Subject: Re: [virtio-dev] [RFC PATCH net-next v2 1/2] virtio_net: Introduce VIRTIO_NET_F_BACKUP feature bit

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 17 Jan 2018 21:02:00 +0200

On Wed, Jan 17, 2018 at 10:15:52AM -0800, Alexander Duyck wrote:
> On Thu, Jan 11, 2018 at 9:58 PM, Sridhar Samudrala
> <[email-masked]> wrote:
> > This feature bit can be used by hypervisor to indicate virtio_net device to
> > act as a backup for another device with the same MAC address.
> >
> > Signed-off-by: Sridhar Samudrala <[email-masked]>
> > ---
> >  drivers/net/virtio_net.c        | 2 +-
> >  include/uapi/linux/virtio_net.h | 3 +++
> >  2 files changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 12dfc5fee58e..f149a160a8c5 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -2829,7 +2829,7 @@ static struct virtio_device_id id_table[] = {
> >         VIRTIO_NET_F_GUEST_ANNOUNCE, VIRTIO_NET_F_MQ, \
> >         VIRTIO_NET_F_CTRL_MAC_ADDR, \
> >         VIRTIO_NET_F_MTU, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS, \
> > -       VIRTIO_NET_F_SPEED_DUPLEX
> > +       VIRTIO_NET_F_SPEED_DUPLEX, VIRTIO_NET_F_BACKUP
> >
> >  static unsigned int features[] = {
> >         VIRTNET_FEATURES,
> > diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
> > index 5de6ed37695b..c7c35fd1a5ed 100644
> > --- a/include/uapi/linux/virtio_net.h
> > +++ b/include/uapi/linux/virtio_net.h
> > @@ -57,6 +57,9 @@
> >                                          * Steering */
> >  #define VIRTIO_NET_F_CTRL_MAC_ADDR 23  /* Set MAC address */
> >
> > +#define VIRTIO_NET_F_BACKUP      62    /* Act as backup for another device
> > +                                        * with the same MAC.
> > +                                        */
> >  #define VIRTIO_NET_F_SPEED_DUPLEX 63   /* Device set linkspeed and duplex */
> >
> >  #ifndef VIRTIO_NET_NO_LEGACY
> 
> I'm not a huge fan of the name "backup" since that implies that the
> Virtio interface is only used if the VF is not present, and there are
> multiple instances such as dealing with east/west or
> broadcast/multicast traffic where it may be desirable to use the
> para-virtual interface rather then deal with PCI overhead/bottleneck
> to send the packet.

Right now hypervisors mostly expect that yes, only one at a time is
used.  E.g. if you try to do multicast sending packets on both VF and
virtio then you will end up with two copies of each packet.

To me the east/west scenario looks like you want something
more similar to a bridge on top of the virtio/PT pair.

So I suspect that use-case will need a separate configuration bit,
and possibly that's when you will want something more powerful
such as a full bridge.

> What if instead of BACKUP we used the name SIDE_CHANNEL? Basically it
> is a bit of double entendre as we are using the physical MAC address
> to provide configuration information, and then in addition this
> interface acts as a secondary channel for passing frames to and from
> the guest rather than just using the VF.
> 
> Just a thought.
> 
> Thanks.
> 
> - Alex

I just feel that's a very generic name, not conveying enough information
about how they hypervisor expects the pair of devices to be used.

-- 
MST