netdev - Re: [RFC v2 -next 1/2] virtio: Start feature MTU support

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Date:	Wed, 16 Mar 2016 11:23:14 -0700
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Aaron Conole <aconole@...hat.com>
Cc:	netdev@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 -next 1/2] virtio: Start feature MTU support

On Tue, 15 Mar 2016 17:04:12 -0400
Aaron Conole <aconole@...hat.com> wrote:

> --- a/include/uapi/linux/virtio_net.h
> +++ b/include/uapi/linux/virtio_net.h
> @@ -55,6 +55,7 @@
>  #define VIRTIO_NET_F_MQ	22	/* Device supports Receive Flow
>  					 * Steering */
>  #define VIRTIO_NET_F_CTRL_MAC_ADDR 23	/* Set MAC address */
> +#define VIRTIO_NET_F_MTU 25	/* Device supports Default MTU Negotiation */
>  
>  #ifndef VIRTIO_NET_NO_LEGACY
>  #define VIRTIO_NET_F_GSO	6	/* Host handles pkts w/ any GSO type */
> @@ -73,6 +74,8 @@ struct virtio_net_config {
>  	 * Legal values are between 1 and 0x8000
>  	 */
>  	__u16 max_virtqueue_pairs;
> +	/* Default maximum transmit unit advice */
> +	__u16 mtu;
>  } __attribute__((packed));
>  
>  /*

You can't change user visible headers without breaking ABI.
This structure might be used by other user code. Also how can this
work if host is using old size of structure.