04 05:36:33 ==
This is a proposed backport containing 7 patches in mbox format. I tried to pick patches sanely, and all I had to do was basically resolving merge conflicts.

I have to admit I have no extensive experience in doing such invasive backports, and my knowledge of the QEMU SCSI stack is very limited. I would be happy if the Ubuntu folks would have a good look at this, and if possible improve on it.