netdev - Re: [Bridge] [PATCH] macvlan: add tap device backend

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Message-ID: <20090810193227.GA16820@redhat.com>
Date:	Mon, 10 Aug 2009 22:32:27 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"Fischer, Anna" <anna.fischer@...com>,
	"Paul Congdon (UC Davis)" <ptcongdon@...avis.edu>,
	"drobbins@...too.org" <drobbins@...too.org>,
	"herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"bridge@...ts.linux-foundation.org" 
	<bridge@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ogerlitz@...taire.com" <ogerlitz@...taire.com>,
	"evb@...oogroups.com" <evb@...oogroups.com>,
	"davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [Bridge] [PATCH] macvlan: add tap device backend

On Mon, Aug 10, 2009 at 09:04:54PM +0200, Arnd Bergmann wrote:
> On Monday 10 August 2009, Fischer, Anna wrote:
> > If you compare macvtap with traditional QEMU networking interfaces that
> > are typically used in current bridged setups, then yes, performance will be
> > different. However, I think that this is not necessarily a fair 
> > comparison, and the performance difference does not come from the 
> > bridge being slow, but simply because you have implemented a better
> > solution to connect a virtual interface to a backend device that
> > can be assigned to a VM. There is no reason why you could not do this
> > for a bridge port as well.
> 
> It's not necessarily the bridge itself being slow (though some people
> claim it is) but more the bridge preventing optimizations or making
> them hard.
> 
> You already mentioned hardware filtering by unicast and multicast
> mac addresses, which macvlan already does (for unicast) but which would be
> relatively complex with a bridge due to the way it does MAC address
> learning.
> 
> If we want to do zero copy receives, the hardware will on top of
> this have to choose the receive buffer based on the mac address,
> with the buffer provided by the guest. I think this is not easy
> with macvlan but doable, while I have no idea where you would start
> using the bridge code.
> 
> 	Arnd <><

Similar thing for zero copy sends. You need to know when
the buffers have been consumed to notify userspace,
and this is very hard with a generic bridge in the middle.


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MST
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