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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] [PATCH 3/4] qemu:virtio-net: Add support for
	qemu_vlan_rxfilter
Date: Fri, 13 Feb 2009 17:04:49 +0000
References: <20090210212841.9760.96780.stgit@kvm.aw>
	<200902131617.54616.paul@codesourcery.com>
	<20090213164601.GK18471@shareable.org>
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To: Jamie Lokier <jamie@shareable.org>
Cc: qemu-devel@nongnu.org, Alex Williamson <alex.williamson@hp.com>, kvm@vger.kernel.org

> > 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. Devi=
ces=20
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:
>
> =A0 =A0 =A0"1 filter requested and installed in the kernel"
> =A0 =A0 =A0">1 filter requested, none installed in the kernel"
> =A0 =A0 =A0"0 filters requested, none installed in the kernel"

Absolutely not. This is the reason we have separate the "request an incomin=
g=20
filter" API from the "provide an outgoing filter" callback. It allows the=20
vlan code to arbitrate in the middle. A vlan is a bus network, not a set of=
=20
point-point connections. I haven't checked whether the proposed patch gets=
=20
this right. I suspect it probably doesn't.

This is why the initial patch that had clients talking to each other direct=
ly=20
was completely wrong.

Paul