Date:	Sat, 19 Jun 2010 20:48:31 +0200
From:	Joakim Tjernlund <joakim.tjernlund@...nsmode.se>
To:	Lennert Buytenhek <buytenh@...tstofly.org>
Cc:	netdev@...r.kernel.org
Subject: Re: Distributed Switch Architecture(DSA)

Lennert Buytenhek <buytenh@...tstofly.org> wrote on 2010/06/19 18:56:43:
>
> On Sat, Jun 19, 2010 at 04:22:18PM +0200, Joakim Tjernlund wrote:
>
> > > > > > OK. With DSA, how does one configure VLANs, policing and
> > > > > > parameters in the HW switch that don't map or exist in the
> > > > > > linux bridge?
> > > > >
> > > > > The idea is to use existing kernel interface for this as much as
> > > > > possible.  So e.g. if you do:
> > > > >
> > > > >    vconfig add lan1 123
> > > > >    vconfig add lan2 123
> > > > >    brctl addbr br123
> > > > >    brctl addif br123 lan1.123
> > > > >    brctl addif br123 lan2.123
> > > > >
> > > > > Then the DSA code (or some userspace netlink listener helper, or some
> > > > > combination of both) should ideally also detect that VLAN 123 on
> > > > > interfaces lan1 and lan2 are to be bridged together, and program the
> > > > > switch chip accordingly.  I think all VLAN configurations that at least
> > > > > the Marvell hardware supports can be expressed this way.
> > > >
> > > > Yes, but I image that this breaks down when you want to do something
> > > > a bit more advanced. For example I don't think linux VLANs supports
> > > > "shared VLAN learning"(SVL) and to configure a HW switch to do SVL
> > > > one would first have to impl. that in Linux VLAN and then add the DSA
> > > > code to get the config to the switch.
> > >
> > > Yes.  But that's really the best way to do it, in my humble opinion.
> >
> > I will buy that for the moment. I can't see a better way either if
> > you truly want to integrate a HW switch into linux. I just wish
> > Linux VLANs had some support for SVL too
>
> You know how to fix that. :)

Possibly :)

>
>
> > > If you don't go the host networking stack integration route, you end
> > > up with something like the vendor drivers.  Which work fine for most
> > > scenarios.. until you want to do something like talking TCP/IP using
> > > the host TCP stack over some of the switch ports, at which point the
> > > lack of host networking stack integration comes to bite you.
> >
> > Just doing STP will bite you :)
>
> Most people deal with this by running a userland STP daemon that uses
> raw sockets to inject manually (i.e. in userspace) DSA-tagged packets
> onto the eth0 (or whatever) interface.  This "works" (for some
> definitions of 'works') for UDP apps such as a DHCP server as well --
> this crappy approach unfortunately only really breaks down for TCP.
>
>
> > > > Not sure how one would express whether VLAN tags should be stripped
> > > > off or not when egressing the HW switch's physical port.
> > >
> > > If you transmit a packet onto 'lan', it will be sent to the switch chip
> > > with an "untagged" DSA tag.  If you transmit a packet onto 'lan.123',
> > > it will be sent to the switch chip with a "tagged" DSA tag.  See
> > > net/dsa/tag_dsa.c for details.
> >
> > Ah, now I get it, thanks.
> > However, how does this work for LAN to LAN pkgs? LAN1 and LAN2 could be
> > in the same VLAN but one is implicit(port) VLAN and the
> > other is explicit.
>
> If you tell the HW switch to forward these packets, they will never
> appear at the CPU interface, so the DSA tagging/untagging doesn't enter
> the picture.

"tell the HW switch"? Doesn't DSA do that already? If not, what
is the point of DSA then if it doesn't use the native forwarding
capabilities of the HW switch?

>
>
> > How do I config the HW switch to do that?
>
> Tell the switch that the vlan is native on one of the ports but not on
> the other.  It's been a while since I looked at the chip docs but there
> are ways of doing this.

The current DSA impl. does not support this? There should be some
way to manage this within the DSA framework.

>
>
> > > > Furthermore, suppose one have a big HW switch, 48 ports, and lots of
> > > > VLANs in that HW switch one would have to create a lot of virtual I/Fs
> > > > and VLANs in linux just to configure the HW switch. This wastes
> > > > resources on the CPU.
> > >
> > > Where the 'resource waste' is on the order of a couple of tens or
> > > hundreds of kilobytes of RAM.  If this is a problem for your host
> > > CPU, I think you have bigger problems anyway.
> >
> > That is not a very good argument, this is how bloat builds.
>
> If you have a better way of getting all the features while spending
> less resources, please step forward with your ideas.  The current design
> is the best I could come up with, but I'm sure it's not optimal in its
> current form.

I don't, I am not that familiar with the inner working of Linux
networking code.

>
>
> > > > > To configure things like ingress/egress rate limiting and such in the
> > > > > switch chip for which there is no Linux counterpart interface, I suppose
> > > > > some sysfs interface or so might suffice.
> > > >
> > > > Yes, there are aspects of a HW switch that doesn't map into DSA currently.
> > > > Perhaps one should add some framework to support this?
> > >
> > > Sounds good.
> >
> > Any idea how such an framework should look like? What transport
> > mechanism is suitable to talk to a user space daemon?
>
> Have a look at netlink.

I was afraid you would say that, I have no experience with netlink :)

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