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

On Mon, Sep 10, 2012 at 08:42:15AM +0200, Paolo Bonzini wrote:

Il 10/09/2012 08:24, Michael S. Tsirkin ha scritto:

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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?

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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.