Re: [Qemu-devel] [PATCH 3/4] qemu:virtio-net: Add support for qemu_vlan_rxfilter

[Paul Brook <paul@codesourcery.com>]

> > A simple "Something changed, please try your filter again" callback
> > automatically covers all these cases.
>
> It doesn't, if tap has no memory of how many clients require a filter.

I'm talking about a callback for devices requesting an inbound filter. Devices 
implementing outgoing (device->vlan) filters just do as they're told.

> If tap just answers "YES and installs the kernel filter" or "NO and
> doesn't install the kernel filter" and doesn't remember how many
> clients need a filter, then:
>...
> In other words, tap needs to distinguish three states:
>
>      "1 filter requested and installed in the kernel"
>      ">1 filter requested, none installed in the kernel"
>      "0 filters requested, none installed in the kernel"

Absolutely not. This is the reason we have separate the "request an incoming 
filter" API from the "provide an outgoing filter" callback. It allows the 
vlan code to arbitrate in the middle. A vlan is a bus network, not a set of 
point-point connections. I haven't checked whether the proposed patch gets 
this right. I suspect it probably doesn't.

This is why the initial patch that had clients talking to each other directly 
was completely wrong.

Paul