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[Xen-devel] [RFC v3 5/6] xen-netback: use a random MAC address and force bridge root block

From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx>

The purpose of using a static MAC address of FE:FF:FF:FF:FF:FF
was to prevent our backend interfaces from being used by the
bridge and nominating our interface as a root port on the bridge.
This was possible given that the bridge code will use the lowest MAC
address for a port once a new interface gets added to the bridge.

Sticking to a static MAC address is undesirable for a few reasons:

  a) using a static MAC address by default on all interfaces can
     lead to possible conflicts with IPv6 SLAAC and DAD

  b) The bridge code has a generic bridge port 'root block' feature
     to allow interfaces to opt out from root bridge port nominations.
     We want to help stop spreading the use of a high MAC address
     as a hack to do root port block, and instead get folks to
     use the proper APIs for this.

This modifies xen-netback to use a random MAC address with the xen OUI
prefix and enables the bridge root block feature since initialization.
Although toggling the root block feature requires the iproute2 bridge
tool [0] or sysfs [1] xen-netback users would only need to toggle this
off in the case that the net_device is required to be a candidate for
root port nomination on the bridge. This is an acceptable compromise
in order to avoid the possible conflicts with IPv6 SLAAC and DAD.

[0] bridge link set dev vif2.0 root_block off
[1] echo 1 > /sys/devices/vif-2-0/net/vif2.0/brport/root_block

Cc: Stephen Hemminger <stephen@xxxxxxxxxxxxxxxxxx>
Cc: bridge@xxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: netdev@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Cc: kvm@xxxxxxxxxxxxxxx
Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxx>
 drivers/net/xen-netback/interface.c | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/drivers/net/xen-netback/interface.c 
index 7669d49..79d71ec 100644
--- a/drivers/net/xen-netback/interface.c
+++ b/drivers/net/xen-netback/interface.c
@@ -42,6 +42,8 @@
+static const u8 xen_oui[3] = { 0x00, 0x16, 0x3e };
 int xenvif_schedulable(struct xenvif *vif)
        return netif_running(vif->dev) && netif_carrier_ok(vif->dev);
@@ -346,15 +348,9 @@ struct xenvif *xenvif_alloc(struct device *parent, domid_t 
        for (i = 0; i < MAX_PENDING_REQS; i++)
                vif->mmap_pages[i] = NULL;
-       /*
-        * Initialise a dummy MAC address. We choose the numerically
-        * largest non-broadcast address to prevent the address getting
-        * stolen by an Ethernet bridge for STP purposes.
-        * (FE:FF:FF:FF:FF:FF)
-        */
-       memset(dev->dev_addr, 0xFF, ETH_ALEN);
-       dev->dev_addr[0] &= ~0x01;
+       eth_hw_addr_random(dev);
+       memcpy(dev->dev_addr, xen_oui, 3);
+       dev->priv_flags |= IFF_BRIDGE_ROOT_BLOCK;
        netif_napi_add(dev, &vif->napi, xenvif_poll, XENVIF_NAPI_WEIGHT);

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