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

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Message-ID: <CACGkMEvK1M_h783QyEXJ5jz25T-Vtkj4=-_dPLzYGwPg8NSU5Q@mail.gmail.com>
Date: Wed, 26 Nov 2025 14:29:58 +0800
From: Jason Wang <jasowang@...hat.com>
To: Jon Kohler <jon@...anix.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, 
	linux-kernel@...r.kernel.org, virtualization@...ts.linux.dev
Subject: Re: [PATCH net v3] virtio-net: avoid unnecessary checksum calculation
 on guest RX

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