Subject: Re: [PATCH v4 2/2] virtio_net: Extend virtio to use VF datapath when available

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 2 Mar 2018 23:36:10 +0200

On Fri, Mar 02, 2018 at 01:11:56PM -0800, Siwei Liu wrote:
> On Thu, Mar 1, 2018 at 12:08 PM, Sridhar Samudrala
> <[email-masked]> wrote:
> > This patch enables virtio_net to switch over to a VF datapath when a VF
> > netdev is present with the same MAC address. It allows live migration
> > of a VM with a direct attached VF without the need to setup a bond/team
> > between a VF and virtio net device in the guest.
> >
> > The hypervisor needs to enable only one datapath at any time so that
> > packets don't get looped back to the VM over the other datapath. When a VF
> > is plugged, the virtio datapath link state can be marked as down. The
> > hypervisor needs to unplug the VF device from the guest on the source host
> > and reset the MAC filter of the VF to initiate failover of datapath to
> > virtio before starting the migration. After the migration is completed,
> > the destination hypervisor sets the MAC filter on the VF and plugs it back
> > to the guest to switch over to VF datapath.
> >
> > When BACKUP feature is enabled, an additional netdev(bypass netdev) is
> > created that acts as a master device and tracks the state of the 2 lower
> > netdevs. The original virtio_net netdev is marked as 'backup' netdev and a
> > passthru device with the same MAC is registered as 'active' netdev.
> >
> > This patch is based on the discussion initiated by Jesse on this thread.
> > https://marc.info/?l=linux-virtualization&m=151189725224231&w=2
> >
> > Signed-off-by: Sridhar Samudrala <[email-masked]>
> > Signed-off-by: Alexander Duyck <[email-masked]>
> > Reviewed-by: Jesse Brandeburg <[email-masked]>
> > ---
> >  drivers/net/virtio_net.c | 683 ++++++++++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 682 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index bcd13fe906ca..f2860d86c952 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -30,6 +30,8 @@
> >  #include <linux/cpu.h>
> >  #include <linux/average.h>
> >  #include <linux/filter.h>
> > +#include <linux/netdevice.h>
> > +#include <linux/pci.h>
> >  #include <net/route.h>
> >  #include <net/xdp.h>
> >
> > @@ -206,6 +208,9 @@ struct virtnet_info {
> >         u32 speed;
> >
> >         unsigned long guest_offloads;
> > +
> > +       /* upper netdev created when BACKUP feature enabled */
> > +       struct net_device *bypass_netdev;
> >  };
> >
> >  struct padded_vnet_hdr {
> > @@ -2236,6 +2241,22 @@ static int virtnet_xdp(struct net_device *dev, struct netdev_bpf *xdp)
> >         }
> >  }
> >
> > +static int virtnet_get_phys_port_name(struct net_device *dev, char *buf,
> > +                                     size_t len)
> > +{
> > +       struct virtnet_info *vi = netdev_priv(dev);
> > +       int ret;
> > +
> > +       if (!virtio_has_feature(vi->vdev, VIRTIO_NET_F_BACKUP))
> > +               return -EOPNOTSUPP;
> > +
> > +       ret = snprintf(buf, len, "_bkup");
> > +       if (ret >= len)
> > +               return -EOPNOTSUPP;
> > +
> > +       return 0;
> > +}
> > +
> 
> What if the systemd/udevd is not new enough to enforce the
> n<phys_port_name> naming? Would virtio_bypass get a different name
> than the original virtio_net?

You mean people using ethX names? Any hardware config change breaks
these, I don't think that can be helped.

> Should we detect this earlier and fall
> back to legacy mode without creating the bypass netdev and ensalving
> the VF?

I don't think we can do this with existing kernel/userspace APIs.

-- 
MST