On Tue, Jun 18, 2024 at 11:17 AM Heng Qi [off-list ref] wrote:

On Tue, 18 Jun 2024 11:10:26 +0800, Jason Wang [off-list ref] wrote:

quoted

On Mon, Jun 17, 2024 at 9:15 PM Heng Qi [off-list ref] wrote:

quoted

The XDP program can't correctly handle partially checksummed
packets, but works fine with fully checksummed packets.

Not sure this is ture, if I was not wrong, XDP can try to calculate checksum.

XDP's interface serves a full checksum,

What do you mean by "serve" here? I mean, XDP can calculate the
checksum and fill it in the packet by itself.

and this is why we disabled the
offloading of VIRTIO_NET_F_GUEST_CSUM when loading XDP.

If we trust the device to disable VIRTIO_NET_F_GUEST_CSUM, any reason
to check VIRTIO_NET_HDR_F_NEEDS_CSUM again in the receive path?

Thanks.

Thanks

quoted

Thanks

quoted

If the
device has already validated fully checksummed packets, then
the driver doesn't need to re-validate them, saving CPU resources.

Additionally, the driver does not drop all partially checksummed
packets when VIRTIO_NET_F_GUEST_CSUM is not negotiated. This is
not a bug, as the driver has always done this.

Fixes: 436c9453a1ac ("virtio-net: keep vnet header zeroed after processing XDP")
Signed-off-by: Heng Qi <redacted>
---
 drivers/net/virtio_net.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index aa70a7ed8072..ea10db9a09fa 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c

@@ -1360,6 +1360,10 @@ static struct sk_buff *receive_small_xdp(struct net_device *dev,
        if (unlikely(hdr->hdr.gso_type))
                goto err_xdp;

+       /* Partially checksummed packets must be dropped. */
+       if (unlikely(hdr->hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM))
+               goto err_xdp;
+
        buflen = SKB_DATA_ALIGN(GOOD_PACKET_LEN + headroom) +
                SKB_DATA_ALIGN(sizeof(struct skb_shared_info));

@@ -1677,6 +1681,10 @@ static void *mergeable_xdp_get_buf(struct virtnet_info *vi,
        if (unlikely(hdr->hdr.gso_type))
                return NULL;

+       /* Partially checksummed packets must be dropped. */
+       if (unlikely(hdr->hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM))
+               return NULL;
+
        /* Now XDP core assumes frag size is PAGE_SIZE, but buffers
         * with headroom may add hole in truesize, which
         * make their length exceed PAGE_SIZE. So we disabled the

@@ -1943,6 +1951,7 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
        struct net_device *dev = vi->dev;
        struct sk_buff *skb;
        struct virtio_net_common_hdr *hdr;
+       u8 flags;

        if (unlikely(len < vi->hdr_len + ETH_HLEN)) {
                pr_debug("%s: short packet %i\n", dev->name, len);

@@ -1951,6 +1960,15 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
                return;
        }

+       /* 1. Save the flags early, as the XDP program might overwrite them.
+        * These flags ensure packets marked as VIRTIO_NET_HDR_F_DATA_VALID
+        * stay valid after XDP processing.
+        * 2. XDP doesn't work with partially checksummed packets (refer to
+        * virtnet_xdp_set()), so packets marked as
+        * VIRTIO_NET_HDR_F_NEEDS_CSUM get dropped during XDP processing.
+        */
+       flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags;
+
        if (vi->mergeable_rx_bufs)
                skb = receive_mergeable(dev, vi, rq, buf, ctx, len, xdp_xmit,
                                        stats);

@@ -1966,7 +1984,7 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
        if (dev->features & NETIF_F_RXHASH && vi->has_rss_hash_report)
                virtio_skb_set_hash(&hdr->hash_v1_hdr, skb);

-       if (hdr->hdr.flags & VIRTIO_NET_HDR_F_DATA_VALID)
+       if (flags & VIRTIO_NET_HDR_F_DATA_VALID)
                skb->ip_summed = CHECKSUM_UNNECESSARY;

        if (virtio_net_hdr_to_skb(skb, &hdr->hdr,
--

2.32.0.3.g01195cf9f