[PATCH v2 0/8] scsi: target: Allow userspace to config cmd submission

[Mike Christie <michael.christie@oracle.com>]

The following patches were made over Linus's tree but apply over Martin's
branches. They allow userspace to configure how fabric drivers submit cmds
to backend drivers.

Right now loop and vhost use a worker thread, and the other drivers submit
from the contexts they receive/process the cmd from. For multiple LUN
cases where the target can queue more cmds than the backend can handle
then deferring to a worker thread is safest because the backend driver can
block when doing things like waiting for a free request/tag. Deferring also
helps when the target has to handle transport level requests from the
recv context.

For cases where the backend devices can queue everything the target sends,
then there is no need to defer to a workqueue and you can see a perf boost of
up to 26% for small IO workloads. For a nvme device and vhost-scsi I can
see with 4K IOs:

fio jobs        1       2       4       8       10
--------------------------------------------------
workqueue
submit           94K     190K    394K    770K    890K

direct
submit          128K    252K    488K    950K    -



v2:
- Use sysfs_emit.
- Add iSCSI target support so now all targets are supported.