Subject: Re: [PATCH net-next v2] virtio-net: enable NETIF_F_GRO_HW only if GRO-related offloads are supported

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 16 Mar 2026 05:15:41 -0400

Thanks! Yes something to improve:

On Mon, Mar 16, 2026 at 03:21:52PM +0800, Di Zhu wrote:
> Although VIRTIO_NET_F_CTRL_GUEST_OFFLOADS is negotiated, which indicates
> the device supports dynamic control of guest offloads, it does not
> necessarily mean the device supports specific hardware GRO features.
> 
> If none of the features defined in GUEST_OFFLOAD_GRO_HW_MASK (such as
> TSO4, TSO6, or UFO) are present in vi->guest_offloads_capable, the
> device effectively lacks the hardware capability to perform GRO.

So what is the user-visible problem this is trying to address?

> 
> So, making NETIF_F_GRO_HW conditional on these feature bits ensures the
> stack does not enable an unsupported hardware offload configuration.

I guess the assumption is that without this, something
enables such a config? Which stack is this and what happens then?

> Fixes: a02e8964eaf9 ("virtio-net: ethtool configurable LRO")
> Signed-off-by: Di Zhu <[email-masked]>

judging by this, has something to do with LRO?

> ---
> /* v2 */
>   -make the modified logic clearer
> ---
>  drivers/net/virtio_net.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 72d6a9c6a5a2..b233c99925e9 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -6781,8 +6781,6 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO4) ||
>  	    virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO6))
>  		dev->features |= NETIF_F_GRO_HW;
> -	if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS))
> -		dev->hw_features |= NETIF_F_GRO_HW;
>  
>  	dev->vlan_features = dev->features;
>  	dev->xdp_features = NETDEV_XDP_ACT_BASIC | NETDEV_XDP_ACT_REDIRECT |
> @@ -7058,6 +7056,10 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	}
>  	vi->guest_offloads_capable = vi->guest_offloads;
>  
> +	if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS) &&
> +	    (vi->guest_offloads_capable & GUEST_OFFLOAD_GRO_HW_MASK))
> +		dev->hw_features |= NETIF_F_GRO_HW;
> +
>  	rtnl_unlock();
>  
>  	err = virtnet_cpu_notif_add(vi);
> -- 
> 2.34.1
>