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

From: Heng Qi <hengqi@linux.alibaba.com>
To: netdev@vger.kernel.org, bpf@vger.kernel.org
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	Jason Wang <jasowang@redhat.com>,
	Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	John Fastabend <john.fastabend@gmail.com>
Subject: [PATCH net-next 2/4] virtio-net: reprobe csum related fields for skb passed by XDP
Date: Mon, 19 Jun 2023 18:57:36 +0800	[thread overview]
Message-ID: <20230619105738.117733-3-hengqi@linux.alibaba.com> (raw)
In-Reply-To: <20230619105738.117733-1-hengqi@linux.alibaba.com>

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 <hengqi@linux.alibaba.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
 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

