scsi: scsi_debug: Support hostwide tags

scsi: scsi_debug: Support hostwide tags

Many SCSI HBAs support a hostwide tagset, whereby each command submitted to
the HW from all submission queues must have a unique tag identifier.

Normally this unique tag will be in the range [0, max queue], where "max
queue" is the depth of each of the submission queues.

Add support for this hostwide tag feature, via module parameter
"host_max_queue". A non-zero value means that the feature is enabled. In
this case, the submission queues are not exposed to upper layer, i.e. from
blk-mq prespective, the device has a single hw queue. There are 2 reasons
for this:

 a. It is assumed that the host can support nr_hw_queues * can_queue
    commands, but this is not true for hostwide tags

 b. For nr_hw_queues != 0, the request tag is not unique over all HW
    queues, and some HBA drivers want to use this tag for the hostwide tag

However, like many SCSI HBA drivers today - megaraid sas being an example -
the full set of HW submission queues are still used in the LLDD driver. So
instead of using a complicated "reply_map" to create a per-CPU submission
queue mapping like megaraid_sas (as it depends on a PCI device + MSIs) -
use a simple algorithm:

    hwq = cpu % queue count

If the host_max_queue param is set non-zero, then the max queue depth is
fixed at this value also.

If and when hostwide shared tags are supported in blk-mq/scsi mid-layer,
then the policy to set nr_hw_queues = 0 for hostwide tags can be revised.

Link: https://lore.kernel.org/r/1594297400-24756-3-git-send-email-john.garry@huawei.com
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>