On Sat, 2024-02-03 at 10:56 +0800, Liang Chen wrote:

On Sat, Feb 3, 2024 at 12:20 AM Jesper Dangaard Brouer [off-list ref] wrote:

quoted

On 02/02/2024 13.11, Liang Chen wrote:

[...]

quoted

quoted

@@ -1033,6 +1039,16 @@ static void put_xdp_frags(struct xdp_buff *xdp)
      }
  }

+static void virtnet_xdp_save_rx_hash(struct virtnet_xdp_buff *virtnet_xdp,
+                                  struct net_device *dev,
+                                  struct virtio_net_hdr_v1_hash *hdr_hash)
+{
+     if (dev->features & NETIF_F_RXHASH) {
+             virtnet_xdp->hash_value = hdr_hash->hash_value;
+             virtnet_xdp->hash_report = hdr_hash->hash_report;
+     }
+}
+

Would it be possible to store a pointer to hdr_hash in virtnet_xdp_buff,
with the purpose of delaying extracting this, until and only if XDP
bpf_prog calls the kfunc?

That seems to be the way v1 works,
https://lore.kernel.org/all/20240122102256.261374-1-liangchen.linux@gmail.com/ (local)
. But it was pointed out that the inline header may be overwritten by
the xdp prog, so the hash is copied out to maintain its integrity.

Why? isn't XDP supposed to get write access only to the pkt
contents/buffer?

if the XDP program can really change the virtio_net_hdr, that looks
potentially dangerous/bug prone regardless of this patch.

Thanks,

Paolo