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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 12 Jun 2018 05:17:26 +0300

On Tue, Jun 12, 2018 at 09:54:44AM +0800, Jason Wang wrote:
> 
> 
> On 2018年06月12日 01:26, Michael S. Tsirkin wrote:
> > On Mon, May 07, 2018 at 04:09:54PM -0700, 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 <[email-masked]>
> > So I do not think we can commit to this interface: we
> > really need to control visibility of the primary device.
> 
> The problem is legacy guest won't use primary device at all if we do this.

And that's by design - I think it's the only way to ensure the
legacy guest isn't confused.

> How about control the visibility of standby device?
> 
> Thanks

standy the always there to guarantee no downtime.

> > However just for testing purposes, we could add a non-stable
> > interface "x-standby" with the understanding that as any
> > x- prefix it's unstable and will be changed down the road,
> > likely in the next release.
> > 
> > 
> > > ---
> > >   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
> > > -- 
> > > 2.14.3