Re: [PATCH/RFC 8/9] Virtual network host switch support

Christian Bornträger
2007-05-11T20:50:55Z

On Friday 11 May 2007 22:21, Anthony Liguori wrote:
> Any feel for the performance relative to the bridging code? The
> bridging code is a pretty big bottle neck in guest=>guest communications
> in Xen at least.

Last time I checked it we had a quite decent guest to guest performance in the
gigabits/sec.
On the downside the switch is quite aggressive with dropping packages as the
inbound buffer of the virtual network adapters has space for 80 packets.
(that can be changed)

>
> > currently tested but not ready yet. We did not use the linux bridging code
to
> > allow non-root users to create virtual networks between guests.
> >
>
> Is that the primary reason? If so, that seems like a rather large
> hammer for something that a userspace suid wrapper could have addressed...

Actually there are some reasons why we did not use the bridging code:

- One thing is, that a lot of OSA network cards do not support promiscous
mode. There is also the issue that a lot of OSA cards are in layer 3 mode (we
get IP packets and no ethernet frames) so bridging wont work to the host
interface.
- non-root switches
- the performance of bridging (we copy directly from one guest buffer to
another without allocating an skb on the host)
- we considered to hook into the qeth driver (for OSA cards) to deal with
layer3 mode.

The first shot was actually a point-to-point driver (guest netif <--> host
netif). We added the switch at a later time.

Hmm, if we can make bridging work (with a decent performance) on s390 that
would reduce the maintainance work for us as this network switch is far from
being complete.

cheers

Christian