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Date: Mon, 10 Sep 2012 09:49:44 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 4/5] virtio-scsi: Add start/stop
 functionality for vhost-scsi
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, kvm-devel <kvm@vger.kernel.org>, Jan Kiszka <jan.kiszka@siemens.com>, Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>, qemu-devel <qemu-devel@nongnu.org>, "Nicholas A. Bellinger" <nab@linux-iscsi.org>, lf-virt <virtualization@lists.linux-foundation.org>, Anthony Liguori <aliguori@linux.vnet.ibm.com>, target-devel <target-devel@vger.kernel.org>, Hannes Reinecke <hare@suse.de>, Zhi Yong Wu <wuzhy@cn.ibm.com>, Christoph Hellwig <hch@lst.de>

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.