Messages in this thread | | | Date | Wed, 19 Feb 2014 14:35:43 +0000 | From | Zoltan Kiss <> | Subject | Re: [Xen-devel] [RFC v2 1/4] bridge: enable interfaces to opt out from becoming the root bridge |
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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@networkplumber.org> wrote: >>> On Fri, 14 Feb 2014 18:59:37 -0800 >>> "Luis R. Rodriguez" <mcgrof@do-not-panic.com> wrote: >>> >>>> From: "Luis R. Rodriguez" <mcgrof@suse.com> >>>> >>>> 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@networkplumber.org> >>>> Cc: bridge@lists.linux-foundation.org >>>> Cc: netdev@vger.kernel.org >>>> Cc: linux-kernel@vger.kernel.org >>>> Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com> >>> >>> 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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