[ovs-dev] OVS in user-space mode

[Jesse Gross]

On Mon, Aug 1, 2011 at 1:31 PM, Madhav Puri <madhav.puri at gmail.com> wrote:
> Hello,
>
> I am trying to walk-thru and understand some of the ovs code that gets
> exercised when ovs is configured for user-space datapath (i.e.
> datapath_type=netdev). I have a few questions on use of tap devices in this
> case for switching data between multiple interfaces.
>
> From the net-device creation function 'netdev_linux_create_tap()',
> definition and the function header, it looks like we open and bind to same
> tap device (/dev/net/tun) for all interfaces i.e. a read from this device
> could give us data available on any of the underlying bound interfaces. If
> the understanding is correct, then in 'dpif_netdev_run()' where we traverse
> the list of all ports and call 'netdev_recv()' we could potentially read
> data on one port which actually came on a different port. Is this
> understanding correct? If so then how do we apply port specific open-flow
> rules in this case? In other words, how do we distinguish between data
> received on one port from another in user-space mode.

/dev/net/tun is simply a device on which to call ioctls - it is not
the actual tap device.  You can create multiple devices with it and
distinguish based on file descriptor.  The comment above
netdev_linux_create_tap() refers to opening the same device multiple
times, which will cause users to share buffers but only within that
device.