[wtf] vtscsi aka virtio_scsi does not see disk at target 7

[Andriy Gapon]

I think I see what's going on, but it was a WTF moment for me.

We use virtio_scsi driver on GCE VMs.  We typically configure VMs with 6 or less
disks, but recently we started to use VMs with 8+ disks.  On some of them, after
a reboot, one disk is missing. Here is an example:

# camcontrol devlist -v
scbus0 on vtscsi0 bus 0:
<Google PersistentDisk 1>          at scbus0 target 1 lun 0 (pass0,da0)
<Google PersistentDisk 1>          at scbus0 target 2 lun 0 (pass1,da1)
<Google PersistentDisk 1>          at scbus0 target 3 lun 0 (pass2,da2)
<Google PersistentDisk 1>          at scbus0 target 4 lun 0 (pass3,da3)
<Google PersistentDisk 1>          at scbus0 target 5 lun 0 (pass4,da4)
<Google PersistentDisk 1>          at scbus0 target 6 lun 0 (pass5,da5)
<Google PersistentDisk 1>          at scbus0 target 8 lun 0 (pass6,da6)
<Google PersistentDisk 1>          at scbus0 target 9 lun 0 (pass7,da7)

We noticed that it is always target 7 that is missing.

After looking at the code, the problem is kind of obvious.
scsi_scan_bus:
                for (i = low_target; i <= max_target; i++) {


                        cam_status status;


                        if (i == initiator_id)


                                continue;

vtscsi_cam_path_inquiry:
        cpi->initiator_id = VTSCSI_INITIATOR_ID;

/*
 * Specification doesn't say, use traditional SCSI default.
 */
#define VTSCSI_INITIATOR_ID     7

>From what I see, on GCE at least, target 7 is a valid target and it is not
reserved for the initiator.  At the same time, I see that target 0 is never used
for disks.  Also, it seems that virtio configuration gives 253 as the maximum
target ID on those instances.

I would appreciate any suggestions on what to use for initiator_id in this
driver.  Thanks!

-- 
Andriy Gapon