Re: [Xen-devel] [RFC v2 1/4] bridge: enable interfaces to opt out from becoming the root bridge

[Zoltan Kiss]

On 19/02/14 09:52, Ian Campbell wrote:
> On Tue, 2014-02-18 at 13:02 -0800, Luis R. Rodriguez wrote:
> > On Sun, Feb 16, 2014 at 10:57 AM, Stephen Hemminger
> > <stephen@xxxxxxxxxxxxxxxxxx> wrote:
> > > On Fri, 14 Feb 2014 18:59:37 -0800
> > > "Luis R. Rodriguez" <mcgrof@xxxxxxxxxxxxxxxx> wrote:
> > >
> > > > From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx>
> > > >
> > > > It doesn't make sense for some interfaces to become a root bridge
> > > > at any point in time. One example is virtual backend interfaces
> > > > which rely on other entities on the bridge for actual physical
> > > > connectivity. They only provide virtual access.
> > > >
> > > > Device drivers that know they should never become part of the
> > > > root bridge have been using a trick of setting their MAC address
> > > > to a high broadcast MAC address such as FE:FF:FF:FF:FF:FF. Instead
> > > > of using these hacks lets the interfaces annotate its intent and
> > > > generalizes a solution for multiple drivers, while letting the
> > > > drivers use a random MAC address or one prefixed with a proper OUI.
> > > > This sort of hack is used by both qemu and xen for their backend
> > > > interfaces.
> > > >
> > > > Cc: Stephen Hemminger <stephen@xxxxxxxxxxxxxxxxxx>
> > > > Cc: bridge@xxxxxxxxxxxxxxxxxxxxxxxxxx
> > > > Cc: netdev@xxxxxxxxxxxxxxx
> > > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > > Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxx>
> > >
> > > This is already supported in a more standard way via the root
> > > block flag.
> >
> > Great! For documentation purposes the root_block flag is a sysfs
> > attribute, added via 3.8 through commit 1007dd1a. The respective
> > interface flag is IFLA_BRPORT_PROTECT and can be set via the iproute2
> > bridge utility or through sysfs:
> >
> > mcgrof@garbanzo ~/linux (git::master)$ find /sys/ -name root_block
> > /sys/devices/pci0000:00/0000:00:04.0/0000:02:00.0/net/eth0/brport/root_block
> > /sys/devices/vif-3-0/net/vif3.0/brport/root_block
> > /sys/devices/virtual/net/vif3.0-emu/brport/root_block
> >
> > mcgrof@garbanzo ~/devel/iproute2 (git::master)$ cat
> > /sys/devices/vif-3-0/net/vif3.0/brport/root_block
> > 0
> > mcgrof@garbanzo ~/devel/iproute2 (git::master)$ sudo bridge link set
> > dev vif3.0 root_block on
> > mcgrof@garbanzo ~/devel/iproute2 (git::master)$ cat
> > /sys/devices/vif-3-0/net/vif3.0/brport/root_block
> > 1
> >
> > So if we'd want to avoid using the MAC address hack alternative to
> > skip a root port userspace would need to be updated to simply set this
> > attribute after adding the device to the bridge. Based on Zoltan's
> > feedback there seems to be use cases to not enable this always for all
> > xen-netback interfaces though as such we can just punt this to
> > userspace for the topologies that require this.
> >
> > The original motivation for this series was to avoid the IPv6
> > duplicate address incurred by the MAC address hack for avoiding the
> > root bridge. Given that Zoltan also noted a use case whereby IPv4 and
> > IPv6 addresses can be assigned to the backend interfaces we should be
> > able to avoid the duplicate address situation for IPv6 by using a
> > proper random MAC address *once* userspace has been updated also to
> > use IFLA_BRPORT_PROTECT. New userspace can't and won't need to set
> > this flag for older kernels (older than 3.8) as root_block is not
> > implemented on those kernels and the MAC address hack would still be
> > used there. This strategy however does put a requirement on new
> > kernels to use new userspace as otherwise the MAC address workaround
> > would not be in place and root_block would not take effect.
>
> Can't we arrange things in the Xen hotplug scripts such that if the
> root_block stuff isn't available/doesn't work we fallback to the
> existing fe:ff:ff:ff:ff usage?
>
> That would avoid concerns about forward/backwards compat I think. It
> wouldn't solve the issue you are targeting on old systems, but it also
> doesn't regress them any further.

I agree, I think this problem could be better handled from userspace: if 
it can set root_block then change the default MAC to a random one, if it 
can't, then stay with the default one. Or if someone doesn't care about 
STP but DAD is still important, userspace can have a force_random_mac 
option somewhere to change to a random MAC regardless of root_block 
presence.

Zoli
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