Subject: Re: [PATCH net-next 2/4] virtio-net: reprobe csum related fields for skb passed by XDP

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 19 Jun 2023 07:27:26 -0400

On Mon, Jun 19, 2023 at 06:57:36PM +0800, Heng Qi wrote:
> Currently, the VIRTIO_NET_F_GUEST_CSUM (corresponds to NETIF_F_RXCSUM
> for netdev) feature of the virtio-net driver conflicts with the loading
> of XDP, which is caused by the problem described in [1][2], that is,
> XDP may cause errors in partial csumed-related fields which can lead
> to packet dropping.
> 
> In addition, when communicating between vm and vm on the same host, the
> receiving side vm will receive packets marked as
> VIRTIO_NET_HDR_F_NEEDS_CSUM, but after these packets are processed by
> XDP, the VIRTIO_NET_HDR_F_NEEDS_CSUM and skb CHECKSUM_PARTIAL flags will
> be cleared, causing the packet dropping.
> 
> This patch introduces a helper function, which will try to solve the
> above problems in the subsequent patch.
> 
> [1] commit 18ba58e1c234 ("virtio-net: fail XDP set if guest csum is negotiated")
> [2] commit e59ff2c49ae1 ("virtio-net: disable guest csum during XDP set")
> 
> Signed-off-by: Heng Qi <[email-masked]>
> Reviewed-by: Xuan Zhuo <[email-masked]>

squash this and patch 1 into where the helpers are used.

in particular so we don't get warnings with bisect.

> ---
>  drivers/net/virtio_net.c | 38 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 38 insertions(+)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 36cae78f6311..07b4801d689c 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1663,6 +1663,44 @@ static int virtnet_flow_dissect_udp_tcp(struct virtnet_info *vi, struct sk_buff
>  	return 0;
>  }
>  
> +static int virtnet_set_csum_after_xdp(struct virtnet_info *vi,
> +				      struct sk_buff *skb,
> +				      __u8 flags)
> +{
> +	int err;
> +
> +	/* When XDP program is loaded, for example, the vm-vm scenario
> +	 * on the same host, packets marked as VIRTIO_NET_HDR_F_NEEDS_CSUM
> +	 * will travel. Although these packets are safe from the point of
> +	 * view of the vm, to avoid modification by XDP and successful
> +	 * forwarding in the upper layer, we re-probe the necessary checksum
> +	 * related information: skb->csum_{start, offset}, pseudo-header csum.
> +	 *
> +	 * This benefits us:
> +	 * 1. XDP can be loaded when there's _F_GUEST_CSUM.
> +	 * 2. The device verifies the checksum of packets , especially
> +	 *    benefiting for large packets.
> +	 * 3. In the same-host vm-vm scenario, packets marked as
> +	 *    VIRTIO_NET_HDR_F_NEEDS_CSUM are no longer dropped after being
> +	 *    processed by XDP.
> +	 */
> +	if (flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) {
> +		err = virtnet_flow_dissect_udp_tcp(vi, skb);
> +		if (err < 0)
> +			return -EINVAL;
> +
> +		skb->ip_summed = CHECKSUM_PARTIAL;
> +	} else if (flags && VIRTIO_NET_HDR_F_DATA_VALID) {
> +		/* We want to benefit from this: XDP guarantees that packets marked
> +		 * as VIRTIO_NET_HDR_F_DATA_VALID still have correct csum after they
> +		 * are processed.
> +		 */
> +		skb->ip_summed = CHECKSUM_UNNECESSARY;
> +	}
> +
> +	return 0;
> +}
> +
>  static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
>  			void *buf, unsigned int len, void **ctx,
>  			unsigned int *xdp_xmit,
> -- 
> 2.19.1.6.gb485710b