[ovs-dev] [PATCH 07/10] netdev-dummy: Queue packets only to one rx queue.

[Ben Pfaff]

On Wed, Mar 23, 2016 at 01:13:06AM +0000, Daniele Di Proietto wrote:
> 
> On 22/03/2016 17:57, "Ben Pfaff" <blp at ovn.org> wrote:
> 
> >On Mon, Mar 14, 2016 at 06:18:17PM +0300, Ilya Maximets wrote:
> >> This is unused functionality and it will harm to
> >> multiqueue implementation.
> >> 
> >> Signed-off-by: Ilya Maximets <i.maximets at samsung.com>
> >
> >This change doesn't make any sense to me.  An equivalent change to the
> >Linux kernel would mean that if two copies of "tcpdump -i eth0" were
> >running then only one of them would get any packets.
> 
> I independently decided to make a very similar code change with the
> same semantics to netdev-dummy, so maybe there's something I don't
> understand as well :-)
> 
> netdev-dummy is like a physical NIC, the wire is a stream object
> (or the netdev-dummy/receive appctl, but only in one direction).
> 
> When a multi queue NIC receives a packet from the wire it should
> put it in just one of its rings, right?

I don't understand the confusion.

Suppose I call netdev_rxq_open() on a netdev that's backed by
netdev-linux, say for eth0.  That calls into
netdev_linux_rxq_construct(), which opens a Linux AF_PACKET socket.
>From then on, I can call netdev_rxq_recv() on the netdev_rxq that I got
(call it rxq0), and get a copy of each packet received on eth0.

Suppose I call netdev_rxq_open() on the same netdev again.  I get an
independent netdev_rxq socket for eth0 (call it rxq1).  I can call
netdev_rxq_recv() on both rxq0 and rxq1 and I'll get two copies of the
same stream of packets.

netdev-dummy is implementing the same thing.  struct netdev_dummy has a
list of open "netdev_rxq"s as its member named 'rxes'.  Each of those
gets a copy of all the received packets.

So...am I missing something?