Re: [PATCH][RFC] net/bridge: add basic VEPA support

On Monday 10 August 2009, Fischer, Anna wrote:

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Subject: Re: [PATCH][RFC] net/bridge: add basic VEPA support

On Friday 07 August 2009, Paul Congdon (UC Davis) wrote:

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As I understand the macvlan code, it currently doesn't allow two VMs

on the

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same machine to communicate with one another.

There are patches to do that. I think if we add that, there should be
a way to choose the behavior between either bridging between the
guests or VEPA.

If you implement this direct bridging capability between local VMs for 
macvlan, then would this not break existing applications that currently
use it? It would be quite a significant change to how macvlan works 
today. I guess, ideally, you would want to have macvlan work in 
separate modes, e.g. traditional macvlan, bridging, and VEPA.

Right, that's what I meant with my sentence above. I'm not sure
if we need to differentiate traditional macvlan and VEPA though.
AFAICT, the only difference should be the handling of broadcast
and multicast frames returning from the hairpin turn. Since this
does not happen with a traditional macvlan, we can always send them
to all macvlan ports except the source port.

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I could imagine a hairpin mode on the adjacent bridge making this
possible, but the macvlan code would need to be updated to filter
reflected frames so a source did not receive his own packet.

Right, I missed this point so far. I'll follow up with a patch
to do that.

Can you maybe point me to the missing patches for macvlan that you
have mentioned in other emails, and the one you mention above? 
E.g. enabling multicast distribution and allowing local bridging etc.
I could not find any of those in the archives. Thanks.

The patch from Eric Biederman to allow macvlan to bridge between
its slave ports is at

http://kerneltrap.org/mailarchive/linux-netdev/2009/3/9/5125774

I could not find any patches for the other features (or bugs).

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This is the interesting part of the discussion. The bridge and macvlan
drivers certainly have an overlap in functionality and you can argue
that you only need one. Then again, the bridge code is a little crufty
and we might not want to add much more to it for functionality that can
be implemented in a much simpler way elsewhere. My preferred way would
be to use bridge when you really need 802.1d MAC learning, netfilter-
bridge
and STP, while we put the optimizations for stuff like VMDq, zero-copy
and multiqueue guest adapters only into the macvlan code.

I can see this being a possible solution. 

My concern with putting VEPA into macvlan instead of the bridging code
is that there will be more work required to make it usable for other
virtualization solution as macvtap will only work for KVM type setups.

Right, I understand.

Basically, VEPA capabilities would rely on someone developing further
drivers to connect macvlan to different backend interfaces, e.g. one for
KVM (macvtap), one for Xen PV drivers, one for virtio, and whatever else
is out there, or will be there in the future. The bridging code is
already very generic in that respect, and all virtualization layers
can deal with connecting interfaces to a bridge. 

Our extensions to the bridging code to enable VEPA for the Linux kernel
are only very minimal code changes and would allow to make VEPA available
to most virtualization solutions today.

I don't object to having VEPA supported in the bridge code at all.
I think your patch is simple enough so it won't hurt in the bridge
code. If Stephen prefers to do VEPA only in one component, we should
probably make it possible for that component to act as a bridge between
1+n existing interfaces as well. You can almost do that with the regular
macvlan and the bridge driver, like

      / macvlan0 - br0 - tap0
eth0 -- macvlan1 - br1 - tap1
      \ macvlan2 - br2 - tap2

Here, you can have two guests attached to tap devices (or xen net ...)
and the macvlan driver doing the VEPA. Of course this is not how bridge
works -- you would have the same mac addresses on two sides of
the bridge.

So we could have another macvlan backend (let's call it macvbridge)
so you can do this:

      / macvlan0 - 'qemu -net raw'
eth0 -- macvtap0 - 'qemu -net tap,fd=3 3<>/dev/net/macvtap0'
      \ macvbr0 -- tap0 - 'qemu -net tap'

The macvbr driver could this way be used to associate an existing
network device to a slave of a macvlan port. Not sure if this
has any significant advantage over your bridge patches, it does
have the obvious disadvantage that someone needs to implement
it first, while your patch is there ;-)

	Arnd <><