Subject: Re: [PATCH v2 1/1] virtio_net: notify MAC address change on device initialization

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 24 Jan 2023 05:15:17 -0500

On Mon, Jan 23, 2023 at 01:00:22PM +0100, Laurent Vivier wrote:
> In virtnet_probe(), if the device doesn't provide a MAC address the
> driver assigns a random one.
> As we modify the MAC address we need to notify the device to allow it
> to update all the related information.
> 
> The problem can be seen with vDPA and mlx5_vdpa driver as it doesn't
> assign a MAC address by default. The virtio_net device uses a random
> MAC address (we can see it with "ip link"), but we can't ping a net
> namespace from another one using the virtio-vdpa device because the
> new MAC address has not been provided to the hardware.

And then what exactly happens? Does hardware drop the outgoing
or the incoming packets? Pls include in the commit log.

> Signed-off-by: Laurent Vivier <[email-masked]>
> ---
>  drivers/net/virtio_net.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 7723b2a49d8e..4bdc8286678b 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3800,6 +3800,8 @@ static int virtnet_probe(struct virtio_device *vdev)
>  		eth_hw_addr_set(dev, addr);
>  	} else {
>  		eth_hw_addr_random(dev);
> +		dev_info(&vdev->dev, "Assigned random MAC address %pM\n",
> +			 dev->dev_addr);
>  	}
>  
>  	/* Set up our device-specific information */
> @@ -3956,6 +3958,18 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	pr_debug("virtnet: registered device %s with %d RX and TX vq's\n",
>  		 dev->name, max_queue_pairs);
>  
> +	/* a random MAC address has been assigned, notify the device */
> +	if (!virtio_has_feature(vdev, VIRTIO_NET_F_MAC) &&
> +	    virtio_has_feature(vi->vdev, VIRTIO_NET_F_CTRL_MAC_ADDR)) {

Maybe add a comment explaining that we don't fail probe if
VIRTIO_NET_F_CTRL_MAC_ADDR is not there because
many devices work fine without getting MAC explicitly.

> +		struct scatterlist sg;
> +
> +		sg_init_one(&sg, dev->dev_addr, dev->addr_len);
> +		if (!virtnet_send_command(vi, VIRTIO_NET_CTRL_MAC,
> +					  VIRTIO_NET_CTRL_MAC_ADDR_SET, &sg)) {
> +			dev_warn(&vdev->dev, "Failed to update MAC address.\n");

Here, I'm not sure we want to proceed. Is it useful sometimes?
I note that we deny with virtnet_set_mac_address.

> +		}
> +	}
> +
>  	return 0;

Also, some code duplication with virtnet_set_mac_address here.

Also:
	When using the legacy interface, \field{mac} is driver-writable
	which provided a way for drivers to update the MAC without
	negotiating VIRTIO_NET_F_CTRL_MAC_ADDR.

How about factoring out code in virtnet_set_mac_address
and reusing that?

This will also handle corner cases such as VIRTIO_NET_F_STANDBY
which are not currently addressed.

>  free_unregister_netdev:
> -- 
> 2.39.0