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

[Anthony Liguori <aliguori@us.ibm.com>]

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

I know this breaks the model of separate backends and frontends but 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.  Having a dedicated 
backend type that only has one possible option and can only be used by one 
device is a bit silly too.

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

I would just take qemu_vhost_scsi_opts and make them device properties.

Regards,

Anthony Liguori

>