linux-kernel - Re: [PATCH net v3] virtio-net: avoid unnecessary checksum calculation on guest RX

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Message-ID: <8FC82034-6D22-4CDC-B444-60F67A25514C@nutanix.com>
Date: Wed, 26 Nov 2025 16:11:48 +0000
From: Jon Kohler <jon@...anix.com>
To: Jason Wang <jasowang@...hat.com>
CC: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Andrew Lunn
	<andrew+netdev@...n.ch>,
        "David S. Miller" <davem@...emloft.net>,
        Eric
 Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni
	<pabeni@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Xuan Zhuo
	<xuanzhuo@...ux.alibaba.com>,
        Eugenio Pérez
	<eperezma@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "virtualization@...ts.linux.dev" <virtualization@...ts.linux.dev>
Subject: Re: [PATCH net v3] virtio-net: avoid unnecessary checksum calculation
 on guest RX



> On Nov 26, 2025, at 1:29 AM, Jason Wang <jasowang@...hat.com> wrote:
> 
> On Wed, Nov 26, 2025 at 5:46 AM Jon Kohler <jon@...anix.com> wrote:
>> 
>> 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, but only from tun side, as
>> virtio_net_hdr_tnl_from_skb() is used also by the virtio_net driver,
>> which in turn must not use VIRTIO_NET_HDR_F_DATA_VALID on tx.
>> 
>> 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>
>> ---
> 
> Acked-by: Jason Wang <jasowang@...hat.com>
> 
> (Should this go -stable?)
> 
> Thanks

It could, sure. This made it into 6.17 branch.

Would you like me to send a separate patch with a Cc: stable
or could someone just edit the commit msg when they queue
this?