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

["Christian Bornträger" <cborntra-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org>]

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

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