Subject: Re: Multicast and receive filtering in TUN/TAP

From: Max Krasnyansky <[email-masked]>
Date: Thu, 10 Jul 2008 09:57:56 -0700

Christian Borntraeger wrote:
> Am Donnerstag, 10. Juli 2008 schrieb Max Krasnyansky:
> [...]
>> The second question is do you guys think that QEMU/KVM/LGUEST/etc would 
>> benefit if receive filtering was done by the host OS. Here is a specific 
>> example of what I'm talking about.
>> We can do what qemu/hw/e1000.c:receive_filter() does in the _host_ 
>> context (that function currently runs in the guest context). By looking 
>> at libvirt, typical QEMU based setup is that you have a single bridge 
>> and all the TAPs from different VMs are hooked up to that bridge. What 
>> that means is that if one VM is getting MC traffic or when the bridge 
>> sees MACADDR that is not in its tables the packets get delivered to all 
>> the VMs. ie We have to wake all of the up only to so that they could 
>> drop that packet. Instead, we could setup filters in the host's side of 
>> the TAP device.
>> Does that sound like something useful for QEMU/KVM ?
>> If yes we can talk about the API. If not then I'll just nuke it.
> 
> Max,
> 
> I know that on s390 the shared OSA network card have multicast filter 
> capabilities. So I guess it is worthwile for a virtualization environments 
> with lots of guests. I also think, that this kind of filtering should be 
> straightforward to implement with the qemu e1000 code. Qemu already knows the 
> multicast addresses.
Sure. It's straightforward to do inside QEMU, and it's already doing it.
The question is should we do it in the host context instead and avoid some
wakeups.

> Thing is, we are heading towards virtio. 
Even for Windows ?

> Unfortunately, virtio_net currently  does not offer a method to register multicast addresses.
I haven't looked at the virtio stuff much, I was assuming that the host side
of it is still the TUN driver. Is it not ?

Max