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@xxxxxxxxxxxxxxxxx>
Reviewed-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
---
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
