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[Xen-devel] [RFC v3 6/6] tun: add initialization root block support

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

The networking bridge module allows us to specify a
root block preference on net_devices but this feature
is a bridge port primitive. The bridge module assumes
that once a device is added as a slave to the brige
that it can be considered for the the root port.
Furthermore ndo_add_slave() only lets us pass on the
net_device, but drivers that want to root block
since the beginning need specify their root block
preference prior to register_netdevice(). This means
the tun driver must expose a knob to userspace to
enable toggling this feature prior to adding the
device to a bridge to prevent any undesired link
bouncing issues on the bridge.

This should be useful for TAP interfaces which are used
for virtualization.

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/tun.c           | 6 +++++-
 include/uapi/linux/if_tun.h | 1 +
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 8fe9cb7..e763fad 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1693,6 +1693,9 @@ static int tun_set_iff(struct net *net, struct file 
*file, struct ifreq *ifr)
                if (err < 0)
                        goto err_free_flow;
+               if (ifr->ifr_flags & IFF_ROOT_BLOCK)
+                       dev->priv_flags |= IFF_BRIDGE_ROOT_BLOCK;
                err = register_netdevice(tun->dev);
                if (err < 0)
                        goto err_detach;
@@ -1898,7 +1901,8 @@ static long __tun_chr_ioctl(struct file *file, unsigned 
int cmd,
                 * This is needed because we never checked for invalid flags on
                 * TUNSETIFF. */
                return put_user(IFF_TUN | IFF_TAP | IFF_NO_PI | IFF_ONE_QUEUE |
-                               IFF_VNET_HDR | IFF_MULTI_QUEUE,
+                               IFF_VNET_HDR | IFF_MULTI_QUEUE |
+                               IFF_ROOT_BLOCK,
                                (unsigned int __user*)argp);
        } else if (cmd == TUNSETQUEUE)
                return tun_set_queue(file, &ifr);
diff --git a/include/uapi/linux/if_tun.h b/include/uapi/linux/if_tun.h
index e9502dd..f814a85 100644
--- a/include/uapi/linux/if_tun.h
+++ b/include/uapi/linux/if_tun.h
@@ -73,6 +73,7 @@
 /* read-only flag */
 #define IFF_PERSIST    0x0800
 #define IFF_NOFILTER   0x1000
+#define IFF_ROOT_BLOCK 0x2000
 /* Socket options */

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