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Date:   Wed, 20 Dec 2017 16:57:37 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Baron <jbaron@...mai.com>
Cc:     qemu-devel@...gnu.org, linux-kernel@...r.kernel.org,
        jasowang@...hat.com
Subject: Re: [PATCH net-next 1/2] virtio_net: allow hypervisor to indicate
 linkspeed and duplex setting

On Thu, Dec 14, 2017 at 02:33:53PM -0500, Jason Baron wrote:
> If the hypervisor exports the link and duplex speed, let's use that instead
> of the default unknown speed. The user can still overwrite it later if
> desired via: 'ethtool -s'. This allows the hypervisor to set the default
> link speed and duplex setting without requiring guest changes and is
> consistent with how other network drivers operate. We ran into some cases
> where the guest software was failing due to a lack of linkspeed and had to
> fall back to a fully emulated network device that does export a linkspeed
> and duplex setting.
> 
> Implement by adding a new VIRTIO_NET_F_SPEED_DUPLEX feature flag, to
> indicate that a linkspeed and duplex setting are present.
> 
> Signed-off-by: Jason Baron <jbaron@...mai.com>
> Cc: "Michael S. Tsirkin" <mst@...hat.com>
> Cc: Jason Wang <jasowang@...hat.com>
> ---
>  drivers/net/virtio_net.c        | 11 ++++++++++-
>  include/uapi/linux/virtio_net.h |  4 ++++
>  2 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 6fb7b65..e7a2ad6 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2671,6 +2671,14 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	netif_set_real_num_rx_queues(dev, vi->curr_queue_pairs);
>  
>  	virtnet_init_settings(dev);
> +	if (virtio_has_feature(vdev, VIRTIO_NET_F_SPEED_DUPLEX)) {
> +		vi->speed = virtio_cread32(vdev,
> +					offsetof(struct virtio_net_config,
> +					speed));
> +		vi->duplex = virtio_cread8(vdev,
> +					offsetof(struct virtio_net_config,
> +					duplex));
> +	}
>  
>  	err = register_netdev(dev);
>  	if (err) {

How are we going to validate speed values? Imagine host
using a new 1000Gbit device and exposing that to guest.

Need to think what do we want guest to do.
I think that ideally we'd say it's a 100Gbit device.

For duplex, force to one of 3 valid values?


> @@ -2796,7 +2804,8 @@ static struct virtio_device_id id_table[] = {
>  	VIRTIO_NET_F_CTRL_RX, VIRTIO_NET_F_CTRL_VLAN, \
>  	VIRTIO_NET_F_GUEST_ANNOUNCE, VIRTIO_NET_F_MQ, \
>  	VIRTIO_NET_F_CTRL_MAC_ADDR, \
> -	VIRTIO_NET_F_MTU, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS
> +	VIRTIO_NET_F_MTU, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS, \
> +	VIRTIO_NET_F_SPEED_DUPLEX
>  
>  static unsigned int features[] = {
>  	VIRTNET_FEATURES,
> diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
> index fc353b5..acfcf68 100644
> --- a/include/uapi/linux/virtio_net.h
> +++ b/include/uapi/linux/virtio_net.h
> @@ -36,6 +36,7 @@
>  #define VIRTIO_NET_F_GUEST_CSUM	1	/* Guest handles pkts w/ partial csum */
>  #define VIRTIO_NET_F_CTRL_GUEST_OFFLOADS 2 /* Dynamic offload configuration. */
>  #define VIRTIO_NET_F_MTU	3	/* Initial MTU advice */
> +#define VIRTIO_NET_F_SPEED_DUPLEX 4	/* Host set linkspeed and duplex */
>  #define VIRTIO_NET_F_MAC	5	/* Host has given MAC address. */
>  #define VIRTIO_NET_F_GUEST_TSO4	7	/* Guest can handle TSOv4 in. */
>  #define VIRTIO_NET_F_GUEST_TSO6	8	/* Guest can handle TSOv6 in. */

I think I'd prefer a high feature bit - low bits are ones that can
be backported to legacy interfaces, so I think we should hang on to
these for fixing issues that break communication completely (like the
mtu).


> @@ -76,6 +77,9 @@ struct virtio_net_config {
>  	__u16 max_virtqueue_pairs;
>  	/* Default maximum transmit unit advice */
>  	__u16 mtu;
> +	/* Host exported linkspeed and duplex */
> +	__u32 speed;
> +	__u8 duplex;
>  } __attribute__((packed));
>  
>  /*
> -- 
> 2.6.1

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