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

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

On Tue, Sep 11, 2012 at 08:46:34AM -0500, Anthony Liguori wrote:
> On 09/10/2012 01:24 AM, Michael S. Tsirkin wrote:
> >On Mon, Sep 10, 2012 at 08:16:54AM +0200, Paolo Bonzini wrote:
> >>Il 09/09/2012 00:40, Michael S. Tsirkin ha scritto:
> >>>On Fri, Sep 07, 2012 at 06:00:50PM +0200, Paolo Bonzini wrote:
> >>>>Il 07/09/2012 08:48, Nicholas A. Bellinger ha scritto:
> >>>>>Cc: Stefan Hajnoczi<stefanha@linux.vnet.ibm.com>
> >>>>>Cc: Zhi Yong Wu<wuzhy@linux.vnet.ibm.com>
> >>>>>Cc: Michael S. Tsirkin<mst@redhat.com>
> >>>>>Cc: Paolo Bonzini<pbonzini@redhat.com>
> >>>>>Signed-off-by: Nicholas Bellinger<nab@linux-iscsi.org>
> >>>>>---
> >>>>>  hw/virtio-pci.c  |    2 ++
> >>>>>  hw/virtio-scsi.c |   49 +++++++++++++++++++++++++++++++++++++++++++++++++
> >>>>>  hw/virtio-scsi.h |    1 +
> >>>>>  3 files changed, 52 insertions(+), 0 deletions(-)
> >>>>
> >>>>Please create a completely separate device vhost-scsi-pci instead (or
> >>>>virtio-scsi-tcm-pci, or something like that).  It is used completely
> >>>>differently from virtio-scsi-pci, it does not make sense to conflate the
> >>>>two.
> >>>
> >>>Ideally the name would say how it is different, not what backend it
> >>>uses. Any good suggestions?
> >>
> >>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"?  Isn't it just a different backend?
> >
> >If yes then it should be a backend option, like it is
> >for virtio-net.
> 
> I don't mean to bike shed here so don't take this as a nack on
> making it a backend option, but in retrospect, the way we did
> vhost-net was a mistake even though I strongly advocated for it to
> be a backend option.
> 
> The code to do it is really, really ugly.  I think it would have
> made a lot more sense to just make it a device and then have it not
> use a netdev backend or any other kind of backend split.
> 
> For instance:
> 
> qemu -device vhost-net-pci,tapfd=X

We'd have to duplicate all tap options such as upscript then,
and educate users that vhost-net-pci is in fact same as virtio-net-pci
just faster. They have enough trouble guessing "-net-pci"
in virtio-net-pci.

IMHO a simple -device virtio-net-pci,vhost=on
would have been the right thing to do in retrospect.

> since vhost-net absolutely requires a tap fd, I think it's better in
> the long run to not abuse the netdev backend to prevent user
> confusion.

In practice adding an option (even if it was in the wrong place)
did not result in user confusion. Also in practice, renaming "virtio"
to virtio-net-pci etc did create confusion.


> Having a dedicated backend type that only has one
> possible option and can only be used by one device is a bit silly
> too.

By now we can just enable it by default. This was always the
idea.

> So I would be in favor of dropping/squashing 3/5 and radically
> simplifying how this was exposed to the user.

Let's just make sure we don't have implementation tail wagging
the user interface dog :)

> 
> I would just take qemu_vhost_scsi_opts and make them device properties.
> 
> Regards,
> 
> Anthony Liguori
> 
> >