Re: [Qemu-devel] [PATCH v4 06/16] net: Remove vlan qdev property

[Zhi Yong Wu]

On Mon, Jun 11, 2012 at 10:42 PM, Stefan Hajnoczi <address@hidden> wrote:
> On Mon, Jun 11, 2012 at 3:24 PM, Zhi Yong Wu <address@hidden> wrote:
>> On Mon, Jun 11, 2012 at 4:57 PM, Stefan Hajnoczi
>> <address@hidden> wrote:
>>> On Mon, Jun 11, 2012 at 08:28:57AM +0200, Paolo Bonzini wrote:
>>>> Il 09/06/2012 05:04, Zhi Yong Wu ha scritto:
>>>> >>>> This commit looks suspicious because it removes a user-visible qdev
>>>> >>>> property but we're trying to preserve backward compatibility.  This
>>>> >>>> command-line will break:
>>>> >>>>
>>>> >>>> x86_64-softmmu/qemu-system-x86_64 -net user,vlan=1 -device 
>>>> >>>> virtio-net-pci,vlan=1
>>>> >>>>
>>>> >>>> Instead of dropping the qdev_prop_vlan completely the
>>>> >>>> hw/qdev-properties.c code needs to call net/hub.h external functions
>>>> >>>> to implement equivalent functionality:
>>>> >>>>
>>>> >>>> 1. Setting the vlan=<id> property looks up the hub port and assigns
>>>> >>>> the NICConf->peer field.
>>>> >>>> 2. Getting the vlan property looks up the hub id (i.e. vlan id) given
>>>> >>>> the peer.  If the peer is not a hub port the result is -1.
>>>> >>>>
>>>> >>>> When I wrote this patch I missed the big picture and forgot about
>>>> >>>> backwards compatibility :(.
>>>> >>>>
>>>> > To be honest, i am concerned if anyone uses this syntax. Since the
>>>> > feature will finally be discarded, i suggest that we don't support
>>>> > this now. If someone complains this later, we can fix it. If nobody
>>>> > complains, that is what we hope.
>>>>
>>>> I think you're missing the big picture of this series, which is exactly
>>>> _not_ to discard the VLAN feature, but just to rewrite it in a better way.
>>>>
>>>> That said, I agree that this is a somewhat fringe usage; most people
>>>> will use -net nic,model=virtio,vlan=1 rather than "-device".  We may get
>>>> by with dropping it.  I have no strong opinion either way.
>>>
>>> Either we keep backwards compatibility or we don't.  Taking a middle
>>> path where we preserve only some of the "VLAN" syntax is confusing and
>>> inconsistent.
>> in terms of technology, i fully agree with you, but in terms of
>> usefulness and our business, it will waste our effort, time and is
>> meaningless if nobody or no customers use this syntax. As i said, if
>> someone complain this later, we can fix it.
>
> When users upgrade QEMU versions and find their setup is now broken
> QEMU's reputation will be damaged.  You can't build critical systems
> on top of software which keeps changing and breaking.  Fixing it after
> a user hits the problem is not okay, users won't trust us if we do
> that.
OK, i will try to work on this.
>
> We need to be disciplined when it comes to backwards compatibility.
> Either we support the "VLAN" feature or we drop it.  We already had
> this discussion in another thread, here's what Anthony had to say:
>
> "Dropping features is only something that should be approached lightly and
> certainly not something that should be done just because you don't like a
> particular bit of code."
>
> http://permalink.gmane.org/gmane.comp.emulators.qemu/153600
>
> Stefan



-- 
Regards,

Zhi Yong Wu