[PATCH 00/16 V2] vhost: fix scsi cmd handling and IOPs

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

The following patches were made over Michael's vhost branch here:
https://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git/log/?h=vhost
 
The patches also apply to Linus's or Martin's trees if you apply
https://patchwork.kernel.org/patch/11790681/
which was merged into mst's tree already.

The following patches are a follow up to this post:
https://patchwork.kernel.org/cover/11790763/
which originally was fixing how vhost-scsi handled cmds so we would
not get IO errors when sending more than 256 cmds.

In that patchset I needed to detect if a vq was in use and for this
patch:
https://patchwork.kernel.org/patch/11790685/
it was suggested to add support for VHOST_RING_ENABLE. While doing
that though I hit a couple problems:

1. The patches moved how vhost-scsi allocated cmds from per lio
session to per vhost vq. To support both VHOST_RING_ENABLE and
where userspace didn't support it, I would have to keep around the
old per session/device cmd allocator/completion and then also maintain
the new code. Or, I would still have to use this patch
patchwork.kernel.org/cover/11790763/ for the compat case so there
adding the new ioctl would not help much.

2. For vhost-scsi I also wanted to prevent where we allocate iovecs
for 128 vqs even though we normally use a couple. To do this, I needed
something similar to #1, but the problem is that the VHOST_RING_ENABLE
call would come too late.

To try and balance #1 and #2, these patches just allow vhost-scsi
to setup a vq when userspace starts to config it. This allows the
driver to only fully setup (we still waste some memory to support older
setups but do not have to preallocate everything like before) what
is used plus I do not need to maintain 2 code paths.

Note that in this posting I am also including additional patches
that create multiple vhost worker threads, because I wanted to see
if people felt that maybe to support that and for this enablement
issue we want a completely a new ioctl.


V2:
- fix use before set cpu var errors
- drop vhost_vq_is_setup
- include patches to do a worker thread per scsi IO vq