Re: [Qemu-devel] [PATCH] qemu: Introduce VIRTIO_NET_F_STANDBY feature bit to virtio_net

["Michael S. Tsirkin" <mst@redhat.com>]

I don't think this is sufficient.

If both primary and standby devices are present, a legacy guest without
support for the feature might see two devices with same mac and get
confused.

I think that we should only make primary visible after guest acked the
backup feature bit.

And on reset or when backup is cleared in some other way, unplug the
primary.

Something like the below will do the job:

Primary device is added with a special "primary-failover" flag.
A virtual machine is then initialized with just a standby virtio
device. Primary is not yet added.

Later QEMU detects that guest driver device set DRIVER_OK.
It then exposes the primary device to the guest, and triggers 
a device addition event (hot-plug event) for it.

If QEMU detects guest driver removal, it initiates a hot-unplug sequence
to remove the primary driver.  In particular, if QEMU detects guest
re-initialization (e.g. by detecting guest reset) it immediately removes
the primary device.

We can move some of this code to management as well, architecturally it
does not make too much sense but it might be easier implementation-wise.

HTH

On Mon, Jun 04, 2018 at 06:41:48PM -0700, Samudrala, Sridhar wrote:
> Ping on this patch now that the kernel patches are accepted into davem's net-next tree.
> https://patchwork.ozlabs.org/cover/920005/
> 
> 
> On 5/7/2018 4:09 PM, Sridhar Samudrala wrote:
> > This feature bit can be used by hypervisor to indicate virtio_net device to
> > act as a standby for another device with the same MAC address.
> > 
> > I tested this with a small change to the patch to mark the STANDBY feature 'true'
> > by default as i am using libvirt to start the VMs.
> > Is there a way to pass the newly added feature bit 'standby' to qemu via libvirt
> > XML file?
> > 
> > Signed-off-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
> > ---
> >   hw/net/virtio-net.c                         | 2 ++
> >   include/standard-headers/linux/virtio_net.h | 3 +++
> >   2 files changed, 5 insertions(+)
> > 
> > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > index 90502fca7c..38b3140670 100644
> > --- a/hw/net/virtio-net.c
> > +++ b/hw/net/virtio-net.c
> > @@ -2198,6 +2198,8 @@ static Property virtio_net_properties[] = {
> >                        true),
> >       DEFINE_PROP_INT32("speed", VirtIONet, net_conf.speed, SPEED_UNKNOWN),
> >       DEFINE_PROP_STRING("duplex", VirtIONet, net_conf.duplex_str),
> > +    DEFINE_PROP_BIT64("standby", VirtIONet, host_features, VIRTIO_NET_F_STANDBY,
> > +                      false),
> >       DEFINE_PROP_END_OF_LIST(),
> >   };
> > diff --git a/include/standard-headers/linux/virtio_net.h b/include/standard-headers/linux/virtio_net.h
> > index e9f255ea3f..01ec09684c 100644
> > --- a/include/standard-headers/linux/virtio_net.h
> > +++ b/include/standard-headers/linux/virtio_net.h
> > @@ -57,6 +57,9 @@
> >   					 * Steering */
> >   #define VIRTIO_NET_F_CTRL_MAC_ADDR 23	/* Set MAC address */
> > +#define VIRTIO_NET_F_STANDBY      62    /* Act as standby for another device
> > +                                         * with the same MAC.
> > +                                         */
> >   #define VIRTIO_NET_F_SPEED_DUPLEX 63	/* Device set linkspeed and duplex */
> >   #ifndef VIRTIO_NET_NO_LEGACY