macvtap/tun: cross-endian support for little-endian hosts

The VNET_LE flag was introduced to fix accesses to virtio 1.0 headers
that are always little-endian. It can also be used to handle the special
case of a legacy little-endian device implemented by a big-endian host.

Let's add a flag and ioctls for big-endian devices as well. If both flags
are set, little-endian wins.

Since this is isn't a common usecase, the feature is controlled by a kernel
config option (not set by default).

Both macvtap and tun are covered by this patch since they share the same
API with userland.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

drivers/net/Kconfig

@@ -244,6 +244,20 @@ config TUN
 
 	  If you don't know what to use this for, you don't need it.
 
+config TUN_VNET_CROSS_LE
+	bool "Support for cross-endian vnet headers on little-endian kernels"
+	default n
+	---help---
+	  This option allows TUN/TAP and MACVTAP device drivers in a
+	  little-endian kernel to parse vnet headers that come from a
+	  big-endian legacy virtio device.
+
+	  Userspace programs can control the feature using the TUNSETVNETBE
+	  and TUNGETVNETBE ioctls.
+
+	  Unless you have a little-endian system hosting a big-endian virtual
+	  machine with a legacy virtio NIC, you should say N.
+
 config VETH
 	tristate "Virtual ethernet pair device"
 	---help---