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 10:46:55 -0400

On Mon, Mar 16, 2026 at 01:57:22PM +0000, Zhud wrote:
>  > 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?
> 
> Yes, of course, but there's still a difference between "best-effort" and "no-effort." Right?

I am not saying this does not improve the user experience.
But let us set the expectations correctly.

What this does (I think):

	When a virtio device does not have either GUEST_TSO6 or
	GUEST_TSO4 offloads, this means it can't really do
	hardware GRO.

	however, the driver will set NETIF_F_GRO_HW whenever
	the device allows control over offload support - even
	if the offloads that can be controlled have nothing
	to do with GRO.

	As a result, in such a setup, rx-gro-hw reported for the device
	is too optimistic.  Improve the situation by masking off
	NETIF_F_GRO_HW.

	Out of abundance of caution, this does not change the
	current behaviour for hardware with just v6 or just v4 GRO:
	current interfaces do not allow
	distinguishing between v6/v4 GRO, so we can't expose
	them to userspace precisely.

Also:
> Fixes: a02e8964eaf9 ("virtio-net: ethtool configurable LRO")

are you sure it's right?

> > 
> > > > > 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
> > > > > > >
> > > > > >
> > > > >
> > > >
> > >
> > 
>