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[80.230.79.166]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-43b41dd93c0sm10828039f8f.10.2026.03.16.07.46.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 16 Mar 2026 07:46:58 -0700 (PDT) Date: Mon, 16 Mar 2026 10:46:55 -0400 From: "Michael S. Tsirkin" To: Zhud Cc: "jasowang@redhat.com" , "xuanzhuo@linux.alibaba.com" , "eperezma@redhat.com" , "andrew+netdev@lunn.ch" , "davem@davemloft.net" , "edumazet@google.com" , "kuba@kernel.org" , "pabeni@redhat.com" , "willemb@google.com" , "netdev@vger.kernel.org" , "virtualization@lists.linux.dev" , Jing Li , Zhiwei Ying Subject: Re: [PATCH net-next v2] virtio-net: enable NETIF_F_GRO_HW only if GRO-related offloads are supported Message-ID: <20260316095919-mutt-send-email-mst@kernel.org> References: <20260316072152.910857-1-zhud@hygon.cn> <20260316051327-mutt-send-email-mst@kernel.org> <607818aba89a44d88afa213f39611451@hygon.cn> <20260316063139-mutt-send-email-mst@kernel.org> <20260316092707-mutt-send-email-mst@kernel.org> <4e3ce2d6120545719a286d3daafbef38@hygon.cn> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <4e3ce2d6120545719a286d3daafbef38@hygon.cn> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: If4D507s1Sc2PQ8tW3qaqW8o4fsLPN557QvNKT6rpkY_1773672420 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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 > > > > > > > > > > > > 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 > > > > > > > > > > > > > > > > > > > > > > > > > > > >