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 06:47:54 -0400

On Mon, Mar 16, 2026 at 10:18:04AM +0000, Zhud wrote:
> > 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?
> 
> A key concern is that once a user enables NETIF_F_GRO_HW via ethtool, 
> they might manually disable software GRO (ethtool -K eth0 gro off) assuming the 
> hardware is now handling the aggregation.

Thanks!
Sorry could you be even more specific please?
Is this a theoretical concern or did some users encounter this?
Note that NETIF_F_GRO_HW is best effort anyway: e.g.
it can apply only to TCPv6 and v4 will still need software.

> Secondly, while we haven't encountered a specific hardware failure yet, 
> enabling a hardware offload feature that the DPU does not physically support 
> introduces the risk of undefined hardware behavior

This would be a major concern but I don't get it - how would one trigger this?
It seems that guest_offloads_capable only includes offloads actually
supported.

> > >
> > > 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?
> > 
> 
> Sorry for the confusion, let me clarify the intent.
> The 'stack' here refers to the ethtool interface and the netset (ioctl/netlink) path. 

A bit more detail about the specific set of commands that leads to
confusion in the commit log would be helpful.
Thanks!

> > 
> > > 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
> > >
> > 
>