Andrew and Nikolay reported connectivity issues with Cilium's service
load-balancing in case of vmxnet3.

If a BPF program for native XDP adds an encapsulation header such as
IPIP and transmits the packet out the same interface, then in case
of vmxnet3 a corrupted packet is being sent and subsequently dropped
on the path.

vmxnet3_xdp_xmit_frame() which is called e.g. via vmxnet3_run_xdp()
through vmxnet3_xdp_xmit_back() calculates an incorrect DMA address:

  page = virt_to_page(xdpf->data);
  tbi->dma_addr = page_pool_get_dma_addr(page) +
                  VMXNET3_XDP_HEADROOM;
  dma_sync_single_for_device(&adapter->pdev->dev,
                             tbi->dma_addr, buf_size,
                             DMA_TO_DEVICE);

The above assumes a fixed offset (VMXNET3_XDP_HEADROOM), but the XDP
BPF program could have moved xdp->data. While the passed buf_size is
correct (xdpf->len), the dma_addr needs to have a dynamic offset which
can be calculated as xdpf->data - (void *)xdpf, that is, xdp->data -
xdp->data_hard_start.

Fixes: 54f00cce ("vmxnet3: Add XDP support.")
Reported-by: Andrew Sauber <andrew.sauber@isovalent.com>
Reported-by: Nikolay Nikolaev <nikolay.nikolaev@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Nikolay Nikolaev <nikolay.nikolaev@isovalent.com>
Acked-by: Anton Protopopov <aspsk@isovalent.com>
Cc: William Tu <witu@nvidia.com>
Cc: Ronak Doshi <ronak.doshi@broadcom.com>
Link: https://patch.msgid.link/a0888656d7f09028f9984498cc698bb5364d89fc.1728931137.git.daniel@iogearbox.netSigned-off-by: Paolo Abeni <pabeni@redhat.com>