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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 11 Jun 2018 20:26:04 +0300

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.

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