Re: [Qemu-devel] [PATCH 4/5] virtio-scsi: Add start/stop functionality for vhost-scsi

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Sep 10, 2012 at 08:42:15AM +0200, Paolo Bonzini wrote:
> Il 10/09/2012 08:24, Michael S. Tsirkin ha scritto:
> >> > I chose the backend name because, ideally, there would be no other
> >> > difference.  QEMU _could_ implement all the goodies in vhost-scsi (such
> >> > as reservations or ALUA), it just doesn't do that yet.
> >> > 
> >> > Paolo
> > Then why do you say "It is used completely differently from
> > virtio-scsi-pci"?
> 
> It is configured differently (and I haven't seen a proposal yet for how
> to bridge the two), it does not interoperate, it has right now a
> different set of features.
> 
> The "does not interoperate" bit is particularly important.  Say QEMU
> were to implement persistent reservations (right now only a vhost-scsi
> feature).  Then QEMU and vhost-scsi PR would not be interchangeable, a
> reservation made by QEMU would not be visible in vhost and vice versa.

So this is backend stuff, right?

> > Isn't it just a different backend?
> > 
> > If yes then it should be a backend option, like it is
> > for virtio-net.
> 
> You mean a -drive option?

Yes.

> That would mean adding the logic to configure
> vhost-scsi to the QEMU block layer, that's a completely different project...
> 
> Paolo

This is an implementation detail. You can make it -drive option
but still have all the actual logic outside block layer.
All you need in block is option parsing code.
Please take a look at how -net does this:
we did *not* add all logic to qemu net layer.