Re: [virtio-dev] Re: [PATCH v7 net-next 2/4] net: Introduce generic failover module

[Alexander Duyck <alexander.duyck@gmail.com>]

On Fri, Apr 20, 2018 at 8:34 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Fri, Apr 20, 2018 at 08:21:00AM -0700, Samudrala, Sridhar wrote:
>> > > + finfo = netdev_priv(failover_dev);
>> > > +
>> > > + primary_dev = rtnl_dereference(finfo->primary_dev);
>> > > + standby_dev = rtnl_dereference(finfo->standby_dev);
>> > > +
>> > > + if (slave_dev != primary_dev && slave_dev != standby_dev)
>> > > +         goto done;
>> > > +
>> > > + if ((primary_dev && failover_xmit_ready(primary_dev)) ||
>> > > +     (standby_dev && failover_xmit_ready(standby_dev))) {
>> > > +         netif_carrier_on(failover_dev);
>> > > +         netif_tx_wake_all_queues(failover_dev);
>> > > + } else {
>> > > +         netif_carrier_off(failover_dev);
>> > > +         netif_tx_stop_all_queues(failover_dev);
>> > And I think it's a good idea to get stats from device here too.
>>
>> Not sure why we need to get stats from lower devs here?
>
> link down is often indication of a hardware problem.
> lower dev might stop responding down the road.
>
>> > > +static const struct net_device_ops failover_dev_ops = {
>> > > + .ndo_open               = failover_open,
>> > > + .ndo_stop               = failover_close,
>> > > + .ndo_start_xmit         = failover_start_xmit,
>> > > + .ndo_select_queue       = failover_select_queue,
>> > > + .ndo_get_stats64        = failover_get_stats,
>> > > + .ndo_change_mtu         = failover_change_mtu,
>> > > + .ndo_set_rx_mode        = failover_set_rx_mode,
>> > > + .ndo_validate_addr      = eth_validate_addr,
>> > > + .ndo_features_check     = passthru_features_check,
>> > xdp support?
>>
>> I think it should be possible to add it be calling the lower dev ndo_xdp routines
>> with proper checks. can we add this later?
>
> I'd be concerned that if you don't xdp userspace will keep poking
> at lower devs. Then it will stop working if you add this later.

The failover device is better off not providing in-driver XDP since
there are already skbs allocated by the time that we see the packet
here anyway. As such generic XDP is the preferred way to handle this
since it will work regardless of what lower devices are present.

The only advantage of having XDP down at the virtio or VF level would
be that it performs better, but at the cost of complexity since we
would need to rebind the eBPF program any time a device is hotplugged
out and then back in. For now the current approach is in keeping with
how bonding and other similar drivers are currently handling this.

Thanks.

- Alex