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 09:30:04 -0400

On Mon, Mar 16, 2026 at 12:57:00PM +0000, Zhud wrote:
> > 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.
> 
> This might not be the best example, but I want to draw an analogy to show how
> this hardware offload capability can be misleading. For instance, if I enable GRO_HW
> expecting to see lower CPU usage when receiving packets, but it doesn't happen, that
> would be very confusing.

It still can happen if hardware does not offload the specific traffic,
yes?

> > > 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.
> 
> You're absolutely right. Upon rechecking the code, virtnet_set_features already ensures 
> that only bits within vi->guest_offloads_capable are sent to the device.
> Thank you for pointing that out.
> 
> > > > >
> > > > > 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
> > > > >
> > > >
> > >
> > 
>