linux-kernel - [PATCH net-next] virtio-net: avoid unnecessary checksum calculation on guest RX

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Message-ID: <20251125175117.995179-1-jon@nutanix.com>
Date: Tue, 25 Nov 2025 10:51:16 -0700
From: Jon Kohler <jon@...anix.com>
To: "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Eugenio Pérez <eperezma@...hat.com>,
        Paolo Abeni <pabeni@...hat.com>, virtualization@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Cc: Jon Kohler <jon@...anix.com>
Subject: [PATCH net-next] virtio-net: avoid unnecessary checksum calculation on guest RX

Commit a2fb4bc4e2a6 ("net: implement virtio helpers to handle UDP
GSO tunneling.") inadvertently altered checksum offload behavior
for guests not using UDP GSO tunneling.

Before, tun_put_user called tun_vnet_hdr_from_skb, which passed
has_data_valid = true to virtio_net_hdr_from_skb.

After, tun_put_user began calling tun_vnet_hdr_tnl_from_skb instead,
which passes has_data_valid = false into both call sites.

This caused virtio hdr flags to not include VIRTIO_NET_HDR_F_DATA_VALID
for SKBs where skb->ip_summed == CHECKSUM_UNNECESSARY. As a result,
guests are forced to recalculate checksums unnecessarily.

Restore the previous behavior by ensuring has_data_valid = true is
passed in the !tnl_gso_type case.

Cc: Paolo Abeni <pabeni@...hat.com>
Fixes: a2fb4bc4e2a6 ("net: implement virtio helpers to handle UDP GSO tunneling.")
Signed-off-by: Jon Kohler <jon@...anix.com>
---
 include/linux/virtio_net.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index b673c31569f3..570c6dd1666d 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -394,7 +394,7 @@ virtio_net_hdr_tnl_from_skb(const struct sk_buff *skb,
 	tnl_gso_type = skb_shinfo(skb)->gso_type & (SKB_GSO_UDP_TUNNEL |
 						    SKB_GSO_UDP_TUNNEL_CSUM);
 	if (!tnl_gso_type)
-		return virtio_net_hdr_from_skb(skb, hdr, little_endian, false,
+		return virtio_net_hdr_from_skb(skb, hdr, little_endian, true,
 					       vlan_hlen);

 	/* Tunnel support not negotiated but skb ask for it. */
-- 
2.43.0